Why final instance class variable in Java?

Question

If instance variable is set final its value can not be changed like

public class Final {

    private final int b;

    Final(int b) {
        this.b = b; 
         


        

Answer 1

This are the usages of the keyword final that I know:

In the class declaration

This means that the class: cannot be sub classed

public final class SomeClass {
//...
}

In a global variable

This means that once a value is assigned to it, it cannot change.

public class SomeClass  {
     private final int value = 5;
}

There will be a compile error if you don't assign the value, but what you can do is use composition to give a value.

public class SomeClass  {
     private final int value;
     public SomeClass(int value) {
       this.value=value
      }
}

In an object parameter

This means that the passed object cannot be changed

public class SomeClass {
   public void someMethod(final String value) {
      //...
   }
}

In local variables

This means that the value cannot change once assigned

public class SomeClass {
   public void someMethod(final String value) {
      final double pi = 3.14;
   }
}

In methods

This means that the method cannot be overriden

 public class SomeClass {
           public final void someMethod() {
              //...
           }
        }

In collections & maps

This means that the collection cannot be reinitialized, but it doesn't mean that the elements are inmutable, each of the elements will not be afected by the keyword final

 public class SomeClass {
       final HashMap someMap = new HashMap();
  }