java 11个有用的Lambda表达式示例

英语原文地址：https://www.javaguides.net/2020/03/java-lambda-expressions-examples.html

作者：Ramesh Fadatare

翻译：高行行

在本教程中，我们将看到Java中约11个有用的Lambda表达式示例。

Java 8 Lambda表达式语法

Java Lambda表达式语法

(argument-list) -> {body}

Java lambda表达式包含三个组件。

1. 参数列表：也可以为空或非空。
2. 箭头标记：用于链接参数列表和表达式主体。
3. 正文：包含lambda表达式的表达式和语句。

Java Lambda表达式示例

1. Java简单Lambda表达式示例

在这里，我们使用lambda表达式实现Drawable功能接口方法：

interface Drawable{  
    public void draw();  
}  
public class LambdaExpressionExample {  
    public static void main(String[] args) {  
        int width=10;         
        //with lambda 
        Drawable withLambda=()->{  
            System.out.println("Drawing "+width);  
        };  
        withLambda.draw();  
    }  
}

输出：

Drawing 10

2. Java Lambda表达式示例：无参数

请参考代码中的注释，该注释指示具有Lambda表达式和不具有Lambda表达式的代码。

interface Sayable {
    public String say();
}
public class JLEExampleNoParameter {
    public static void main(String[] args) {
        // without lambda expression
        Sayable sayable = new Sayable() {
            @Override
            public String say() {
                return "Return something ..";
            }
        };
        sayable.say();
        // with lambda expression
        Sayable withLambda = () -> {
            return "Return something ..";
        };
        withLambda.say();
    }
}

3. Java Lambda表达式示例：单参数

请参考代码中的注释，该注释指示具有Lambda表达式和不具有Lambda表达式的代码。

interface Printable {
    void print(String msg);
}
public class JLEExampleSingleParameter {
    public static void main(String[] args) {
     // without lambda expression
         Printable printable = new Printable() {
            @Override
            public void print(String msg) {
               System.out.println(msg);
            }
         };
         printable.print(" Print message to console....");
         // with lambda expression
         Printable withLambda = (msg) -> System.out.println(msg);
         withLambda.print(" Print message to console....");
     }
}

输出：

 Print message to console....
 Print message to console....

4. Java Lambda表达式示例：多个参数

请参考代码中的注释，该注释指示具有Lambda表达式和不具有Lambda表达式的代码。

interface Addable{  
    int add(int a,int b);  
}  
public class JLEExampleMultipleParameters {
 public static void main(String[] args) {
     // without lambda expression
  Addable addable = new Addable() {
   @Override
   public int add(int a, int b) {
    return a + b;
   }
  };
  addable.add(10, 20);
  // with lambda expression
   // Multiple parameters in lambda expression  
        Addable withLambda = (a,b)->(a+b);  
        System.out.println(withLambda.add(10,20));  
        // Multiple parameters with data type in lambda expression  
        Addable withLambdaD = (int a,int b) -> (a+b);  
        System.out.println(withLambdaD.add(100,200));  
 }
}

5. Java Lambda表达式示例：多个语句

interface IAvarage{  
    double avg(int[] array);  
}  
public class JLEExampleMultipleStatements {
    public static void main(String[] args) {
        // without lambda expression, IAvarage implementation using anonymous class  
         IAvarage avarage = new IAvarage() {
            @Override
            public double avg(int[] array) {
                double sum = 0;
                int arraySize = array.length;
                System.out.println("arraySize : " + arraySize);
                for (int i = 0; i < array.length; i++) {
                    sum = sum + array[i]; 
                } 
                System.out.println("sum : " + sum);
                return (sum/ arraySize);
           }
        };
        int[] array = {1,4,6,8,9};
        System.out.println(avarage.avg(array));
        // with a lambda expression
        // You can pass multiple statements in lambda expression 
        IAvarage withLambda = (withLambdaArray) -> {
            double sum = 0;
            int arraySize = withLambdaArray.length;
            System.out.println("arraySize : " + arraySize);
            for (int i = 0; i < withLambdaArray.length; i++) {
                sum = sum + withLambdaArray[i]; 
            }
            System.out.println("sum : " + sum);
            return (sum/ arraySize);
       };
        int[] withLambdaArray = {1,4,6,8,9};
        System.out.println(withLambda.avg(withLambdaArray)); 
    }
}

6. Java Lambda表达式示例：创建线程

public class JLEExampleRunnable {
    public static void main(String[] args) {
    //without lambda, Runnable implementation using anonymous class  
    Runnable runnable = new Runnable() {
        @Override
        public void run() {
            System.out.println(" Runnable example without lambda exp.");
        }
    };
    Thread thread = new Thread(runnable);
    thread.start();
     //with lambda 
    Runnable withLambda = () -> System.out.println(" Runnable example with lambda exp.");
    Thread thread1 = new Thread(withLambda);
    thread1.start();
  }
}

输出：

 Runnable example without lambda exp.
 Runnable example with lambda exp.

