What’s the meaning of “final” in java?

The following is adapted from this link.

import java.util.ArrayList;
import java.util.List;

class Test {
    private final List foo;
    // private static final List foo = new ArrayList();
    // Results in compile time error.

    public Test() {
        foo = new ArrayList();
        foo.add("foo"); // Modification-1
    }

    public void setFoo(List foo) {
       //this.foo = foo; Results in compile time error.
    }
    
    public static void main(String[] args) 
      {
          Test t = new Test();
          t.foo.add("bar"); // Modification-2
          System.out.println("print - " + t.foo);
      }
}

In the above example, a ‘Test’ constructor is defined, and given a ‘setFoo’ method. foo is an instance variable. When we create Test class object, then the instance variable foo will be copied inside the object of Test class, and instantiated to an ArrayList with “foo” content. The value of a final variable can only be assigned one time. So this.foo = foo' in the ‘setFoo’ method will cause a compile time error.

If private final List foo; is changed to private static final List foo = new ArrayList(); The compiler will yell at you. That is because, in this case, foo is a static variable. A static variable is shared by all instances of that class. If every new instance of that class can see the foo variable, and try to re-assign it to a new ArrayList when constructor is called, the compiler will start to yell.

Modification-1 and Modification-2 are both allowed. Because those code were only adding contents to foo. They are not re-assigning foo to something else. In other words, final is only about the reference itself, and not about the contents of the referenced object.

The above examples gives a clear explanation of final variable.

Other rules for final key word:
final classes cannot be subclassed.
final method cannot be overridden by subclasses, but can override.