Declaring and Creating Objects

In Java, you have to declare new objects and then create them with the new operator. For example, here's how I create an object of the Java String class, passing the text "Welcome to Java" to that class's constructor:

 public class ch10_12  {     public static void main(String[] args)     {  String greeting1;   greeting1 = new String("Welcome to Java");  .     .     . 

Note that I first declared the greeting1 object, giving the object's class, String , as its type. Then I create the object with the new operator.

Overloaded Constructors

Classes can have different constructors that handle different types of data. For example, I passed a string to the String class's constructor in the previous example, but I can also pass an array of characters this way:

 public class ch10_12  {     public static void main(String[] args)     {         String greeting1, greeting2, greeting3;         greeting1 = new String("Welcome to Java");  char characters[] = {'W', 'e', 'l', 'c', 'o', 'm', 'e',   ' ', 't', 'o', ' ', 'J', 'a', 'v', 'a'};   greeting2 = new String(characters);  .         .         .     } } 

Constructors and methods that can take different argument lists are said to be overloaded.

Overloading

To overload a constructor or method, you just define it a number of times, each with a different argument list. You can also overload operators. For example, the + operator is overridden for the String class, which means that you can add strings with the + operator.

Assigning Objects

You can also assign one object to another using the = assignment operator:

 public class ch10_12  {     public static void main(String[] args)     {         String greeting1, greeting2, greeting3;         greeting1 = new String("Welcome to Java");         char characters[] = {'W', 'e', 'l', 'c', 'o', 'm', 'e',                      ' ', 't', 'o', ' ', 'J', 'a', 'v', 'a'};         greeting2 = new String(characters);  greeting3 = greeting2;  .         .         .     } } 

To end this example, I'll print all the strings we've created:

Listing ch10_12.java

 public class ch10_12 {     public static void main(String[] args)     {         String greeting1, greeting2, greeting3;         greeting1 = new String("Welcome to Java");         char characters[] = {'W', 'e', 'l', 'c', 'o', 'm', 'e',                      ' ', 't', 'o', ' ', 'J', 'a', 'v', 'a'};         greeting2 = new String(characters);         greeting3 = greeting2;  System.out.println(greeting1);   System.out.println(greeting2);   System.out.println(greeting3);  } } 

Here's what this application looks like when run:

 %java ch10_12  Welcome to Java Welcome to Java Welcome to Java 

That's how to declare and create objects in Java. It's similar to the way you declare and create simple variables , with the added power of configuring objects by passing data to a class's constructor.