A variable that is declared as final and not initialized is called a blank final variable. A blank final variable forces the constructors to initialize it.

In other words
The final property of class must have a value assigned before object is created. So the last point where you can assign value to them is constructor.

This is used often for immutable objects.

 public class Foo {

  private final Bar bar;

  public Foo(Bar bar) {
    this.bar = bar;
  }

  public Bar getBar() {
   return new Bar(bar);
 } 
}