CS 46A - Lecture 5

Pre-class reading

• Sections 2.7 - 2.10
• Did you take the quiz?

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);

  

Lecture 5 Clicker Question 1

Consider these methods:

• String.toUpperCase
• Rectangle.grow
• PrintStream.println

Which of them are mutators?

1. None of them
2. Only Rectangle.grow
3. Rectangle.grow and PrintStream.println
4. All of them

The API Documentation

• API: Application Programming Interface
• API documentation: lists classes and methods in the Java library
• http://docs.oracle.com/javase/7/docs/api/index.html

The API Documentation of the Standard Java Library

The API Documentation for the Rectangle Class

Method Summary

Detailed Method Description

The detailed description of a method shows:

• The action that the method carries out
• The parameters that the method receives
• The value that it returns (or the reserved word void if the method doesn’t return any value)

Packages

• Package: a collection of classes with a related purpose
• 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

Lecture 5 Clicker Question 2

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.)

1. " Hello, Space ! "
2. " Hello,Space! "
3. "Hello, Space !"
4. "Hello,Space!"

Lecture 5 Clicker Question 3

Find the Random class in the API documentation. What statement do you need to add to your program in order to use that class?

1. import Random;
2. import java.util;
3. import java.util.Random;
4. import java.*.Random;

Implementing a Test Program

1. Provide a tester 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.
6. Display the values that you expect to get.

Display the Values that You Expect to Get

import java.awt.Rectangle; 

public class MoveTester
{
   public static void main(String[] args)
   {
      Rectangle box = new Rectangle(5, 10, 20, 30);

      // Move the rectangle
      box.translate(15, 25);

      // Print information about the moved rectangle 
      System.out.print("x: "); 
      System.out.println(box.getX());
      System.out.println("Expected: 20"); 

      System.out.print("y: "); 
      System.out.println(box.getY());
      System.out.println("Expected: 35");   
   }
}

Expected Values

• In this course, we will do this all the time.

• System.out.println(someResult);
  System.out.println("Expected: 42");

  or

  System.out.print("SomeLabel: ");
  System.out.println(someResult);
  System.out.println("Expected: 42");

• Do this in your own programs
• It makes you think what your program is supposed to do
• When the output looks like

  42
  Expected: 42

  you can relax

Lecture 5 Clicker Question 4

We want to  write a tester for trim. Which of these is best?

1. System.out.println("Hello");
   System.out.println("Expected: Hello");

2. System.out.println(" Hello ".trim());
   System.out.println("Expected: Hello");

3. System.out.println(" Hello ".trim());
   System.out.println("Expected:  Hello ");

4. System.out.print("|"); 
   System.out.print(" Hello ".trim());
   System.out.println("|\nExpected: |Hello|");
   

Object References

• Object reference: describes the location of an object
• 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;

Copying Numbers

int luckyNumber = 13;
int luckyNumber2 = luckyNumber;

Copying Numbers

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

Copying Object References

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

Copying Object References

Rectangle box = new Rectangle(5, 10, 20, 30);
Rectangle box2 = box;

Copying Object References

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

Lecture 5 Clicker Question 5

To which strings do greeting and greeting2 refer after the following code segment:

String greeting = "Hello";
String greeting2 = greeting;
greeting2.toUpperCase();

1. greeting and greeting2 refer to "Hello"
2. greeting refers to "Hello", greeting2 to "HELLO"
3. greeting and greeting2 refer to "HELLO"
4. None of the above because there is a syntax error