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

In Java we use 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);
  }
}

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

You are always allowed to initialize a final variable. The compiler makes sure that you can do it only once.

Note that calling methods on an object stored in a final variable has nothing to do with the semantics of final. In other words: final is only about the reference itself, and not about the contents of the referenced object.

Java has no concept of object immutability; this is achieved by carefully designing the object, and is a far-from-trivial endeavor.