UNIT 5
Writing Classes
IN THIS UNIT
Summary: This unit is the continuation of Unit 2, “Using Objects.” In this unit you will learn how to design your own classes. Full understanding of designing classes, writing constructors, and writing methods are key to doing well on the AP Computer Science A Exam.
Key Ideas
Java is an object-oriented programming language whose foundation is built on classes and objects.
A class describes the characteristics of any object that is created from it.
An object is a virtual entity that is created using a class as a blueprint.
Objects are created to store and manipulate information in your program.
An instance variable is used to store an attribute of an object.
A method is an action that an object can perform.
The keyword new is used to create an object.
A reference variable stores the address of the object, not the object itself.
Objects are passed by reference, while primitives are passed by value.
Data encapsulation is a way of hiding user information.
Overloaded constructors have the same name but different parameter lists.
Overloaded methods have the same name but different parameter lists.
Static variables are called class variables and are shared among all objects of the same class.
Static final variables are called class constants and, once given a value, they cannot be changed during the running of the program.
The keyword this is used to refer to the current object.
Scope describes the region of the program in which a variable is known.
The class Declaration
When you design your own class, you must create a file that contains the class declaration. The name of the class is identified in this declaration. By Java naming convention, a class name always starts with an uppercase letter and is written in camel case. Classes are designated public so users can create objects from that class.
Example
Create a class called the Circle class:
Instance Variables
The virtual attributes that describe an object of a class are called its instance variables. A class can have as many instance variables of any data type as it wants as long as they describe some characteristic or feature of an object of the class. Unlike local primitive variables, default values are assigned to primitive instance variables when they are created (int variables are 0, doubles are 0.0, booleans are false, and objects are null). The phrase has-a refers to the instance variables of a class as in: an objectName has-a instanceVariableName.
Example
Every Circle object has-a radius so create an instance variable called radius:
private Versus public Visibility
The words private and public are called visibility modifiers (or access level modifiers). Access to attributes should be kept internal to the class, so instance variables are designated as private. Using the word private in an instance variable declaration ensures that other classes do not have access to the data stored in the variable without asking the object to provide it. Using the word public in the class declaration ensures that any programmer can make objects from that class.
private Versus public
On the AP Computer Science A Exam, always give instance variables private access visibility. This ensures that the data in these variables is hidden.
Constructors
Constructors are the builders of the virtual objects from a class. They are used in combination with the keyword new to create an object from the class. Constructors have the same name as the class and are typically listed near the top of a class. Constructors are designated public. Constructors are used to set the initial state of an object, which should include initial values for all of its instance variables.
No-Argument Constructor
The constructor is the code that is called when you create a new object. When you define a new class, it comes with no-argument constructor. Depending on the IDE that you use, you may or may not see it. It is generally recommended that you write one anyway, especially when you are first learning classes.
The No-Argument Constructor
The no-argument constructor gets its name because no information is passed to this constructor when creating a new object. It is the constructor that allows you to make a generic object from a class.
Parameterized Constructors and Parameters
If you will know some information about an object prior to creating it, then you may want to write a parameterized constructor in your class. You use the parameterized constructor to give initial values to the instance variables when creating a brand-new object. The parameters that are defined in the parameterized constructor are placed in a parameter list and are on the same line as the declaration of the constructor.
The process of sending the initial values to the constructor is called passing a parameter to the constructor. The actual value that is passed to the constructor is called an argument (or actual parameter) and the variable that receives the value inside the constructor is called the formal parameter. You include a line of code in the parameterized constructor that assigns the value of the formal parameter to the instance variable.
If you choose to write a parameterized constructor for a class, then the default, no-argument constructor vanishes. This is why it is generally recommended that you simply write your own.
Example
Write the no-argument constructor and one parameterized constructor for the Circle class.
The parameterized constructor must have a parameter variable for the radius of a circle.
Parameters in the Parameter List
A parameter is a variable that is located in a parameter list in the constructor declaration. A pair of parentheses after the name of the constructor encloses the entire parameter list.
For example, the parameterized constructor for the Circle class has one parameter variable in its parameter list. The name of the parameter is rad (short for radius) and its data type is a double. The no-argument constructor for the Circle class does not have any parameters in its parameter list.
Methods
The real power of an object-oriented programming language takes place when you start to manipulate objects. A method defines an action that allows you to do these manipulations. A method has two options. It may simply perform a service of some kind, or it may compute a value and give the answer back. The action of giving a value back is called returning a value.
Methods that simply perform some action are called void methods. Methods that return a value are called return (or non-void) methods. Return methods must define what data type they will be returning and include a return statement in the method. A return method can return any kind of data type. In non-void methods, a return expression compatible with the return type is evaluated, and a copy of that value is returned. But if the return expression is a reference to an object, a copy of that reference is returned, not a copy of the object. This is referred to as “return by value.”
