7. Java 8: Method Enhancements

Question 1

Java 8 now supports two new method types for interfaces, what are these?

Answer 1

Static methods

Default methods

Question 2

Considering the following class, what is the result:
public interface DoAble {
   public static void doStuff() {
      System.out.println("hello");
   }
}

public class Task implements DoAble {}

new Task().doStuff();

Answer 2

Compiler error

Normally you can reference static methods using a object reference. Static methods for interface however can only be called using a Class reference to the interface. So the only valled call to doStuff() in this example is: DoAble.doStuff();

Question 3

What is a default method in a interface?

Answer 3

A interface can now provide a default implementation for methods. This way any implementing classes are not required to implement all of the methods (only the non-default ones).

Question 4

Given the following interface and class:
interface Greeter { 
default void greet() { System.out.prinltn("hello"); }
}
class GreeterImpl implements Greeter{
void greet() {
// 1
}
}

Replace line 1 to call the default method from the interface.

Answer 4

Greeter.super.greet();

Question 5

What is the result of the following:
interface Greeter { 
default void greet() { System.out.prinltn("hello"); }
}
interface Greeter2 { 
default void greet() { System.out.prinltn("hello"); }
}
class GreeterImpl implements Greeter, Greeter2{
}

Answer 5

Compilation fails.

GreeterImpl should provide a override for the method greet because two defaults are found.

Question 6

What is the difference between a default method inside a interface and a regular method inside a class?

Answer 6

A default method can only access the arguments and not any instance methods or variables (because an interface itself doesn’t have any instance methods or variables.

Question 7

What is the result of the following:
public interface MyInterface {
default boolean equals(Object o) {
return true;
}
}

Answer 7

Won’t compile.

You cannot use default methods to override any of the Object methods.