Object Basics and Simple Data Objects

1
Lesson 3

• Object Basics and Simple Data Objects
• The Life Cycle of an Object
• Characters and Strings
• Arrays

2
The Life Cycle of an Object

• Creating Objects
• CreateObjectDemo.java
• Point.java
• Rectangle.java
• Point origin_one new Point(23, 94)
• Rectangle rect_one new Rectangle(origin_one,
  100, 200)
• Rectangle rect_two new Rectangle(50, 100)

3
The Life Cycle of an Object

• Declaration
• Point origin_one
• Instantiation and Initialization
• Point origin_one new Point(23, 94)

4
The Life Cycle of an Object

• Instantiation and Initialization
• Rectangle rect_one new Rectangle(origin_one,
  100, 200)

Question How can you show rect_one?
5
Using Objects

• Manipulate or inspect its variables.
• Call its methods.
• objectReference.variableName
• System.out.println("Width of rect_one "
  rect_one.width)
• objectReference.methodName(argumentList)
• System.out.println("Area of rect_one "
  rect_one.area())
• rect_two.move(40, 72)

6
Cleaning Up Unused Objects

• Garbage Collection
• An object is eligible for garbage collection when
  there are no more references to that object.
• References that are held in a variable are
  usually dropped when the variable goes out of
  scope. Or, you can explicitly drop an object
  reference by setting the variable to the special
  value null
• Garbage Collector
• Automatic
• But can be run explicitly by calling the gc
  method in the System class

7
Characters and Strings

• Character class
• String class
• StringBuffer class.

8
Character Class

• An object of Character type contains a single
  character value.
• You use a Character object instead of a primitive
  char variable when an object is required-for
  example
• when passing a character value into a method that
  changes the value or when placing a character
  value into a data structure, such as a vector,
  that requires objects.
• Example Code

9
Two String Classes

• The Java platform provides two classes, String
  and StringBuffer, that store and manipulate
  strings-character data consisting of more than
  one character.
• The String class provides for strings whose value
  will not change.
• The StringBuffer class provides for strings that
  will be modified you use string buffers when you
  know that the value of the character data will
  change. You typically use string buffers for
  constructing character data dynamically for
  example, when reading text data from a file.
• Because strings are constants, they are more
  efficient to use than are string buffers and can
  be shared. So it's important to use strings when
  you can.

• Example Code

10
Creating Strings and StringBuffers

• Different ways to create a String
• String palindrome "Dot saw I was Tod"
• char helloArray 'h', 'e', 'l', 'l', 'o'
• String helloString new String(helloArray)
• You must always use new to create a string buffer
  
• StringBuffer dest new StringBuffer(len)

11
Accessor Methods

• Getting the Length of a String or a String Buffer
• String palindrome "Dot saw I was Tod"
• int len palindrome.length()
• A string buffer's length is the number of
  characters it contains a string buffer's
  capacity is the number of character spaces that
  have been allocated.
• The String class doesn't have a capacity method,
  because a string cannot change.

12
Accessor Methods

• Getting Characters from a String or a String
  Buffer
• String anotherPalindrome "Niagara. O roar
  again!"
• char aChar anotherPalindrome.charAt(9)
• String roar anotherPalindrome.substring(11,
  15)

13
Java Strings Are First-Class Objects

• By convention, C and C strings are
  null-terminated array of characters there is no
  real entity in C and C that is a string. Java
  strings are first-class objects.
• Example code which illustrates the difference
  between strings in Java and C/C

14
Arrays

• An array is a structure that holds multiple
  values of the same type.
• The length of an array is established when the
  array is created (at runtime).
• After creation, an array is a fixed-length
  structure.

15
Creating and Using Arrays

• Declaring a Variable to Refer to an Array
• Creating an Array
• Accessing an Array Element
• Getting the Size of an Array
• Example Code
• Array Initializers
• boolean answers true, false, true, true,
  false

16
Arrays of Objects Arrays of Arrays

• Arrays can hold reference types as well as
  primitive types.
• Example Code
• Arrays of Arrays
• Example Code

17
Exercise

• Write a Java program which will generate 10
  random strings, sort them and print the output.