Using the word public in the method declaration ensures that the method is available for other objects from other classes to use. A method declared private is only accessible within the class.
The return Statement on the AP Computer Science A Exam
Methods that are supposed to return a value must have a reachable return statement. Methods that are void do not have a return statement. This is a big deal on the free-response questions of the AP Computer Science A Exam as you will lose points if you include a return statement in a void method or a constructor.
Accessor and Mutator Methods
When first learning how to write methods for a class, beginning programmers learn two types of methods:
1. Methods that allow you to access the values stored in the instance variables
2. Methods that allow you to modify the values of the instance variables
Methods that return the value of an instance variable are called accessor (or getter) methods. Methods that change the value of an instance variable are called mutator (or modifier or setter) methods. Methods that don’t perform either one of these duties are simply called methods.
Is My Carbonated Soft Drink a Soda or a Pop?
They both refer to the same thing, it just depends on where you live. In a similar way, the methods that retrieve the instance variables of a class can be referred to as accessor or getter methods. The methods that change the value of an instance variable can be called modifier, mutator, or setter methods.
Declaring a Method
Methods are declared using a line of code called a method declaration statement. The declaration includes the access modifier, the return type, the name of the method, and the parameter list. The method signature only includes the name of the method and the parameter list.
General Form of a Method Declaration
One-Track Mind
Methods should have only one goal. The name of the method should clearly describe the purpose of the method.
toString Method
It’s a good idea to override the toString() method that is provided by the Object class. By doing this, the class designer (that’s you) has control over what is printed each time a user calls the System.out.print() method by displaying a meaningful representation of the attributes rather than simply a reference to the memory location (what good is that?).
In the CircleRunner class below, if the toString() method was not overidden, then the call to System.out.println(myCircle) would print Circle@7852e922 (or some other value referring to the location in memory).
Example
Using the Circle class already written, write an accessor method for the radius, a mutator method for the radius, a method that calculates and returns the area of the circle, and a toString method that returns a string describing the circles attributes.
Java Ignores Spaces
Java ignores all spacing that isn’t relevant. The compiler doesn’t care how pretty the code looks in the IDE; it just has to be syntactically correct. On the AP exam, you will sometimes see code that has been streamlined to save space on the page.
For example, the getRadius method for the Circle class could appear like this on the exam:
Putting It All Together: The Circle and CircleRunner Classes
I’m now going to combine the Circle class that we just created with another class to demonstrate code that follows an example of two classes interacting in an object-oriented setting. The Circle class defined how to make a Circle object. The CircleRunner class is where the programmer creates virtual Circle objects and then manipulates them.
Summary of the Circle Class
Every circle has-a radius, so the Circle class defines an instance variable called radius. The class also has two constructors as ways to build a Circle object. The no-argument constructor is used whenever you don’t know the radius at the time that you create the Circle object. The radius gets the default value for a double, which is 0.0. The parameterized constructor is used if you know the radius of the circle at the time you construct the Circle object. You pass the radius when you call the parameterized constructor and the constructor assigns the value of its parameter to the instance variable.
The Circle class also has four methods:
1. The first method, getRadius, returns the radius from that object. Since the method is getting the radius for the programmer, it is called an accessor (or getter) method.
2. The second method, setRadius, is a void method that sets the value of the radius for the object. It is referred to as a modifier (or mutator or setter) method. This method has a parameter, which is a variable that receives a value that is passed to it when the method is called (or invoked) and assigns this value to the instance variable.
3. The third method, getArea, calculates and returns the area of the circle using the object’s radius.
4. Finally, the fourth method, toString returns a string containing the radius and area of the circle.
Summary of the CircleRunner Class
The Circle class can’t do anything all by itself. To be useful, Circle objects are created in a class that is outside of the Circle class. Each object that is created from a class is called an instance of the class and the act of constructing an object is called instantiating an object. A class that contains a main method is sometimes called a runner class. Other times, they are called client programs. The main method is the point of entry for a program. That means that when the programmer runs the program, Java finds the main method first and executes it. You will not have to write main methods on the AP exam.
Notice that the Circle class does not contain a public static void main(String[] args). The purpose of the Circle class is to define how to build and manipulate a Circle object; however, Circle objects are created and manipulated in a different class that contains a main method.
How to Make a New Object from a Class
When you want to make an object from a class, you have to call one of the class’s constructors. However, you may have more than one constructor, including the no-argument constructor or one or more parameterized constructors. The computer decides which constructor to call by looking at the parameter list in the declaration of each constructor. Whichever constructor matches the call to the constructor is used to create the object.
Examples
Example
Write a runner class that instantiates three Circle objects. Make the first Circle object have a radius of 10, while the second and third Circles will get the default value. Manipulate the radii of the Circles and print their areas. Call the toString method with each of the three Circle objects to print each circles radius and area.
Every Object from the Same Class Is a Different Object
Every object that is constructed from a class has its own instance variables. Two different objects can have the same state, which means that their instance variables have the same values.
