Explain the importance of final & blank final keyword in java

1) final variable : When the variable in a class is declared as final, then its value remains the same. Its considered as a constant. It cannot be changed. When a variable is declared final, then it needs to be initialized only once, using an assignment operator or in initializer. If the final variable is not assigned a value at the time of declaration, then it is considered as "blank final" variable. In this case, it need to be mandatorily initialzed in a constructor or in a static block in the class.

Java compiler will ensure that the blank final is not used until it is assigned a value and that once assigned a value, the now final variable cannot be reassigned another value.

public class FinalVariables 
{
 public static final double declaredFinal 3.85;  
 public final double radius;
 
 FinalVariables(double r) {
  radius = r;
 }
}

2) final method : When a method is made final, then this method cannot be overloaded in case some other class is extending the class containing the final method.

public class Base 
{
    public final void m2() {...}
}
 
public class Derived extends Base 
{
    public void m2() {...}  // not allowed, gives compilation error
}

3) final class : When a class is made final, then it cannot be subclassed.

public final class FinalClass {...}
 
public class ThisIsWrong extends FinalClass {...} // not allowed, gives compilation error