java 中遍历Map的几种方法

方法分为两类：

一类是基于map的Entry；map.entrySet();

一类是基于map的key；map.keySet()

而每一类都有两种遍历方式：

a.利用迭代器 iterator；

b.利用for-each循环；

代码举例如下：

package cn.wzb;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.Map.Entry;

//循环遍历map的方法
public class TestMap {
public static void main(String[] args) {
Map<String, Integer> map = new HashMap<String, Integer>();
map.put("a", 1);
map.put("b", 2);
map.put("c", 3);

// JDK1.4中
// 遍历方法一 hashMap.entrySet() 遍历
System.out.println("方法一");
Iterator<Entry<String, Integer>> it = map.entrySet().iterator();
while (it.hasNext()) {
Map.Entry<String, Integer> entry = it.next();
String key = entry.getKey();
Integer value = entry.getValue();
System.out.println("key = " + key + "; value = " + value);
}

System.out.println("");

// JDK1.5中,应用新特性For-Each循环 // 遍历方法二
System.out.println("方法二");
for (Map.Entry<String, Integer> entry : map.entrySet()) {
String key = entry.getKey();
String value = entry.getValue().toString();
System.out.println("key =" + key + " value = " + value);
}
System.out.println("");

// 遍历方法三 hashMap.keySet().iterator() 遍历
System.out.println("方法三");
for (Iterator<String> i = map.keySet().iterator(); i.hasNext(); ) {
String key = i.next();
System.out.println(" key = " + key + "; value = " + map.get(key));
}

/*
for (Iterator<Integer> i = map.values().iterator(); i.hasNext(); ) {
Integer value = i.next();
System.out.println(value));// 循环输出value
}
*/
/*
for(Integer value : map.values() ) {
System.out.println("value = " + value);
}
*/


System.out.println("");
// 遍历方法四 Hashmap.keySet()遍历
System.out.println("方法四");
for (String key : map.keySet()) {
System.out.println("key = " + key + "; value = " + map.get(key));
}
System.out.println();

// java如何遍历Map <String, ArrayList> map = new HashMap <String, ArrayList<Integer> >();
System.out.println("java 遍历Map <String, ArrayList<Integer> map = new HashMap <String, ArrayList<Integer>>();");
System.out.println("方法一：");
Map<String, List<Integer> > mapList = new HashMap <String, List<Integer> >();
List<Integer> list = new ArrayList<Integer>();
list.add(1);
list.add(2);
list.add(3);
list.add(4);
list.add(5);
mapList.put("aaa", list);
mapList.put("bbb", list);

Set<String> keys = mapList.keySet();
Iterator<String> iterator = keys.iterator();
while (iterator.hasNext()) {
String key = iterator.next();
System.out.println(key + " : ");

List<Integer> arrayList = mapList.get(key);
for (Integer i : arrayList) {
System.out.print(i + ", ");
}
System.out.println();
}

System.out.println("方法二：");

for (Map.Entry<String, List<Integer> > entry : mapList.entrySet()) {
String key = entry.getKey();
System.out.println(key + " : ");

List<Integer> values = (List<Integer>) entry.getValue();
for (Integer value : values) {
System.out.print(value + ", ");
}
System.out.println();
}
}
}

package cn.wzb;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.Map.Entry;

//循环遍历map的方法
public class TestMap {
    public static void main(String[] args) {
        Map<String, Integer> map = new HashMap<String, Integer>();
        map.put("a", 1);
        map.put("b", 2);
        map.put("c", 3);

        // JDK1.4中
        // 遍历方法一 hashMap.entrySet() 遍历
        System.out.println("方法一");
        Iterator<Entry<String, Integer>> it = map.entrySet().iterator();
        while (it.hasNext()) {
            Map.Entry<String, Integer> entry =  it.next();
            String key = entry.getKey();
             Integer value = entry.getValue();
            System.out.println("key = " + key + "; value = " + value);
        }

        System.out.println("");

        // JDK1.5中,应用新特性For-Each循环  // 遍历方法二
        System.out.println("方法二");
        for (Map.Entry<String, Integer> entry : map.entrySet()) {
            String key = entry.getKey();
            String value = entry.getValue().toString();
            System.out.println("key =" + key + " value = " + value);
        }
        System.out.println("");

        // 遍历方法三 hashMap.keySet().iterator() 遍历
        System.out.println("方法三");
        for (Iterator<String> i = map.keySet().iterator(); i.hasNext(); ) {
             String key = i.next();
            System.out.println(" key = " + key + "; value = " + map.get(key));
        }

       /*
        for (Iterator<Integer> i = map.values().iterator(); i.hasNext(); ) {
            Integer value = i.next();
            System.out.println(value));// 循环输出value
        }
        */
        /*
        for(Integer value : map.values() ) {
            System.out.println("value = " + value);
        }
        */


        System.out.println("");
        // 遍历方法四  Hashmap.keySet()遍历
        System.out.println("方法四");
        for (String key : map.keySet()) {
            System.out.println("key = " + key + "; value = " + map.get(key));
        }
        System.out.println();

        // java如何遍历Map <String, ArrayList> map = new HashMap <String, ArrayList<Integer> >();
        System.out.println("java  遍历Map <String, ArrayList<Integer> map = new HashMap <String, ArrayList<Integer>>();");
        System.out.println("方法一：");
        Map<String, List<Integer> > mapList  = new HashMap <String, List<Integer> >();
        List<Integer> list = new ArrayList<Integer>();
        list.add(1);
        list.add(2);
        list.add(3);
        list.add(4);
        list.add(5);
        mapList.put("aaa", list);
        mapList.put("bbb", list);

        Set<String> keys = mapList.keySet();
        Iterator<String> iterator = keys.iterator();
        while (iterator.hasNext()) {
            String key = iterator.next();
            System.out.println(key + " : ");

            List<Integer> arrayList = mapList.get(key);
            for (Integer i : arrayList) {
                System.out.print(i + ", ");
            }
            System.out.println();
        }

        System.out.println("方法二：");

        for (Map.Entry<String, List<Integer> > entry : mapList.entrySet()) {
            String key = entry.getKey();
            System.out.println(key + " : ");

            List<Integer> values = (List<Integer>) entry.getValue();
            for (Integer value : values) {
                System.out.print(value + ", ");
            }
            System.out.println();
        }
    }
}