Lecture-2 | Java8 | @FunctionalInterface

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java8 playlist:
https://youtube.com/playlist?list=PLEUBd9Jw4iIC4t4hpK3zDHk6IoJG4o3rc

Notes:

Interface - before java8

data member - public static final
methods - public abstract
cannot contains method body - no static or default methods

Functional Interface
- Must contains only one abstract method
- also known as SAM interface

- Before java8 also, we have SAM Interfaces like
Comparator - int compare(T o1, T o2);
Runnable - public abstract void run();
Then what is the change?
- Now we refer it as functional interface

eg- public interface FunctionalInterface{
public void method();//It contains only one abstract method or single abstract method
}//No Error

@FunctionalInterface//Error
public interface Interface1 {
public void method1();
public void method2();
}//Error bcz a FunctionalInterface cannot hold more than 1 abstract method

public interface Interface1 {
public void method1();
public void method2();
}//No Error - Bcz here it will behave as normal interface
- Can also contain static and default methods
eg- public interface FunctionalInterface{
public void method();
public static void method2(){
}
public default void method3(){
}
}

- @FunctionalInterface – to force compiler to check if functional Interface or not

@FunctionalInterface
eg- public interface FunctionalInterface{
public void method();
}No Error

@FunctionalInterface
eg- public interface FunctionalInterface{
public void method();
public void method2();
}//Error

We can remove @FunctionalInterface and still it will be FunctionalInterface

- FunctionalInterface can contain declaration of Object's class public method

@FunctionalInterface
eg- public interface FunctionalInterface{
public String toString();
}//No Error

@FunctionalInterface
eg- public interface FunctionalInterface{
public String toString();
public void finalize();
}//Error