6.3 lambda expressions, 6.3 lambda

6.3 lambda expressions, 6.3 lambda

6.3.1 A lambda expression is a piece of code that can be passed. It can be executed once or multiple times later.

Think (how to complete the work at a specified interval and put the work in the actionreceivmed method of an ActionListener :)

class Worker implements ActionListener{    public void actionPerformed(ActionEvent event )    {      //do some work      }}

To execute this code repeatedly, You can construct an instance of the Worker class and submit the instance to a Timer object. The focus here is that the actionreceivmed method contains the code that you want to execute later.

Think (how to sort with a custom comparator. To sort strings by length rather than the default dictionary order, you can input a Comparator object to the sort method :)

class lengthComparator implements Comparator<String>{    public int compare(String first,String second)    {        return first.length()-second.length();    }}...Arrays.sort(strings,new lengthComparator());

In common: It is to pass a code block to an object (one is a timer or an sort method ). This code will be called at a certain time in the future.

6.3.2 lambda expression syntax

(String first, String second)
-> First. length ()-second. length ()

This is the first lambda expression I have seen. A lambda expression is a code block and the variable specification of the Code must be passed in.

-------- Expressions with parameter variables are called Lambda expressions.

------------------------- Expression form, parameter, arrow (->), and an expression. If the code to be computed cannot be placed in an expression, put the code in {} and contain the displayed return statement. For example:

(String first,String second)->{     if(first.length()<second.length()) return -1;else if (first.length()>second.length())  return 1;else return 0;}

Even if the lambda expression does not have a parameter, empty parentheses are required, just like the method without parameters:

( ->) {for(int i=100;i>=0;i--) System.out.println(i); }

 

If you can export the parameter type of a lambda expression, you can ignore its type. For example:

Comparator<String> comp        =(first,second)            ->first.length()>second.length();

 

If the method has only one parameter and the type of this parameter can be deduced, parentheses can be omitted;

ActionListener listener =event->       System.out.println("The time is "+new Data()); 

 

You do not need to specify the return type of the lambda expression. The return type of a lambda expression is always derived from the context. For example:

(String first,String second)->first.length()-second.length()

The above can be used in the context where int type results are required.

The following example shows how to use a lambda expression in a comparator and an action listener.

 1 package cc.openhome; 2 import java.awt.Toolkit; 3 import java.util.Arrays; 4 import java.util.Date; 5 import javax.swing.JOptionPane; 6 import javax.swing.Timer; 7 public class LambdaTest { 8     public static void main(String[] args) { 9         // TODO code application logic here10         String[] planets =new String[] {"Yi","Er","San","Si","Wu","Liu","Qi","Ba"};11         System.out.println(Arrays.toString(planets));12         System.out.println("Sortd in dictionary order:");13         Arrays.sort(planets);14         System.out.println(Arrays.toString(planets));15         System.out.println("Sorted by length:");16         Arrays.sort(planets,(first,second) -> first.length()-second.length());17         System.out.println(Arrays.toString(planets));18         Timer t =new Timer (1000,event -> {System.out.println("The time is "+new Date());Toolkit.getDefaultToolkit().beep();});19         t.start();20          JOptionPane.showMessageDialog(null, "Quit programe?");21     }  22 }

[Yi, Er, San, Si, Wu, Liu, Qi, Ba] Sortd in dictionary order: [Ba, Er, Liu, Qi, San, Si, Wu, yi] Sorted by length: [Ba, Er, Qi, Si, Wu, Yi, Liu, san] The time is Wed Mar 21 10:00:34 CST 2018The time is Wed Mar 21 10:00:35 CST 2018The time is Wed Mar 21 10:00:36 CST 2018The time is Wed Mar 21 10:00:37 CST 2018The time is Wed Mar 21 10:00:38 CST 2018The time is Wed Mar 21 10:00:39 CST 2018The time is Wed Mar 21 10:00:40 CST 2018The time is Wed Mar 21 10:00:41 CST 2018 successful build (total time: 8 seconds)

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