CS 46A - Lecture 4

Pre-class reading

• Sections 2.4 - 2.6
• Did you take the quiz?

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: declares the methods that you can apply to its objects
• Class determines legal methods:

  String greeting = "Hello, World!";
  int n = greeting.length();
  System.out.length(); // This method call is an error

• Public interface: specifies what you can do with the objects of a class
• Overloaded method: when a class declares two methods with the same name, but different parameters
  • Example: the PrintStream class declares a second method, also called println, as

    public void println(int output)

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

Lecture 4 Clicker Question 1

What does the following program segment print?

String name = "Fred";
System.out.print(name);
System.out.println("name".toUpperCase());

1. Nothing—the code contains a compile-time error

2. FredNAME

3. FredFRED

4. Fred
   FRED

Lecture 4 Clicker Question 2

What does the following program segment print?

String name = "Fred";
name.println();
int length = name.length();
length.println();

1. Nothing—the code contains a compile-time error

2. Fred4

3. Fred
   4

4. FRED
   4

Parameters

• Parameter: an input to a method
• Implicit parameter: the object on which a method is invoked:

  System.out.println(greeting)

• Explicit parameters: all parameters except the implicit parameter:

  System.out.println(greeting)

• Not all methods have explicit parameters:

  greeting.length() // has no explicit parameter

  

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 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

• String method replace 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"

Lecture 4 Clicker Question 3

What is true about the method call river.length()?

1. It has one implicit parameter, no explicit parameter, and no return value
2. It has one implicit parameter, no explicit parameter, and one return value
3. It has no implicit parameter, one explicit parameter, and no return value
4. It has no implicit parameter, one explicit parameter, and oe return value

Lecture 4 Clicker Question 4

What does the following program segment print?

String name = "Fred";
PrintStream out = System.out;
out.println(name.replace("r", "e").replace("e", "o").toLowerCase());

1. Nothing—the code contains a compile-time error

2. frrd

3. food

4. 4

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

new Rectangle(5, 10, 20, 30)

• 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

• Construction: the process of creating a new object
• 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

Lecture 4 Clicker Question 5

What does the following program segment print?

Rectangle rect = new Rectangle(10, 10, 10, 10);
PrintStream out = System.out;
out.println(rect.length());

1. Nothing—the code contains a compile-time error

2. 10

3. 20

4. 40