In this example, the CircleRunner class creates three different Circle objects called myCircle, hisCircle, and herCircle. Notice that the myCircle object has a radius of 10. This is because when the Circle object created it, the computer looked for the parameterized constructor and when it found it, assigned the 10 to the parameter rad. For the hisCircle and herCircle objects, the no-argument constructor was called and the instance variable, radius, was given the default value of 0.0.
Remember that myCircle, hisCircle, and herCircle all refer to different objects that were all created from the same Circle class and each object has its own radius. When each object calls the getArea method, it uses its own radius to compute the area.
The Dot Operator
The dot operator is used to call a method of an object. The name of the object’s reference variable is followed by a period and then the name of the method.
Understanding the Keyword new When Constructing an Object
Beginning Java programmers often have a hard time understanding why the name of the class is used twice when creating an object, such as the Circle objects in the CircleRunner. Let’s analyze the following line of code to find out what’s really happening.
The first use of the word Circle is the data type of the variable. It’s similar to how we defined an int or a double variable. The variable myCircle is called a reference variable of type Circle. It’s similar to a primitive variable in that it holds a value, but it’s way different in the fact that it does not hold the Circle object itself. The value that myCircle holds is the memory address of the soon-to-be-created Circle object.
The second use of the word Circle(10) is a call to the parameterized constructor of the Circle class. The keyword new tells the computer, “Hey we are about ready to make a new object.” The computer looks for a constructor that has a single parameter that matches the data type of the 10. Once it finds this constructor, it gives the 10 to the parameter, rad. The code in the constructor assigns the instance variable, radius, the value of the parameter rad. Then, voilá! The radius of the Circle object is 10.
Finally, the computer looks for a place to store the Circle object in memory (RAM) and when it finds a location, it records the memory address. The construction of the Circle is now complete, and the address of the newly formed Circle object is assigned to the reference variable, myCircle.
Now, anytime you want to manipulate the object that is referenced by myCircle, such as give it a new radius, you are actually telling myCircle to go find the object that it is referencing (also called, pointing to), and then set the radius for that object.
The Reference Variable
The reference variable does not hold the object itself. It holds the address of where the object is stored in RAM (random access memory).
The Reference Variable Versus the Actual Object
The following diagram is meant to help you visualize the relationship between object reference variables and the objects themselves in the computer’s memory. Note: The memory addresses are simulated and only meant to serve as an example of addresses in the computer’s memory.
Question 1: What happens after these two lines of code are executed?
Question 2: What do you suppose happens after this line of code is executed?
If you are following the diagram correctly, you will see that herCircle “takes on the value” of the address of the hisCircle reference variable. Now, herCircle contains the same address that hisCircle contains. This means that both reference variables contain the same address of the same object and are pointing to the same object. The word aliasing is used to describe the situation when two different object reference variables contain the same address of an object.
Look at the object that used to be referenced by herCircle. Nothing is pointing to it. It has been detached from the rest of the world and will be garbage collected by Java. This means that Java will delete it when it is good and ready.
The null Reference
When an object reference variable is created but not given the address of an object, it is considered to have a null reference. This simply means that it is supposed to know where an object lives, but it does not. Whenever you create an object reference variable and don’t assign it the address of an actual object, it has the value of null, which means that it has no value. You can also assign a reference variable the value of null to erase its memory of its former object.
The null Reference
A reference variable that does not contain the address of any object is called a null reference.
Question 3: What do you suppose happens when this statement is executed?
The reference variable hisCircle is now null; it has no value. This means that it does not contain the address of any object. The reference variable herCircle still holds the address of a Circle object and so it can access methods of that object.
Parameters
Primitives Are Passed by Value
Values that are passed to a method or constructor are called actual parameters (or arguments), while the variables in the parameter list of the method declaration are called formal parameters. When an actual parameter is of a primitive type, its value cannot be changed in the method because only the value of the variable is sent to the method.
Said another way, the formal parameter variable becomes a copy of the actual parameter. The method cannot alter the value of the actual parameter variable. The formal parameter variable only receives the value of the actual parameter. The method only has the ability to change the value of the formal parameter variable, not the value of the actual parameter.
Objects Are Passed by Reference
Objects are passed by reference. What does that mean?
Simple answer: It means that when an object is passed to a method, the method has the ability to change the state of the object.
Technical answer: When an actual parameter is an object, it is the object reference that is actually the parameter. This means that when the method receives the reference and stores it in a formal parameter variable, the method now has the ability to change the state of the object because it knows where the object lives. The formal parameter is now an alias of the actual parameter. Essentially, you have just given away the keys to the car.
Example
Demonstrate the difference between passing by reference and passing by value. The state of the object is changed in the method, while the state of the primitive variable is unchanged.
