A brief analysis of Lambda expressions, new features of Java 8

Speaking of Java 8, lambda (closure) and virtual extension method (default method) will first come to mind. This feature has already been hyped by major technology websites, and it is also the first thing we will talk about at the beginning of our Java 8 series. Features (JEP126 http://openjdk.java.net/jeps/126), some libraries of jdk8 have been redesigned using lambda expressions. Understanding it is of great significance to learning the new features of Java 8.

1. Functional interface

Functional interface (functional interface is also called functional interface, but it is actually the same thing). Simply put, a functional interface is an interface that contains only one method. For example, java.lang.Runnable and java.util.Comparator in the Java standard library are typical functional interfaces. Java 8 provides @FunctionalInterface as an annotation. This annotation is not necessary. As long as the interface meets the standards of functional interfaces (that is, an interface that only contains one method), the virtual machine will automatically determine it. However, it is best to use the annotation @FunctionalInterface on the interface to declare it. This prevents other members of the team from mistakenly adding new methods to the interface.
Lambda in Java cannot appear alone. It requires a functional interface to flourish. The lambda expression method body is actually the implementation of the functional interface. The grammar will be discussed below.

2. Lambda syntax

Contains three parts

1. A formal parameter separated by commas in parentheses. The parameter is the parameter of the method in the functional interface

2. An arrow symbol:- >

3. The method body can be an expression or a code block. The method body is the implementation of the method in the functional interface. If it is a code block, it must be wrapped with {} and a return is required. value, but there is an exception. If the return value of the method in the functional interface is void, there is no need to {}

The overall look is like this:

(parameters) -> expression 或者 (parameters) -> { statements; }

Look at a complete example for easy understanding

/**
 * 测试lambda表达式
 *
 * @author benhail
 */
public class TestLambda {
    public static void runThreadUseLambda() {
        //Runnable是一个函数接口，只包含了有个无参数的，返回void的run方法；
        //所以lambda表达式左边没有参数，右边也没有return，只是单纯的打印一句话
        new Thread(() ->System.out.println("lambda实现的线程")).start(); 
    }
    public static void runThreadUseInnerClass() {
        //这种方式就不多讲了，以前旧版本比较常见的做法
        new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println("内部类实现的线程");
            }
        }).start();
    }
    public static void main(String[] args) {
        TestLambda.runThreadUseLambda();
        TestLambda.runThreadUseInnerClass();
    }
}

It can be seen that the code designed using lambda expressions will be more concise and readable.

3. Method reference

is actually a simplified way of writing lambda expression. The referenced method is actually the method body implementation of lambda expression. The syntax is also very simple. On the left is the container (can is the class name, instance name), in the middle is "::", and on the right is the corresponding method name. As shown below:

ObjectReference::methodName

The reference format of a general method is

If it is a static method, it is ClassName::methodName. For example, Object::equals

If it is an instance method, it is Instance::methodName. For example, Object obj=new Object();obj::equals;

constructor. Then it is ClassName::new

Let’s look at a complete example for easy understanding:

import java.awt.FlowLayout;
import java.awt.event.ActionEvent;
import javax.swing.JButton;
import javax.swing.JFrame;
/**
 *
 * @author benhail
 */
public class TestMethodReference {
    public static void main(String[] args) {
        JFrame frame = new JFrame();
        frame.setLayout(new FlowLayout());
        frame.setVisible(true);

        JButton button1 = new JButton("点我!");
        JButton button2 = new JButton("也点我!");

        frame.getContentPane().add(button1);
        frame.getContentPane().add(button2);
        //这里addActionListener方法的参数是ActionListener，是一个函数式接口
        //使用lambda表达式方式
        button1.addActionListener(e -> { System.out.println("这里是Lambda实现方式"); });
        //使用方法引用方式
        button2.addActionListener(TestMethodReference::doSomething);

    }
    /**
     * 这里是函数式接口ActionListener的实现方法
     * @param e 
     */
    public static void doSomething(ActionEvent e) {

        System.out.println("这里是方法引用实现方式");

    }
}

It can be seen that the doSomething method is the implementation of lambda expression. The advantage of this is that if you feel that the lambda method is very long and affects the readability of the code, method reference is a solution

IV. Summary

The above is the entire content of lambda expression syntax. I believe everyone has a certain understanding of lambda expressions. However, if the code is simple, it will not impress many viewers, and neither does Java 8. It is so exciting. In fact, the urgent need for the introduction of lambda in Java 8 is because lambda expressions can simplify multi-thread or multi-core processing of data on collections and provide faster collection processing speed. This will be discussed later on JEP126. Features will be divided into 3 parts. The reason why they are separated is because there are too many things that can be written about this feature. This part makes readers familiar with the syntax and concepts of lambda expressions and method references. The second part is virtual extension methods (default method), the last part is about the processing of large data collections, unraveling the mystery of the most powerful effect of lambda expressions.

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