How does the “final” keyword in Java work? (I can still modify an object.)

Question

In Java we use the final keyword with variables to specify its values are not to be changed. But I see that you can change the value in the constructor/methods of the class. Again, if the variable is static then it is a compilation error.

Here is the code:

import java.util.ArrayList;

import java.util.List;

class Test {

  private final List foo;

  public Test()

  {

      foo = new ArrayList();

      foo.add("foo"); // Modification-1

  }

  public static void main(String[] args) 

  {

      Test t = new Test();

      t.foo.add("bar"); // Modification-2

      System.out.println("print - " + t.foo);

  }

}

The above code works fine and no errors.

Now change the variable as static:

private static final List foo;

Now it is a compilation error. How does this final really work?

Answer 1

You are perpetually authorized to initialize a final variable. The compiler makes it certain that you can perform it only once.

Perceive that calling methods on an object saved in a final variable has nothing to act with the semantics of the final. In other words: the final is simply about the citation itself, and not about the contents of the referenced object.

Java has no notion of object immutability; this is accomplished by deliberately creating the object, and is a far-from-trivial effort.