Practice›Java Fundamentals›Module 11. Modern Java›11.4 Lambdas and streams

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11.4 Lambdas and streams — a function as a value

Lesson

Implementing an interface concisely

A functional interface is an interface with exactly one abstract method. Instead of writing a whole class to implement it, you can use a lambda expression: parameter -> result. The lambda is an implementation of that one method. You then call it through the interface's method name.

Worked example

interface IntOp { int apply(int x); }

IntOp doubler = x -> x * 2;    // implements apply
IntOp square  = x -> x * x;
System.out.println(doubler.apply(5));   // 10
System.out.println(square.apply(4));    // 16

For more than one parameter, put them in parentheses: (a, b) -> a + b. A lambda lets you pass behavior around like a value.

▶ Try it: run this — two lambdas implement the same interface differently.

interface IntOp { int apply(int x); }
public class Main {
    public static void main(String[] args) {
        IntOp doubler = x -> x * 2;
        IntOp square = x -> x * x;
        System.out.println(doubler.apply(5));
        System.out.println(square.apply(4));
    }
}

A functional interface has...

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The lambda x -> x * 2...

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Type exactly what this program prints.

interface IntOp { int apply(int x); }
public class Main { public static void main(String[] a){
    IntOp inc = x -> x + 1;
    System.out.println(inc.apply(7));
    System.out.println(inc.apply(0));
}}

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A lambda with two parameters is written...

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True or false?

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