Java 查漏补缺：函数式接口

Functional interface is a new feature Java 8 introduced. Functional interfaces provides target types for lambda expressions and method references.

// Assignment context
Predicate p = String::isEmpty;

// Method invocation context
stream.filter(e -> e.getSize() > 10)...

// Cast context
stream.map((ToIntFunction) e -> e.getSize())...

特性

• 函数式接口都是表达一种行为
• @FunctionalInterface 保证了函数式接口只有一个抽象方法，但是注解的使用是不必须的

java.util.function

相关的新的函数式接口定义在 java.util.function
包下：

void accept(T t)
R apply(T t)
T get()
boolean test(T t)

实例

class Test
{
    public static void main(String args[])
    {
        // create anonymous inner class object
        new Thread(new Runnable()
        {
            @Override
            public void run()
            {
                System.out.println("New thread created");
            }
        }).start();
    }
}

使用函数式接口后

class Test
{
public static void main(String args[])
{

    // lambda expression to create the object
    new Thread(()->
    {System.out.println("New thread created");}).start();
}
}

定义和使用

@FunctionalInterface
interface Square
{
    int calculate(int x);
}

class Test
{
    public static void main(String args[])
    {
        int a = 5;

        // lambda expression to define the calculate method
        Square s = (int x)->x*x;

        // parameter passed and return type must be
        // same as defined in the prototype
        int ans = s.calculate(a);
        System.out.println(ans);
    }
}

reference

• https://www.geeksforgeeks.org/functional-interfaces-java/