Final variable manipulation in Java

问题:

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?

回答1:

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).

回答2:

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.

回答3:

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.

回答4:

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).

回答5:

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[].

回答6:

You can call any method on it even if the method can change the state of the object the reference is pointing to. E.g

final MyClass myClass = new MyClass();
myClass.setVar(something);

This is fine because myClass itself is not changing, i.e you are not doing myClass = myClass1;.

回答7:

You can manipulate mutable final variables for e.g. of type StringBuffer but you cannot manipulate final variables of immutable types.

In case of mutable variables, new object is not created every time it's value is changed. But in case of of immutable types, whenever you change value, new object is created, so when you make it final, you cannot modify it.

回答8:

Yes the final variable can be modified.

    final StringBuffer s = new StringBuffer();
    // won't work
    s = new StringBuffer();
    //this works
    s.append("hai");

You can't change the reference but the fields of the the object can be modified. for more details

回答9:

You can still change a 'final' variable using Reflection.