Chapter 2

Using Objects

Chapter Goals

• To learn about variables
• To understand the concepts of classes and objects
  
• To be able to call methods
• To be able to browse the API documentation
• To realize the difference between objects and object references

Types and Variables

• Every value has a type
• Variable declaration examples:
  String greeting = "Hello, World!";
  PrintStream printer = System.out;
  int luckyNumber = 13;
  
• Variables
  • Store values
  • Can be used in place of the objects they store

Syntax 2.1: Variable Definition

Identifiers

• Identifier: name of a variable, method, or class
• Rules for identifiers in Java:
  • Can be made up of letters, digits, and the underscore (_) character
  • Cannot start with a digit
  • Cannot use other symbols such as ? or %
  • Spaces are not permitted inside identifiers
  • You cannot use reserved words
  • They are case sensitive
• By convention, variable names start with a lowercase letter
• By convention, class names start with an uppercase letter

Self Check

1. What is the type of the values 0 and "0"?
2. Which of the following are legal identifiers?

   Greeting1
   g
   void
   101dalmatians
   Hello, World
   <greeting>

3. Define a variable to hold your name. Use camel case in the variable name.

Answers

1. int and String
2. Only the first two are legal identifiers
3. String myName = "John Q. Public";

The Assignment Operator

• Assignment operator: =
• Not used as a statement about equality
  
• Used to change the value of a variable
  int luckyNumber = 13;
  luckyNumber = 12;
  

Uninitialized Variables

• Error:
  

int luckyNumber;
System.out.println(luckyNumber);
   // ERROR - uninitialized variable

Syntax 2.2: Assignment

Self Check

4. Is 12 = 12 a valid expression in the Java language?
5. How do you change the value of the greeting variable to "Hello, Nina!"?

Answers

4. No, the left-hand side of the = operator must be a variable
5. greeting = "Hello, Nina!";
   Note that
   String greeting = "Hello, Nina!";
   is not the right answer–that statement defines a new variable

Objects and Classes

• Object: entity that you can manipulate in your programs (by calling methods)
• Each object belongs to a class. For example, System.out belongs to the class PrintStream
  

Methods

• Method: Sequence of instructions that accesses the data of an object
• You manipulate objects by calling its methods
• Class: Set of objects with the same behavior
• Class determines legal methods
  String greeting = "Hello";
  greeting.println() // Error
  greeting.length() // OK
• Public Interface: Specifies what you can do with the objects of a class

A Representation of Two String Objects

String Methods

• length: counts the number of characters in a string

  String greeting = "Hello, World!";
  int n = greeting.length(); // sets n to 13

• toUpperCase: creates another String object that contains the characters of the original string, with lowercase letters converted to uppercase

  String river = "Mississippi";
  String bigRiver = river.toUpperCase(); // sets bigRiver to "MISSISSIPPI"

• When applying a method to an object, make sure method is defined in the appropriate class

  System.out.length(); // This method call is an error

Self Check

6. How can you compute the length of the string "Mississippi"?
7. How can you print out the uppercase version of "Hello, World!"?
8. Is it legal to call river.println()? Why or why not?

Answers

6. river.length() or "Mississippi".length()
7. System.out.println(greeting.toUpperCase());
8. It is not legal. The variable river has type String. The println method is not a method of the String class.

Implicit and Explicit Parameters

• Parameter (explicit parameter): Input to a method. Not all methods have explicit parameters.
  System.out.println(greeting)
  greeting.length() // has no explicit parameter
• Implicit parameter: The object on which a method is invoked
  System.out.println(greeting)

Return Values

• Return value: A result that the method has computed for use by the code that called it
  int n = greeting.length(); // return value stored in n

Passing Return Values

• You can also use the return value as a parameter of another method:
  System.out.println(greeting.length());
• Not all methods return values. Example: println

A More Complex Call

• replace method carries out a search-and-replace operation

  river.replace("issipp", "our") // constructs a new string ("Missouri")

• This method call has
  • one implicit parameter: the string "Mississippi"
  • two explicit parameters: the strings "issipp" and "our"
  • a return value: the string "Missouri"

Method Definitions

• Method definition specifies types of explicit parameters and return value
• Type of implicit parameter = current class; not mentioned in method definition
• Example: Class String defines

  public int length() 
     // return type: int
     // no explicit parameter
  public String replace(String target, String replacement)
     // return type: String; 
     // two explicit parameters of type String

Method Definitions

• If method returns no value, the return type is declared as void

  public void println(String output) // in class PrintStream

• A method name is overloaded if a class has more than one method with the same name (but different parameter types)

  public void println(String output)
  public void println(int output)

Self Check

9. What are the implicit parameters, explicit parameters, and return values in the method call river.length()?
10. What is the result of the call river.replace("p", "s")?
11. What is the result of the call greeting.replace("World", "Dave").length()?
12. How is the toUpperCase method defined in the String class?

Answers

9. The implicit parameter is river. There is no explicit parameter. The return value is 11
10. "Missississi"
11. 12
12. As public String toUpperCase(), with no explicit parameter and return type String.