Comparing Against a null Reference
Recall that a nullPointerException error occurs if you attempt to perform a method on a reference variable that is null. How can you tell if a reference variable is null so you won’t get that error? The answer is to perform a null check. Compare the reference variable using the == comparator or != comparator. Just don’t use the equals method to perform a null check. You’ll get a nullPointerException!
Example
Demonstrate how to perform a null check using the Circle class that was created in Concept 2. If the Circle reference is not null, then compute the area. If the Circle reference is null, then return -1 for the area.
Checking Against null
Never use the equals method to determine if an object reference is null. Only use == or != to check if an object reference variable is null.
Overloaded Constructors
It is possible to have more than one constructor in a class. Constructors are always declared using the name of the class, but their parameter lists can differ. If there are two or more constructors in a class, they are all called overloaded constructors. The computer decides which of the overloaded constructors is the correct one to use based on the parameter list.
Overloaded constructors have the same name; however, they must differ by one of these ways:
• a different number of parameters
• the same number of parameters but at least one is a different type
• the same exact parameter types but in a different order
Example
Declare four constructors for a class called Student. The constructors have the same name as the class, but are different because their parameter lists differ.
Overloaded Methods
Methods that have the same name but different parameters lists are called overloaded methods. The computer decides which of the overloaded methods is the correct one to use based on the parameter list.
Overloaded methods must have the same name and may have a different return type; however, they must differ by one of these ways:
• a different number of parameters
• the same number of parameters but at least one is a different type
• the same exact parameter types but in a different order
Example
Declare four overloaded methods. The methods should have the same name, but different parameter lists as described above.
Blast from the Past
Recall that the substring method from the String class is overloaded. There are two versions of the method, but they are different because their parameter lists are not identical.
static, static, static
static Variables (Class Variables)
There are times when a class needs to have a variable that is shared by all the objects that come from the class. The variable is declared and assigned a value as though it were an instance variable; however, it uses the keyword static in its declaration. Static variables are also called class variables and can be declared private or public.
static final Variables (Class Constants)
Likewise, there are times when a class needs to have a constant that is used by all the objects of the class. As the name constant suggests, it cannot be reassigned a different value at any other place in the program. In fact, a compile-time error will occur if you try to give the constant a different value. Class constants are declared using the keywords static and final in their declaration and use all uppercase letters (and underscores, if necessary). These static final variables are also called class constants.
static Methods (Class Methods)
If a method is labeled static, it can only call other static methods and can only use or modify static variables. Static methods cannot access or change the values of the instance variables or cannot call non-static methods in the class. Class methods are associated with methods of the class, not individual objects of the class.
Example
Suppose you are hired by Starbucks to keep track of the total number of stores. You’ve already decided to have a class called StarbucksStore, but how can you keep track of the total number of stores? One solution is to make a static variable, called storeCount. Then, each time a store is constructed (pun intended), increment this class variable. Access will be given to the storeCount by creating a static method called getStoreCount. Let’s also say that you decide to make a variable that holds the logo for all the stores. You choose to make this variable a constant because every store shares the logo, and none of them should be allowed to change the logo. For this we will make a static final variable called STARBUCKS_LOGO and initialize it to “Mermaid”.
Did you notice how the class method getStoreCount() and constant STARBUCKS_LOGO were called from the main program? Since the method and constant belong to the class and not a particular object of that class, they were called by using the class name StarbucksStore. Does that look familiar? You’ve done something like this already when you called Math.abs() (a static method from the Math class) or INTEGER.MAX_VALUE (a static constant from the Integer class).
Data Encapsulation
Information Hiding
Protecting information by restricting its access is important in software development. Since classes are public, all the objects from the class are public. The way that we hide information or encapsulate data in Java is to use private access modifiers.
Encapsulated Data
It is expected on the AP Computer Science A Exam that all instance variables of a class are declared private. The only way to gain access to them is to use the accessor or mutator methods provided by the class.
What Happens If I Declare an Instance Variable public?
Let’s suppose you just want to find out what happens if you declare an instance variable public. The result is that you can use the dot operator to access the instance variables.
Example
Declare the radius instance variable from the Circle class as public. Notice that the object has now allowed access to the public variable by anyone using the dot operator. Using the dot operator on a private variable produces a compile-time error.
The Integer Class and Its public Fields
Remember how we accessed the largest and smallest Integer? We used the dot operator to just get the value. That’s because MAX_VALUE and MIN_VALUE are public constant fields of the Integer class. You don’t use an accessor method to get them.
Scope
The scope of a variable refers to the region of the program in which the variable is known. Attempting to use a variable outside of its scope produces a compile-time error.
Different types of variables have different scopes.
• The scope of a local variable is within the nearest pair of curly braces that encloses it.
• The scope of a parameter is the method or constructor in which it is declared.
• The scope of an instance variable is the class in which it is defined.
