Final variable manipulation in Java

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Could anyone please tell me what is the meaning of the following line in context of Java:

final variable can still be manipulated unless it's immutable

As far as I know, by declaring any variable as final, you can't change it again, then what they mean with the word immutable in above line?

It means that if your final variable is a reference type (i.e. not a primitive like int), then it's only the reference that cannot be changed. It cannot be made to refer to a different object, but the fields of the object it refers to can still be changed, if the class allows it. For example:

final StringBuffer s = new StringBuffer();

The content of the StringBuffer can still be changed arbitrarily:

s.append("something");

But you cannot say:

s = null;

or

s = anotherBuffer;

On the other hand:

final String s = "";

Strings are immutable - there simply isn't any method that would enable you to change a String (unless you use Reflection - and go to hell).

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If you have a final reference to a Java object you can still manipulate it but cannot change its reference. For instance this code is perfectly legal:

import javax.swing.JLabel;

class Test1 {
    private final static JLabel l = new JLabel("Old text");
    public static void main(String[] args) {
        System.err.println(l.getText());
        l.setText("New Text");
        System.err.println(l.getText());
    }
}

But you can't say:

l = new JLabel("Newest Text");

After the first assignment to l. Note that you can do this though:

import javax.swing.JLabel;

class Test1 {
    public static void main(String[] args) {
        final JLabel l;
        String s = getArbitaryString(); // Assume this method returns a string
        l = new JLabel(s);
        System.err.println(l.getText());
    }
}

This can be done because when l is declared it is not assigned to anything not even null. So you are allowed to assign something to it one time only.

Same thing goes for primitives. You can assign a value to it like this:

class Test1 {
    public static void main(String[] args) {
        final int i;
        i = 2;
    }
}

But now you cannot manipulate it further since the only thing you can do to primitive types is to assign values to them.

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You cannot change what object or value a final variable refers to. You can only assign a final variable once.

This has no effect on whether you can change the state of the object. The object itself can still be manipulated unless it is coded in such a way that this manipulation is forbidden. An immutable object is an object whose state cannot change.

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As others have said, it means that you can manipulate the object the variable points at, but you cannot change the reference (i.e. assign another object to the variable).

Objects that are mutable by design, such as a List can be changed (you can add elements to them) whereas if you have an immutable object such as a String or Integer you won't be able to change it (all the operations the class String supports return a new instance, and don't modify the actual object).

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The one that always kills me?

If you want final variables to actually be as safe as you thought they were, you need a lot of extra code to return a copy of a String[].

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Comments
  • @pramodc84: It's the same for all variables. Most collections are mutable though.
  • So how can I change it with reflection? =)
  • @Daniel Magnusson: use docs.oracle.com/javase/7/docs/api/java/lang/reflect/… to make the private char[] inside String accessible.
  • +1 unless you use Reflection - and go to hell - just love a good humor coming from a bad experience :)))
  • @Viraj: In Java, only variables can be final, not objects. So in your code, final only applies to the reference newTask. But the array object which it refers to can sill have its elements changed.
  • You can but that's not what he meant.
  • Actually, you can't. You would get a java.lang.IllegalAccessException.
  • No you can't. You get java.lang.IllegalAccessException: Can not set final [type] field [fieldname] to [type of new value].
  • It's documented here too: java.sun.com/docs/books/tutorial/reflect/member/… -- "IllegalAccessException when Modifying Final Fields"
  • @polygenelubricants - java.sun.com/javase/6/docs/api/java/lang/reflect/… says this: "If the underlying field is final, the method throws an IllegalAccessException unless setAccessible(true) has succeeded for this field and this field is non-static."