Functional Programming With Java 简明教程

Functional Programming - Functional Interfaces

函数式接口具有单一功能可表现。例如，带有单个方法“compareTo”的 Comparable 接口用于比较目的。Java 8 已经定义了很多函数式接口，以便在 lambda 表达式中广泛使用。以下是 java.util.Function 包中定义的函数式接口列表。

Functional interfaces have a single functionality to exhibit. For example, a Comparable interface with a single method 'compareTo' is used for comparison purpose. Java 8 has defined a lot of functional interfaces to be used extensively in lambda expressions. Following is the list of functional interfaces defined in java.util.Function package.

Functional Interface Example

Predicate <T> 接口是一个函数式接口，具有一个方法 test(Object)，用于返回一个布尔值。此接口表示将对某个对象进行真或假测试。

Predicate <T> interface is a functional interface with a method test(Object) to return a Boolean value. This interface signifies that an object is tested to be true or false.

在任意编辑器中创建以下 Java 程序，例如，在 C:\> JAVA 中创建。

Create the following Java program using any editor of your choice in, say, C:\> JAVA.

Java8Tester.java

import java.util.Arrays;
import java.util.List;
import java.util.function.Predicate;
public class Java8Tester {
   public static void main(String args[]) {
      List<Integer> list = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9);

      // Predicate<Integer> predicate = n -> true
      // n is passed as parameter to test method of Predicate interface
      // test method will always return true no matter what value n has.

      System.out.println("Print all numbers:");

      //pass n as parameter
      eval(list, n->true);

      // Predicate<Integer> predicate1 = n -> n%2 == 0
      // n is passed as parameter to test method of Predicate interface
      // test method will return true if n%2 comes to be zero

      System.out.println("Print even numbers:");
      eval(list, n-> n%2 == 0 );

      // Predicate<Integer> predicate2 = n -> n > 3
      // n is passed as parameter to test method of Predicate interface
      // test method will return true if n is greater than 3.

      System.out.println("Print numbers greater than 3:");
      eval(list, n-> n > 3 );
   }

   public static void eval(List<Integer> list, Predicate<Integer> predicate) {
      for(Integer n: list) {
         if(predicate.test(n)) {
            System.out.println(n + " ");
         }
      }
   }
}

这里我们传递了 Predicate 接口，它采用一个输入并返回布尔值。

Here we’ve passed Predicate interface, which takes a single input and returns Boolean.

Verify the Result

按照如下方式使用 javac 编译器编译类 −

Compile the class using javac compiler as follows −

C:\JAVA>javac Java8Tester.java

现在按照如下方式运行 Java8Tester −

Now run the Java8Tester as follows −

C:\JAVA>java Java8Tester

它应该产生以下输出 −

It should produce the following output −

Print all numbers:
1
2
3
4
5
6
7
8
9
Print even numbers:
2
4
6
8
Print numbers greater than 3:
4
5
6
7
8
9