Documentation
Inline Comments
I’ve been using inline comments throughout and will continue to use them in the rest of the book. They begin with two forward slashes, //. All text that comes after the slashes (on the same line) is ignored by the compiler.
Multiple-Line Comments
When your comment takes longer than one line, you will want to use a multiple-line comment. These comments begin with /* and end with */. The compiler ignores everything in between these two special character sequences.
Javadoc Comments
You will see Javadoc comments (also called documentation comments) on the AP Computer Science A Exam, especially in the FRQ section. These comments begin with /** and end with */. The compiler ignores everything in between the two character sequences.
Javadoc
Javadoc is a tool that generates an HTML-formatted document that summarizes a class (including its constructors, instance variables, methods, etc.) in a readable format. Among programmers, this document itself is often referred to as a Javadoc. The summary is called the API (application program interface) for that set of classes and is extremely useful for programmers.
Javadoc Tags
Similar to a hashtag, the @param is a tag that is used in Javadoc comments. When a programmer uses an @param tag, they state the name of the parameter and provide a brief description of it. Javadoc tags are automatically detected by Javadoc and appear in the API.
The @return is also a Javadoc tag. It is only used in return methods, and the programmer identifies the name of the variable being returned and a brief description of it.
Documentation Comments on the AP Computer Science A Exam
You won’t have to write any comments on the AP exam. You just need to know how to read them.
Precondition and Postcondition
You know what happens when you assume, right? Well, a precondition states what you are allowed to assume about a parameter. It’s a statement in the documentation comment before a constructor or method signature that describes the expectations of the parameter sent to the method. There is no expectation that the method will check to ensure preconditions are satisfied. If a method is called and the preconditions haven’t been met, then the method most likely won’t perform as expected. On the AP Computer Science A Exam, reading the preconditions can help you understand the nature of the parameters. A postcondition is a condition that must always be true after the execution of a section of program code. Postconditions describe the outcome of the execution in terms of what is being returned or the state of an object. Programmers must write method code to satisfy the postconditions when preconditions are met.
Example
This is the same Circle class that was created in Unit 2; however, it now includes the Javadoc tags and precondition/postcondition statements as they may appear on the AP Computer Science A Exam. It also includes an improved way to calculate the area using the Math class.
The Keyword this
The keyword this is a reference to the current object, or rather, the object whose method or constructor is being called. It is also referred to as the implicit parameter.
Example 1
Pass the implicit parameter this to a different method in a class. This usage may be tested on the exam.
Example 2
A common place to use the keyword this is when the instance variables have the same name as the parameters in a constructor. The keyword this is used on the left side of the assignment statements to assign the instance variables the values of the parameters. The left side is the instance variable; the right side is the parameter variable. Using this here says, “Hey, I understand that we have the same name, so these are my instance variables and not the parameters.” This usage of the keyword this is not tested on the exam.
IllegalArgumentException
If you pass an argument to a method and the value of the argument does not meet certain criteria required by the method, an IllegalArgumentException error may be thrown during run-time. Programmers may also choose to write a method that terminates with an IllegalArgumentException if it does not receive the expected input.
〉 Rapid Review
Classes
• A class contains the blueprint, or design, from which individual objects are created.
• Classes tend to represent things that are nouns.
• A class declaration (or class definition) is the line of code that declares the class.
• By naming convention, class names begin with an uppercase letter and use camel case.
• The two access modifiers that are tested on the AP Computer Science A Exam are public and private.
• All classes should be declared public.
• A public class means that the class is visible to all classes everywhere.
Instance Variables
• Instance variables (or fields) are properties that all objects from the same class possess. They are the attributes that help distinguish one object from another in the same class.
• All instance variables should be declared private.
• A class can have as many instance variables as is appropriate to describe all the attributes of an object from the class.
• By naming convention, all instance variables begin with a lowercase letter and use camel case.
• All primitive instance variables receive default values. The default value of an int is 0, the default value of a double is 0.0, the default value for a boolean is false, and the default for reference is null.
• The current values of all of the instance variables of an object determine the state of the object.
• The state of the object is changed whenever any of the instance variables are modified.
Constructors
• Every class, by default, comes with a no-argument (or empty) constructor.
• Parameterized constructors should be created if there are values that should be assigned to the instance variables at the moment that an object is created.
• A class can have as many parameterized constructors as is relevant.
• The main reason to have a parameterized constructor is to assign values to the instance variables.
• Other code may be written in the constructor.
• A parameter is a variable that receives values and is declared in a parameter list of a method or constructor.
Methods
• The methods of a class describe the actions that an object can perform.
• Methods tend to represent things that are verbs.
• By naming convention, method names begin with a lowercase letter and use camel case.
• The name of a method should describe the purpose of the method.
• Methods should not be multi-purpose. They should have one goal.
• A method declaration describes pertinent information for the method such as the access modifier, the return type, the name of the method, and the parameter list.
• Methods that don’t return a value are called void methods.
• Methods that return a value of some data type are called return methods.
