Four methods of traversing key value in map

 Four ways 

• First use keySet() Remove all key value , Then take out the corresponding value—— enhance for Loop traversal

• First use keySet() Remove all key value , Then take out the corresponding value—— Use iterators to traverse

• adopt entrySet To get key-value—— enhance for Loop traversal

• adopt entrySet To get key-value—— Use iterators to traverse

Map yes java The interface ,Map.Entry yes Map An internal interface for .Map Provides some common methods , Such as keySet()、entrySet() Other methods ,keySet() Method return value is Map in key It's worth it Set aggregate ;entrySet() The return value of also returns a Set aggregate , The type of this collection is Map.Entry<K, V>.

Map.Entry yes Map An internal interface for declarations , This interface is generic , Defined as Entry<K,V>. It said Map One of the entities in （ One key-value Yes ）. There are getKey(),getValue Method .

1、 structure HashMap, to hashMap Additive elements

HashMap hashMap = new HashMap();
hashMap.put(" The romance of The Three Kingdoms "," Luo Guanzhong ");
hashMap.put(" Water margin "," Shi Naian ");
hashMap.put(" Journey to the west "," Wu chengen ");
hashMap.put(" A dream of red mansions "," Cao xueqin ");

2、 Method 1 ： First use keySet() Remove all key value , Then take out the corresponding value—— Use iterators to traverse

2.1 Code

/*1、 First use keySet() Remove all key value , Then take out the corresponding value—— enhance for Loop traversal */
System.out.println("====1、 First use keySet() Remove all key value , Then take out the corresponding value—— enhance for Loop traversal ====");
Set keyset = hashMap.keySet();
for(Object key:keyset){
    System.out.println(key+"-"+hashMap.get(key));
}

2.2 Running results

3、 Method 2 ： Take out all... First key, Re pass key Take out the corresponding value—— Use iterators to traverse

3.1 Code

/*2、 First use keySet() Remove all key value , Then take out the corresponding value—— Use iterators to traverse */
System.out.println("====2、 First use keySet() Remove all key value , Then take out the corresponding value—— Use iterators to traverse ====");
Iterator iterator = keyset.iterator();
while(iterator.hasNext()){
    Object key = iterator.next();
    System.out.println(key+"-"+hashMap.get(key));
}

3.2 Running results

4、 Method 3 ： adopt entrySet() get key-value value —— enhance for Loop traversal

4.1 Code

/*3、 adopt entrySet() get key-value value —— enhance for Loop traversal */
System.out.println("====3、 adopt entrySet() get key-value value —— enhance for Loop traversal ====");
Set set = hashMap.entrySet();
for(Object key:set){
    Map.Entry entry = (Map.Entry) key;
    System.out.println(entry.getKey()+"-"+entry.getValue());
}

4.2 Running results

5、 Method four ： adopt entrySet() get key-value value —— Use iterators to traverse

5.1 Code

/*4、 adopt entrySet() get key-value value —— Use iterators to traverse */
System.out.println("====4、 adopt entrySet() get key-value value —— Use iterators to traverse ====");
Set set1 = hashMap.entrySet();
Iterator iterator1 = set1.iterator();
while(iterator1.hasNext()){
    Object itset = iterator1.next();
    Map.Entry entry = (Map.Entry) itset;
    System.out.println(entry.getKey()+"-"+entry.getValue());
}

5.2 Running results

6、 Complete code

import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.Set;
public class MapTraverse {
    public static void main(String[] args){
        HashMap hashMap = new HashMap();
        hashMap.put(" The romance of The Three Kingdoms "," Luo Guanzhong ");
        hashMap.put(" Water margin "," Shi Naian ");
        hashMap.put(" Journey to the west "," Wu chengen ");
        hashMap.put(" A dream of red mansions "," Cao xueqin ");
        /*1、 First use keySet() Remove all key value , Then take out the corresponding value—— enhance for Loop traversal */
        System.out.println("====1、 First use keySet() Remove all key value , Then take out the corresponding value—— enhance for Loop traversal ====");
        Set keyset = hashMap.keySet();
        for(Object key:keyset){
            System.out.println(key+"-"+hashMap.get(key));
        }
        /*2、 First use keySet() Remove all key value , Then take out the corresponding value—— Use iterators to traverse */
        System.out.println("====2、 First use keySet() Remove all key value , Then take out the corresponding value—— Use iterators to traverse ====");
        Iterator iterator = keyset.iterator();
        while(iterator.hasNext()){
            Object key = iterator.next();
            System.out.println(key+"-"+hashMap.get(key));
        }
        /*3、 adopt entrySet() get key-value value —— enhance for Loop traversal */
        System.out.println("====3、 adopt entrySet() get key-value value —— enhance for Loop traversal ====");
        Set set = hashMap.entrySet();
        for(Object key:set){
            Map.Entry entry = (Map.Entry) key;
            System.out.println(entry.getKey()+"-"+entry.getValue());
        }
        /*4、 adopt entrySet() get key-value value —— Use iterators to traverse */
        System.out.println("====4、 adopt entrySet() get key-value value —— Use iterators to traverse ====");
        Set set1 = hashMap.entrySet();
        Iterator iterator1 = set1.iterator();
        while(iterator1.hasNext()){
            Object itset = iterator1.next();
            Map.Entry entry = (Map.Entry) itset;
            System.out.println(entry.getKey()+"-"+entry.getValue());
        }
    }
}

