Java集合框架学习(八) HashMap详解
HashMap介绍
HashMap是一个基于Map的集合类,用于存储Key和Value的键值对。
通常用HashMap<Key, Value> or HashMap<K, V>标识。
这个类不保证元素的顺序。
HashMap和HashTable很类似,除了它是非线程安全和允许null值(null值和null Key)这2个特点。不保证元素顺序意思是它不会按照你插入的顺序返回。它不会对key和value进行排序。
类定义
public class HashMap<K,V>extends AbstractMap<K,V>
implements Map<K,V>, Cloneable, Serializable
例子介绍
package com.dylan.collection;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.Set;
/**
* @author xusucheng
* @create 2018-01-31
**/
public class HashMapExample {
public static void main(String args[]) {
/* This is how to declare HashMap */
HashMap<Integer, String> hmap = new HashMap<Integer, String>();
/*Adding elements to HashMap*/
hmap.put(12, "Chaitanya");
hmap.put(2, "Rahul");
hmap.put(7, "Singh");
hmap.put(49, "Ajeet");
hmap.put(3, "Anuj");
/* Display content using Iterator*/
Set set = hmap.entrySet();
Iterator iterator = set.iterator();
while(iterator.hasNext()) {
Map.Entry mentry = (Map.Entry)iterator.next();
System.out.print("key is: "+ mentry.getKey() + " & Value is: ");
System.out.println(mentry.getValue());
}
/* Get values based on key*/
String var= hmap.get(2);
System.out.println("Value at index 2 is: "+var);
/* Remove values based on key*/
hmap.remove(3);
System.out.println("Map key and values after removal:");
Set set2 = hmap.entrySet();
Iterator iterator2 = set2.iterator();
while(iterator2.hasNext()) {
Map.Entry mentry2 = (Map.Entry)iterator2.next();
System.out.print("Key is: "+mentry2.getKey() + " & Value is: ");
System.out.println(mentry2.getValue());
}
}
}
常用方法
增:put, putAll
删:clear, remove
改:put
查:isEmpty, get, keySet, size, containsKey
如何给HashMap的键值对排序
1.按key排序
package com.dylan.collection;
import java.util.*;
/**
* @author xusucheng
* @create 2018-01-31
**/
public class HashMapSortByKey {
public static void main(String[] args) {
HashMap<Integer, String> hmap = new HashMap<Integer, String>();
hmap.put(5, "A");
hmap.put(11, "C");
hmap.put(4, "Z");
hmap.put(77, "Y");
hmap.put(9, "P");
hmap.put(66, "Q");
hmap.put(0, "R");
System.out.println("Before Sorting:");
Set set = hmap.entrySet();
Iterator iterator = set.iterator();
while(iterator.hasNext()) {
Map.Entry me = (Map.Entry)iterator.next();
System.out.print(me.getKey() + ": ");
System.out.println(me.getValue());
}
Map<Integer, String> map = new TreeMap<Integer, String>(hmap);
System.out.println("After Sorting:");
Set set2 = map.entrySet();
Iterator iterator2 = set2.iterator();
while(iterator2.hasNext()) {
Map.Entry me2 = (Map.Entry)iterator2.next();
System.out.print(me2.getKey() + ": ");
System.out.println(me2.getValue());
}
}
}
2.按value排序
package com.dylan.collection;
import java.util.*;
/**
* @author xusucheng
* @create 2018-01-31
**/
public class HashMapSortByValue {
public static void main(String[] args) {
HashMap<Integer, String> hmap = new HashMap<Integer, String>();
hmap.put(5, "A");
hmap.put(11, "C");
hmap.put(4, "Z");
hmap.put(77, "Y");
hmap.put(9, "P");
hmap.put(66, "Q");
hmap.put(0, "R");
System.out.println("Before Sorting:");
Set set = hmap.entrySet();
Iterator iterator = set.iterator();
while (iterator.hasNext()) {
Map.Entry me = (Map.Entry) iterator.next();
System.out.print(me.getKey() + ": ");
System.out.println(me.getValue());
}