• Methods can return any data type.
• Return methods must include a reachable return statement.
• Methods that return the value of an instance variable are called accessor (or getter) methods.
• Methods that modify the value of an instance variable are called mutator (or modifier or setter) methods.
• The data type of all parameters must be defined in the parameter list.
• Methods declared as public can be used externally to the class.
• Methods declared as private can be used internally to the class.
• When designing a class programmers make decisions about what data to make accessible and modifiable from an external class.
Objects
• An object from a class is a virtual realization of the class.
• Objects store their own data.
• An instance of a class is the same thing as an object of a class.
• The word “instantiate” is used to describe the action of creating an object. Instantiating is the act of constructing a new object.
• The keyword new is used whenever you want to create an object.
• An object reference variable stores the memory address of where the actual object is stored. It can only reference one object.
• The reference variable does not store the object itself.
• Two reference variables are equal only if they refer to the exact same object.
• The word null means no value.
• A null reference describes a reference variable that does not contain an address of any object.
• You can create as many objects as you want from one class. Each is a different object that is stored in a different location in the computer’s memory.
Passing Parameters by Value
• The parameter variables that are passed to a method or constructor are called actual parameters (or arguments).
• The parameter variables that receive the values in a method or constructor are called formal parameters.
• Passing by value means that only the value of the variable is passed.
• Primitive variables are passed by value.
• It is impossible to change the value of actual parameters that are passed by value.
Passing Parameters by Reference
• Passing by reference means that the address of where the object is stored is passed to the method or constructor.
• Objects are passed by reference (including arrays).
• The state of the object can be modified when it is passed by reference.
• It is possible to change the value of actual parameters that are passed by reference.
Overloaded
• Overloaded constructors may have (1) a different number of parameters, (2) the same number of parameters but of a different type, or (3) the same exact parameter types but in a different order.
• Overloaded methods have the same name; however, their method parameter lists are different in some way.
• Overloaded methods may have (1) a different number of parameters, (2) the same number of parameters but of a different type, (3) the same exact parameter types but in a different order.
static
• Static variables are also known as class variables.
• A class variable is a variable that is shared by all instances of a class.
• Changes made to a class variable by any object from the class are reflected in the state of each object for all objects from the class.
• Static final variables are also called class constants.
• Class constants are declared using both the keywords static and final.
• Constants, by naming convention, use uppercase letters and underscores.
• Constants cannot be modified during run-time.
• If a method is declared static, it can only call other static methods and can reference static variables.
Scope
• The scope of a variable refers to the area of the program in which it is known.
• The scope of a local variable is in the block of code in which it is defined.
• The scope of an instance variable is the class in which it is defined.
• The scope of a parameter is the method or constructor in which it is defined.
Documentation
• Documentation tags are used by Javadocs.
• The @param tag is used to describe a parameter.
• The @return tag is used to describe what is being returned by a method.
• A precondition describes what can be expected of the values that the parameters receive.
• A postcondition describes the end result criteria for a method.
Miscellaneous
• Data encapsulation is the act of hiding the values of the instance variables from other classes.
• Declaring instance variables as private encapsulates them.
• Use the keyword this when you want to refer to the object itself.
• An IllegalArgumentException error occurs when a parameter that is passed to a method fails to meet criteria set up by the programmer.
〉 Review Questions
Basic Level
1. The relationship between a class and an object can be described as:
(A) The terms class and object mean the same thing.
(B) A class is a program, while an object is data.
(C) A class is the blueprint for an object and objects are instantiated from it.
(D) An object is the blueprint for a class and classes are instantiated from it.
(E) A class can be written by anyone, but Java provides all objects.
2. Which of the following is a valid constructor declaration for the Cube class?
I. public static Cube()
II. public void Cube()
III. public Cube()
IV. public Cube(int side)
(A) I only
(B) II only
(C) III only
(D) I and II only
(E) III and IV only
Questions 3–6 refer to the following class.
3. Which of the following could be used to instantiate a new Student object called sam?
(A) Student sam = new Student();
(B) sam = new Student();
(C) Student sam = new Student("Sam Smith", 3.5);
(D) new Student sam = ("Sam Smith", 3.5);
(E) new Student(sam);
4. Which of the following could be used in the StudentTester class to print the name associated with the Student object instantiated in problem 3?
(A) System.out.println(getName());
(B) System.out.println(sam.getName());
(C) System.out.println(getName(sam));
(D) System.out.println(Student.name);
(E) System.out.println(getName(Student));
5. Which of the following is the correct way to write a method that changes the value of a Student object’s gpa variable?
(A) 
(B) 
(C) 
(D) 
(E) 
6. Consider the following code segment.
What is printed as a result of executing the code segment?
(A) 3.2
(B) 3.9
(C) 4.0
(D) Nothing will be printed. There will be a compile-time error.
(E) Nothing will be printed. There will be a run-time error.