7、 Application, for example,

7.1 Problem description

Use HashMap add to 3 Employees , requirement

• key : staff id

• value : The employee object And traverse to display salary >18000 The employees' ( There are at least two traversal methods ) The employee class : full name 、 Wages 、 staff id

7.2 Code

import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.Set;

/* Use HashMap add to 3 Employees , requirement 
 key : staff id
 value : The employee object 
 And traverse to display salary >18000 The employees' ( There are at least two traversal methods ) The employee class : full name 、 Wages 、 staff id
*/
public class MapExercise {
    public static void main(String[] args) {
        HashMap hashMap = new HashMap();
        hashMap.put(1, new employee("Tom", 20000.0, 1));
        hashMap.put(2, new employee("Jack", 10000.0, 2));
        hashMap.put(3, new employee("Bob", 30000.0, 3));
        hashMap.put(4, new employee("Marry", 17000.0, 4));
        /*1、 adopt keySet() Get all key value , And get the corresponding value value —— enhance for Loop traversal */
        System.out.println("====1、 adopt keySet() Get all key value , And get the corresponding value value —— enhance for Loop traversal ");
        Set keyset = hashMap.keySet();
        for (Object key : keyset) {
            employee m = (employee) hashMap.get(key);
            if (m.getSalary() > 18000) {
                System.out.println(m);
            }
        }
        /*2、 adopt entrySet() obtain key-value value —— Use iterators to traverse */
        System.out.println("====2、 adopt entrySet() obtain key-value value —— Use iterators to traverse ====");
        Set set = hashMap.entrySet();
        Iterator iterator = set.iterator();
        while (iterator.hasNext()) {
            Object itset = iterator.next();
            Map.Entry entry = (Map.Entry) itset;
            employee m = (employee) entry.getValue();
            if (m.getSalary() > 18000) {
                System.out.println(m);
            }
        }

    }
}
class employee{
    private String name;
    private Double salary;
    private int id;

    public employee(String name, Double salary, int id) {
        this.name = name;
        this.salary = salary;
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Double getSalary() {
        return salary;
    }

    public void setSalary(Double salary) {
        this.salary = salary;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    @Override
    public String toString() {
        return "employee{" +
                "name='" + name + '\'' +
                ", salary=" + salary +
                ", id=" + id +
                '}';
    }
}

7.3 Running results

author ：dengmsjava

https://blog.csdn.net/qq_40839718/article/details/123622684

official account “Java selected ” The published content indicates the source of , All rights reserved （ Those whose copyright cannot be verified or whose source is not indicated all come from the Internet , Reprinted , The purpose of reprinting is to convey more information , The copyright belongs to the original author . If there is any infringement , Please contact the , The author will delete the first time ！

Many people have asked recently , Is there any readers Communication group ！ The way to join is simple , official account Java selected , reply “ Add group ”, You can join the group ！

Java Interview questions （ Wechat applet ）：3000+ The road test questions , contain Java Basics 、 Concurrent 、JVM、 Threads 、MQ series 、Redis、Spring series 、Elasticsearch、Docker、K8s、Flink、Spark、 Architecture design, etc , Brush questions online at any time ！

------ Special recommendation ------

Special recommendation ： Focus on the most cutting-edge information and technology sharing , Official account for preparing for overtaking on curves and various open source projects and efficient software ,「 Big coffee notes 」, Focus on finding good things , It's worth your attention . Click the official account card below to follow .

If the article helps , Click to see , Forward! ！