Map<Integer, String> map = sortByValues(hmap);
System.out.println("After Sorting:");
Set set2 = map.entrySet();
Iterator iterator2 = set2.iterator();
while (iterator2.hasNext()) {
Map.Entry me2 = (Map.Entry) iterator2.next();
System.out.print(me2.getKey() + ": ");
System.out.println(me2.getValue());
}
}
private static HashMap sortByValues(HashMap map) {
List list = new LinkedList(map.entrySet());
// Defined Custom Comparator here
Collections.sort(list, new Comparator() {
public int compare(Object o1, Object o2) {
return ((Comparable) ((Map.Entry) (o1)).getValue())
.compareTo(((Map.Entry) (o2)).getValue());
}
});
// Here I am copying the sorted list in HashMap
// using LinkedHashMap to preserve the insertion order
HashMap sortedHashMap = new LinkedHashMap();
for (Iterator it = list.iterator(); it.hasNext();) {
Map.Entry entry = (Map.Entry) it.next();
sortedHashMap.put(entry.getKey(), entry.getValue());
}
return sortedHashMap;
}
}
序列化/反序列化HashMap
import java.io.*;
import java.util.HashMap;
public class Details
{
public static void main(String [] args)
{
HashMap<Integer, String> hmap = new HashMap<Integer, String>();
//Adding elements to HashMap
hmap.put(11, "AB");
hmap.put(2, "CD");
hmap.put(33, "EF");
hmap.put(9, "GH");
hmap.put(3, "IJ");
try
{
FileOutputStream fos =
new FileOutputStream("hashmap.ser");
ObjectOutputStream oos = new ObjectOutputStream(fos);
oos.writeObject(hmap);
oos.close();
fos.close();
System.out.printf("Serialized HashMap data is saved in hashmap.ser");
}catch(IOException ioe)
{
ioe.printStackTrace();
}
}
}
import java.io.*;
import java.util.HashMap;
import java.util.Map;
import java.util.Iterator;
import java.util.Set;
public class Student
{
public static void main(String [] args)
{
HashMap<Integer, String> map = null;
try
{
FileInputStream fis = new FileInputStream("hashmap.ser");
ObjectInputStream ois = new ObjectInputStream(fis);
map = (HashMap) ois.readObject();
ois.close();
fis.close();
}catch(IOException ioe)
{
ioe.printStackTrace();
return;
}catch(ClassNotFoundException c)
{
System.out.println("Class not found");
c.printStackTrace();
return;
}
System.out.println("Deserialized HashMap..");
// Display content using Iterator
Set set = map.entrySet();
Iterator iterator = set.iterator();
while(iterator.hasNext()) {
Map.Entry mentry = (Map.Entry)iterator.next();
System.out.print("key: "+ mentry.getKey() + " & Value: ");
System.out.println(mentry.getValue());
}
}
}
HashMap和HashTable区别
HashMap和Hashtable都实现了Map接口,但决定用哪一个之前先要弄清楚它们之间的分别。
主要的区别有:线程安全性,同步(synchronization),以及速度。
1. HashMap几乎可以等价于Hashtable,除了HashMap是非synchronized的,并可以接受null(HashMap可以接受为null的键
值(key)和值(value),而Hashtable则不行)。
2. HashMap是非synchronized,而Hashtable是synchronized,这意味着Hashtable是线程安全的,多个线程可以共享一个
Hashtable;
而如果没有正确的同步的话,多个线程是不能共享HashMap的。Java 5提供了ConcurrentHashMap,它是HashTable的替代,
比HashTable的扩展性更好。
3. 另一个区别是HashMap的迭代器(Iterator)是fail-fast迭代器,而Hashtable的enumerator迭代器不是fail-fast的。
所以当有其它线程改变了HashMap的结构(增加或者移除元素),将会抛出ConcurrentModificationException,
但迭代器本身的remove()方法移除元素则不会抛出ConcurrentModificationException异常。但这并不是一个一定发生的行为,
要看JVM。
这条同样也是Enumeration和Iterator的区别。
4. 由于Hashtable是线程安全的也是synchronized,所以在单线程环境下它比HashMap要慢。如果你不需要同步,只需要单一
线程,那么使用HashMap性能要好过Hashtable。
HashMap不能保证随着时间的推移Map中的元素次序是不变的。