7. Java Lambda表达式示例：比较器

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;
public class JLEComparatorExample {
    public static void main(String[] args) {
        List<Person> listOfPerson = new ArrayList<Person>();
        listOfPerson.add(new Person("abc", 27));
        listOfPerson.add(new Person("mno", 26));
        listOfPerson.add(new Person("pqr", 28));
        listOfPerson.add(new Person("xyz", 27));
        // Without lambda expression.
        // Sort list by age
        Comparator<Person> comparator = new Comparator<Person>() {
            @Override
            public int compare(Person o1, Person o2) {
                return o1.getAge() - o2.getAge();
            }
        };
        Collections.sort(listOfPerson, comparator);
        System.out.println(" sort persons by age in ascending order");
        for (Person person : listOfPerson) {
            System.out.println(" Person name : " + person.getName());
        }
        // Witht lambda expression.
        // Sort list by age
        Collections.sort(listOfPerson, (Person o1, Person o2) -> {
            return o1.getAge() - o2.getAge();
        });
        // Use forEach method added in java 8
        System.out.println(" sort persons by age in ascending order");
        listOfPerson.forEach(
           (person) -> System.out.println(" Person name : " + person.getName()));
        }
  }
class Person {
    private String name;
    private int age;
    public Person(String name, int age) {
       super();
       this.name = name;
       this.age = age;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
}

8. Java Lambda表达式示例：事件监听器

import javax.swing.JButton;  
import javax.swing.JFrame;  
import javax.swing.JTextField;  
public class LambdaEventListenerExample {  
    public static void main(String[] args) {  
        JTextField tf=new JTextField();  
        tf.setBounds(50, 50,150,20);  
        JButton b=new JButton("click");  
        b.setBounds(80,100,70,30);  
        // lambda expression implementing here.  
        b.addActionListener(e-> {tf.setText("hello swing");});  
        JFrame f=new JFrame();  
        f.add(tf);f.add(b);  
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);  
        f.setLayout(null);  
        f.setSize(300, 200);  
        f.setVisible(true);  
    }  
}

9.使用forEach（）方法和Lambda表达式示例迭代ArrayList

package net.javaguides.collections;
import java.util.Arrays;
import java.util.Iterator;
import java.util.List;
/**
 * 
 * Java program to demonstrate different ways to Iterate over an ArrayList in Java
 * @author Ramesh Fadatare
 *
 */
public class DifferentWaysListIterateProgram {
    public static void main(String...args) {
        List < String > courses = Arrays.asList("C", "C++", "Core Java", "J2EE", "Spring", "Hibernate", "Python");
        // JDK 8 streaming example lambda expression
        courses.stream().forEach(course - > printCourse(course));
        // JDK 8 streaming example method reference
        courses.stream().forEach(DifferentWaysListIterateProgram::printCourse);
        // JDK 8 for each with lambda
        courses.forEach(course - > printCourse(course));
        // JDK 8 for each
        courses.forEach(DifferentWaysListIterateProgram::printCourse);
    }
    // common method to print course
    private static void printCourse(String course) {
        System.out.println("course name :: " + course);
    }
}

10.使用forEach（）方法和Lambda表达式示例迭代HashSet

package net.javaguides.collections;
import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;
/**
 * Java program to demonstrate different ways to iterate over a Set in Java
 * 
 * @author Ramesh Fadatare
 *
 */
public class DifferentWaysSetIterateProgram {
    public static void main(String...args) {
        Set < String > courses = new HashSet < String > ();
        courses.add("Java");
        courses.add("C");
        courses.add("C++");
        courses.add("Python");
        courses.add("Scala");
        // JDK 8 streaming example lambda expression
        courses.stream().forEach(course - > coursePrinter(course));
        // JDK 8 streaming example method reference
        courses.stream().forEach(DifferentWaysSetIterateProgram::coursePrinter);
        // JDK 8 for each with lambda
        courses.forEach(course - > coursePrinter(course));
        // JDK 8 for each
        courses.forEach(DifferentWaysSetIterateProgram::coursePrinter);
    }
    // common method to print course
    private static void coursePrinter(String course) {
        System.out.println("course name :: " + course);
    }
}

11.使用forEach（）方法和Lambda表达式示例迭代HashMap

package net.javaguides.collections;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
/**
 * This program demonstrate different ways to iterate over a Map in Java
 * 
 * @author Ramesh Fadatare
 *
 */
public class DifferentWaysMapIterateProgram {
    public static void main(String...args) {
        Map < Integer, String > coursesMap = new HashMap < Integer, String > ();
        coursesMap.put(1, "C");
        coursesMap.put(2, "C++");
        coursesMap.put(3, "Java");
        coursesMap.put(4, "J2EE");
        coursesMap.put(5, "Python");
        coursesMap.put(6, "Scala");
        // JDK 8 for each with lambda
        coursesMap.forEach((k, v) - > coursePrinter(k, v));
        // JDK 8 for each method reference
        coursesMap.forEach(DifferentWaysMapIterateProgram::coursePrinter);
    }
    // common method to print map key value
    private static void coursePrinter(Integer number, String brand) {
        System.out.println("course no : " + number + " and course name : " + brand);
    }
}

参考文献

• https://www.javaguides.net/2018/06/different-ways-to-iterate-over-list-set-map-in-java.html
• https://www.javaguides.net/2018/07/java-8-lambda-expressions.html
• https://www.javaguides.net/p/java-8.html