Mutable and Immutable Objects


Mutable Objects: When you have a reference to an instance of an object, the contents of that instance can be altered
Immutable Objects: When you have a reference to an instance of an object, the contents of that instance cannot be altered


Immutability and Instances

To demonstrate this behaviour, we'll use java.lang.String as the immutable class and java.awt.Point as the mutable class.
 Point myPoint = new Point( 0, 0 );
 System.out.println( myPoint );
 myPoint.setLocation( 1.0, 0.0 );
 System.out.println( myPoint );

 String myString = new String( "old String" );
 System.out.println( myString );
 myString.replaceAll( "old", "new" );
 System.out.println( myString );
In case you can't see what the output is, here it is:
 java.awt.Point[0.0, 0.0]
 java.awt.Point[1.0, 0.0]
 old String
 old String
We are only looking at a single instance of each object, but we can see that the contents of myPoint has changed, but the contents of myString did not. To show what happens when we try to change the value ofmyString, we'll extend the previous example.
 String myString = new String( "old String" );
 System.out.println( myString );
 myString = new String( "new String" );
 System.out.println( myString );
The output from this is:
 old String
 new String
Now we find that the value displayed by the myString variable has changed. We have defined immutable objects as being unable to change in value, so what is happening? Let's extend the example again to watch the myString variable closer.
 String myString = new String( "old String" );
 String myCache = myString;
 System.out.println( "equal: " + myString.equals( myCache ) );
 System.out.println( "same:  " + ( myString == myCache ) );

 myString = "not " + myString;
 System.out.println( "equal: " + myString.equals( myCache ) );
 System.out.println( "same:  " + ( myString == myCache ) );
The result from executing this is:
 equal: true
 same:  true
 equal: false
 same:  false
What this shows is that variable myString is referencing a new instance of the String class. The contents of the object didn't change; we discarded the instance and changed our reference to a new one with new contents.

Source: http://www.javaranch.com/journal/2003/04/immutable.htm



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