Predefined Functional Interfaces

Predefined Functional Interfaces

Up to this point, the examples in this chapter have defined their own functional interfaces so that the fundamental concepts behind lambda expressions and functional interfaces could be clearly illustrated. However, in many cases, you won’t need to define your own functional interface because JDK 8 adds a new package called java.util.function that provides several predefined ones. Although we will look at them more closely in Part II, here is a sampling:

The following program shows the Function interface in action by using it to rework the earlier example called BlockLambdaDemo that demonstrated block lambdas by implementing a factorial example. That example created its own functional interface called NumericFunc, but the built-in Function interface could have been used, as this version of the program illustrates:

    //Use the Function built-in functional interface.

    //Import the Function interface.

import java.util.function.Function;

class UseFunctionInterfaceDemo {

public static void main(String args[])

{

     //This block lambda computes the factorial of an int value.

     //This time, Function is the functional interface.

     Function<Integer, Integer> factorial = (n) -> {

int result = 1;

for(int i=1; i <= n; i++) result = i * result;

return result;

};

System.out.println("The factoral of 3 is " + factorial.apply(3)); 

System.out.println("The factoral of 5 is " + factorial.apply(5));

}

}

It produces the same output as previous versions of the program.