1. Final variable:

Once we declare a variable as a final we can't re-assign the value of the variable.

Syntax:

final data_type variable_name = value;

Example: 

package quipohouse;
class FinalVariable {
    public static void main(String[] args) {
        final int X = 5;
        X = 10;  // Error: Cannot reassign a final variable
        System.out.println(X);
    }
}

Output: 

Error: Cannot assign a value to final variable 'X'

2. Final method:

Whenever we declare a method as a final our extended or inherited class can't override it.

Syntax:

class Class_Name {
    final return_type method_name() {
        // code
    }
}

Example:

package quipohouse;
class A {
    final void finalMethod() {
        System.out.println("Inside Final Method");
    }
}
class B extends A {
    // Error: Cannot override final method
    final void finalMethod() {  
        System.out.println("Inside Final Method");
    }
    
    public static void main(String[] args) {
        B b = new B();
        b.finalMethod();
    }
}

Output:

Error: LinkageError occurred while loading main class B
	java.lang.IncompatibleClassChangeError: class B overrides final method A.finalMethod()V

 3. Final class:

Whenever we declare a class as a final it can't be extended or inherited to sub-classes.

Syntax:

final class Class_Name
{
   //code
}

Example:

package quipohouse;
final class A {
}
class B extends A {      
 //gives error as final classes cant be extended
	public static void main(String[] args) {
	}
}

Output:

The type B cannot subclass the final class A