Number Types

• Integers: short, int, long
  13
• Floating point numbers: float, double
  1.3
  0.00013
• When a floating-point number is multiplied or divided by 10, only the position of the decimal point changes; it "floats". This representation is related to the "scientific" notation 1.3 × 10^-4.
  1.3E-4       // 1.3 × 10^-4 written in Java
• Numbers are not objects; numbers types are primitive types

Arithmetic Operations

• Operators: + - *
  10 + n
  n - 1
  10 * n       // 10 × n
• As in mathematics, the * operator binds more strongly than the + operator
  x + y * 2    // means the sum of x and y * 2
  (x + y) * 2  // multiplies the sum of x and y with 2

Self Check

13. Which number type would you use for storing the area of a circle?
14. Why is the expression 13.println() an error?
15. Write an expression to compute the average of the values x and y.

Answers

13. double
14. An int is not an object, and you cannot call a method on it
15. (x + y) * 0.5

Rectangular Shapes and Rectangle Objects

• Objects of type Rectangle describe rectangular shapes
  

Rectangular Shapes and Rectangle Objects

• A Rectangle object isn't a rectangular shape–it is an object that contains a set of numbers that describe the rectangle
  

Constructing Objects

• Detail:
  1. The new operator makes a Rectangle object
  2. It uses the parameters (in this case, 5, 10, 20, and 30) to initialize the data of the object
  3. It returns the object
• Usually the output of the new operator is stored in a variable

  Rectangle box = new Rectangle(5, 10, 20, 30);

Constructing Objects

• The process of creating a new object is called construction
• The four values 5, 10, 20, and 30 are called the construction parameters
• Some classes let you construct objects in multiple ways

  new Rectangle()
     // constructs a rectangle with its top-left corner
     // at the origin (0, 0), width 0, and height 0

Syntax 2.3: Object Construction

new ClassName(parameters)

Self Check

16. How do you construct a square with center (100, 100) and side length 20?
17. What does the following statement print?
    System.out.println(new Rectangle().getWidth());

Answers

16. new Rectangle(90, 90, 20, 20)
17. 0

Accessor and Mutator Methods

• Accessor method: does not change the state of its implicit parameter
  double width = box.getWidth();
• Mutator method: changes the state of its implicit parameter
  box.translate(15, 25);
  

Self Check

18. Is the toUpperCase method of the String class an accessor or a mutator?
19. Which call to translate is needed to move the box rectangle so that its top-left corner is the origin (0, 0)?

Answers

18. An accessor–it doesn't modify the original string but returns a new string with uppercase letters
19. box.translate(-5, -10), provided the method is called immediately after storing the new rectangle into box

Implementing a Test Program

1. Provide a new class
2. Supply a main method
3. Inside the main method, construct one or more objects
4. Apply methods to the objects
5. Display the results of the method calls

Importing Packages

• Java classes are grouped into packages
• Import library classes by specifying the package and class name:
  import java.awt.Rectangle;
• You don't need to import classes in the java.lang package such as String and System

Syntax 2.4 : Importing a Class from a Package

File MoveTester.java

Output

   After moving, the top-left corner is:
   20
   35

Self Check

20. The Random class is defined in the java.util package. What do you need to do in order to use that class in your program?
21. Why doesn't the MoveTester program print the width and height of the rectangle?

Answers

20. Add the statement import java.util.Random; at the top of your program
21. Because the translate method doesn't modify the shape of the rectangle

Testing Classes in an Interactive Environment

The API Documentation

• API: Application Programming Interface
• Lists classes and methods in the Java library
• http://java.sun.com/j2se/1.5/docs/api/index.html

The API Documentation of the Standard Java Library

The API Documentation for the Rectangle Class

Javadoc Method Summary

translate Method Documentation

Self Check

22. Look at the API documentation of the String class. Which method would you use to obtain the string "hello, world!" from the string "Hello, World!"?
23. In the API documentation of the String class, look at the description of the trim method. What is the result of applying trim to the string " Hello, Space ! "? (Note the spaces in the string.)

Answers

22. toLowerCase
23. "Hello, Space !"–only the leading and trailing spaces are trimmed

Object References

• Describe the location of objects
• The new operator returns a reference to a new object
  Rectangle box = new Rectangle();
• Multiple object variables can refer to the same object
  Rectangle box = new Rectangle(5, 10, 20, 30);
  Rectangle box2 = box;
  box2.translate(15, 25);
• Primitive type variables ≠ object variables

Object Variables and Number Variables

Copying Numbers

   int luckyNumber = 13;
   int luckyNumber2 = luckyNumber;
   luckyNumber2 = 12;

Copying Object References

   Rectangle box = new Rectangle(5, 10, 20, 30);
   Rectangle box2 = box;
   box2.translate(15, 25);

Self Check

24. What is the effect of the assignment greeting2 = greeting?
25. After calling greeting2.toUpperCase(), what are the contents of greeting and greeting2?

Answers

24. Now greeting and greeting2 both refer to the same String object.
25. Both variables still refer to the same string, and the string has not been modified. Recall that the toUpperCase method constructs a new string that contains uppercase characters, leaving the original string unchanged.

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