Questions 7–11 refer to the following information.
Consider the following method.
7. The following code segment appears in another method in the same class.
What is printed as a result of executing the code segment?
(A) 2
(B) 9
(C) 23
(D) 40
(E) 49
8. The following code segment appears in another method in the same class.
What is printed as a result of executing the code segment?
(A) 40 49 40
(B) 9 2 40
(C) 40 49 49
(D) Run-time error: a and b cannot contain two values at once.
(E) Compile-time error; duplicate variable.
9. Consider the following calculation of the area of a circle.
Math.PI can best be describes as:
(A) A method in the Math class.
(B) A method in the class containing the given line of code.
(C) A public static final double in the Math class.
(D) A private static final double in the Math class.
(E) A private static final double in the class containing the given line of code.
10. A programmer is designing a BankAccount class. In addition to the usual information needed for a bank account, she would like to have a variable that counts how many BankAccount objects have been instantiated. She asks you how to do this. You tell her:
(A) It cannot be done, since each object has its own variable space.
(B) She should use a class constant.
(C) She should call a mutator method to change the numAccounts variable held in each object.
(D) She should create a static variable and increment it in the constructor as each object is created.
(E) Java automatically keeps a tally. She should reference the variable maintained by Java.
11. Having multiple methods in a class with the same name but with a different number or different types of parameters is called:
(A) abstraction
(B) method overloading
(C) encapsulation
(D) method visibility
(E) parameterization
Advanced Level
12. Consider the following method.
Which statement about this method is true?
(A) mystery returns the smallest of the three integers a, b, and c.
(B) mystery always returns the value of a.
(C) mystery always returns both values a and c.
(D) mystery sometimes returns all three values a, b, and c.
(E) mystery will not compile, because the return statement appears to be unreachable under some conditions.
13. The method from the problem above is rewritten as shown.
Now, which statement about the method is true?
(A) mystery2 returns the smallest of the three integers a, b, and c.
(B) mystery2 always returns the value of c.
(C) mystery2 returns either the values of both a and c or both b and c.
(D) mystery2 sometimes returns all three values a, b, and c.
(E) mystery2 will not compile, because the return statement appears to be unreachable under some conditions.
14. Consider the following class declaration.
The following code segment appears in the main method of another class.
What is printed as a result of executing the code segment?
(A) 30
(B) 15
(C) 7.75
(D) 7.5
(E) 7
15. Consider the following method declaration.
Which of these method declarations would correctly overload the given method?
I. public double workaround(int a, double b)
II. public int workaround(double b, int a)
III. public int workaround(String s, double b)
IV. public double workaround(int a, double b, String s)
V. public int workaround(int b, double a)
(A) I only
(B) IV only
(C) II and III only
(D) II, III, and IV only
(E) All of the above
16. Free-Response Practice: Dream Vacation Class
Everyone fantasizes about taking a dream vacation.
Write a full DreamVacation class that contains the following:
a. An instance variable for the name of the destination
b. An instance variable for the cost of the vacation (dollars and cents)
c. A no-argument constructor
d. A parameterized constructor that takes in both the name of the vacation and the cost
e. Accessor (getter) methods for both instance variables
f. Modifier (setter) methods for both instance variables
17. Free-Response Practice: The Height Class
The purpose of the Height class is to hold a measurement in feet and inches, facilitate additions to that measurement, and keep the measurement in simplest form (inches < 12). A Height object can be instantiated with a measurement in feet and inches, or in total inches.
Write the entire Height class that includes the following:
a. Instance variables int feet for the number of feet and int inches for the number of inches. These instance variables must be updated anytime they are changed so that they hold values in simplest form. That is, inches must always be less than 12 (see method described in part C below).
b. Two constructors, one with two parameters representing feet and inches, and one with a single parameter representing inches. The parameters do not necessarily represent the measurement simplest form.
c. A method called simplify() that recalculates the number of feet and inches, if necessary, so that the number of inches is less than 12.
d. An add(int inches) method that takes the number of inches to add and updates the appropriate instance variables.
e. An add(Height ht) method that takes a parameter that is a Height object and adds that object’s inches and feet to this object’s instance variables, updating if necessary.
f. Accessor (getter) methods for the instance variables inches and feet.
〉 Answers and Explanations
Bullets mark each step in the process of arriving at the correct solution.
1. The answer is C.
• A class is a blueprint for constructing objects of the type described by the class definition.
2. The answer is E.
• The general form of a constructor declaration is:
• Notice that there is no return type, not even void.
• In this example, the name of the class is Cube. III is a correct declaration for a no-argument constructor and IV is a correct declaration for a constructor with one parameter.
3. The answer is C.
• The general form of an instantiation statement is:
• In this example, the type is Student and the variable name is sam.
• We have to look at the constructor code to see that the parameter list consists of a String followed by a double.
