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对于Map来说,遍历的方式都是一样的,大概都是有四种形式

直接遍历

返回keySet()

返回Values()

返回entrySet()

对于第四种方式可能会除了返回的可以直接打印外,还可以通过返回Map.Entry类来依次遍历该集合返回key和value值

import java.util.*;

public class HashMapDemo {

public static void main(String[] args) {

Map hm = new HashMap<>();

hm.put("姓名", "Jack");

hm.put("age", 18);

hm.put("身高", '?');

hm.put("身高", 178);

// 键和值都允许为 null

hm.put(null, null);

// 第一种遍历方式 : 直接输出该对象

System.out.println("直接打印");

System.out.println(hm);

// 第二种遍历方式 : 通过返回键集

System.out.println("keySet()");

Set keySet = hm.keySet();

Iterator iter = keySet.iterator();

while (iter.hasNext()) {

Object next = iter.next();

System.out.println(next + "," + hm.get(next));

}

/*

首先返回的都是集合类型,除了可以使用iterator进行遍历外,还可以使用增强for循环也是非常方便的.

其实增强for循环就是上面形式的简写.本质也是上面的形式.

*/

// 增强for循环

System.out.println("增强for循环");

for (Object s :

keySet) {

System.out.println(s + "," + hm.get(s));

}

// 第三种遍历方式 : 通过返回值集

System.out.println("value()");

Collection values = hm.values();

Iterator iter1 = values.iterator();

while (iter1.hasNext()) {

System.out.println(iter1.next());

}

// 第四种遍历方式 : 通过返回key-value集

System.out.println("entrySet()集");

Iterator iter2 = hm.entrySet().iterator();

while (iter2.hasNext()) {

System.out.println(iter2.next());

}

/*

第四种方式的细化,将返回的key-value集分别取出

*/

while (iter2.hasNext()) {

Map.Entry next = (Map.Entry) iter2.next();

System.out.println(next.getKey() + " . " + next.getValue());

}

/*

第四种方式细化的简化形式.

*/

Set<Map.Entry> entrySet = hm.entrySet();

for (Map.Entry e : entrySet) {

System.out.println(e.getKey() + " , " + e.getValue());

}

}

}

上面代码的反编译 :

import java.io.PrintStream;

import java.util.*;

public class HashMapDemo

{

public HashMapDemo()

{

}

public static void main(String args[])

{

Map hm = new HashMap();

hm.put("姓名", "Jack");

hm.put("age", Integer.valueOf(18));

hm.put("身高", Character.valueOf('?'));

hm.put("身高", Integer.valueOf(178));

hm.put(null, null);

System.out.println("直接打印");

System.out.println(hm);

System.out.println("keySet()");

Set keySet = hm.keySet();

Object next;

for (Iterator iter = keySet.iterator(); iter.hasNext();

System.out.println((new StringBuilder()).append(next).append(",").append(hm.get(next)).toString()))

next = iter.next();

System.out.println("增强for循环");

Object s;

for (Iterator iterator = keySet.iterator(); iterator.hasNext();

System.out.println((new StringBuilder()).append(s).append(",").append(hm.get(s)).toString()))

s = iterator.next();

System.out.println("value()");

Collection values = hm.values();

for (Iterator iter1 = values.iterator(); iter1.hasNext(); System.out.println(iter1.next()));

System.out.println("entrySet()集");

Iterator iter2;

for (iter2 = hm.entrySet().iterator(); iter2.hasNext(); System.out.println(iter2.next()));

java.util.Map.Entry next;

for (; iter2.hasNext();

System.out.println((new StringBuilder()).append(next.getKey()).append(" . ").append(next.getValue()).toString()))

next = (java.util.Map.Entry)iter2.next();

Set entrySet = hm.entrySet();

java.util.Map.Entry e;

for (Iterator iterator1 = entrySet.iterator(); iterator1.hasNext();

System.out.println((new StringBuilder()).append(e.getKey()).append(" , ").append(e.getValue()).toString()))

e = (java.util.Map.Entry)iterator1.next();

}

}

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原文链接：https://blog.csdn.net/qq_36852780/article/details/98470429