• The only answer that has the correct form with the parameters in the correct order is:
4. The answer is B.
• We do not have direct access to the instance variables of an object, because they are private. We have to ask the object for the data we need with an accessor method. In this case, we need to call the getName method.
• We need to ask the specific object we are interested in for its name. The general syntax for doing that is:
• Placing the call in the print statement gives us:
5. The answer is E.
• A modifier or setter method has to take a new value for the variable from the caller and assign that value to the variable. The new value is given to the method through the use of a parameter.
• A modifier method does not return anything. Why would it? The caller has given the value to the method. The method doesn’t have to give it back.
• So we need a void method that has one parameter. It has to change the value of gpa to the value of the parameter. Option E shows the only method that does that.
6. The answer is D.
• Option A is incorrect. It correctly states that each object has its own variable space, and each object’s variables will be different. However, it is not correct about it being impossible.
• Option B is incorrect. It says to use a constant, which doesn’t make sense because she wants to be able to update its value as objects are created.
• Option C is incorrect. There is no one mutator method call that will access the instance variables of each object of the class.
• Option D is correct. A static variable or class variable is a variable held by the class and accessible to every object of the class.
• Option E is simply not true.
7. The answer is C.
• The instantiation statements give us 2 objects: s1 and s2.
• s1 references (points to) a Student object with the state information: name = "Emma Garcia", gpa = 3.9
• s2 references (points to) a Student object with the state information: name = "Alice Garrett", gpa = 3.2
• When we execute the statement s2 = s1, s2 changes its reference. It now references the exact same object that s1 references, with the state information: name = "Emma Garcia", gpa = 3.9
• s2.setGpa(4.0); changes the gpa in the object that both s1 and s2 reference, so when s1.getGpa(); is executed, that’s the value it retrieves and that’s the value that is printed.
8. On the AP exam, you are often asked to write an entire class, like you were in this question. Check your answer against our version below. The comments are to help you read our solution. Your solution does not need comments.
Make Variable Names Meaningful
It’s OK if you use different variable names from what we use in the solutions to the chapter questions. Just be sure that the names are meaningful.
9. The answer is D.
• When the method call calculate(var, count) is executed, the arguments 9 and 2 are passed by value. That is, their values are copied into the parameters a and b, so a = 9 and b = 2 at the beginning of the method.
• a = 9 - 2 = 7
• int c = 2
• b = 7 * 7 = 49
• a = 49 – (7 + 2) = 40 and that is what will be returned.
10. The answer is B.
• When the method call calculate(a, b) is executed, the arguments 9 and 2 are passed by value, that is their values are copied into the parameters a and b. The parameters a and b are completely different variables from the a and b in the calling method and any changes made to a and b inside the method do not affect a and b in the calling method in any way.
• We know from problem #1 that calculate(9, 2) will return 40.
• 9 2 40 will be printed.
11. The answer is C.
• All instance variables in the classes we write must be declared private. In fact, the AP exam readers may deduct points if you do not declare your variables private. However, some built-in Java classes provide us with useful constants by making their fields public.
• We access these fields by using the name of the class followed by a dot, followed by the name of the field. We can tell that it is a constant because its name is written entirely in capital letters. Some other public constants that we use are Integer.MAX_VALUE and Integer.MIN_VALUE.
• Note that we can tell this is not a method call, because the name is not followed by (). The Math.pow(radius, 2) is a method call to a static method of the Math class.
12. The answer is E.
• The intent of the mystery method is to find and return the smallest of the three integers. We know that all possible conditions are covered by the if statements and one of them has to be true, but the compiler does not know that. The compiler just sees that all of the return statements are in if clauses, and so it thinks that if all the conditions are false, the method will not return a value. The compiler generates an error message telling us that the method must return a value.
13. The answer is A.
• The first time a return statement is reached, the method returns to the caller. No further code in the method will be executed, so no further returns will be made.
14. The answer is E.
• There are a lot of calls here, but only two that will affect the value of val, since the only time val is changed is in the constructor.
• It’s important to notice that val is a static variable. That means that the class maintains one copy of the variable, rather than the objects each having their own copy. Since there is only one shared copy, changes made to val by any object will be seen by all objects.
• The first time the constructor is called is when the word “object” is instantiated. val = 31 when the constructor is called and this call to the constructor changes its value to 15 (integer division).
• The second time the constructor is called is when the sentence object is instantiated. This time val = 15 when the constructor is called and this call to the constructor changes its value to 7 (integer division).
15. The answer is D.
• Options I and V are incorrect. An overloaded method must have a different parameter list so that the compiler knows which version to use.
• Options II, III, and IV are valid declarations for overloaded methods. They can all be distinguished by their parameter lists.
16. The answer is B.
• Method overloading allows us to use the same method name with different parameters.
17. The Height class
Note: There are many ways to write the simplify method. If you aren’t sure if yours works, type it into your IDE. Here’s one alternate solution:
