Java Map接口

本文概述

• Java Map层次结构
• Map.Entry接口

映射包含基于键的值, 即键和值对。每个键和值对称为一个条目。映射包含唯一键。

如果你必须根据关键字搜索, 更新或删除元素, 则“地图”很有用。

Java Map层次结构

有两个用于在Java中实现Map的接口：Map和SortedMap, 以及三个类：HashMap, LinkedHashMap和TreeMap。 Java Map的层次结构如下：

映射不允许重复的键, 但是你可以有重复的值。 HashMap和LinkedHashMap允许空键和值, 但是TreeMap不允许任何空键或值。

不能遍历Map, 因此你需要使用keySet()或entrySet()方法将其转换为Set。

Map界面的有用方法

Map.Entry接口

Entry是Map的子接口。因此, 将通过Map.Entry名称对其进行访问。它返回地图的集合视图, 其元素属于此类。它提供了获取关键和价值的方法。

Map.Entry接口的方法

Java Map示例：非泛型（旧样式）

//Non-generic
import java.util.*;
public class MapExample1 {
public static void main(String[] args) {
	Map map=new HashMap();
	//Adding elements to map
	map.put(1, "Amit");
	map.put(5, "Rahul");
	map.put(2, "Jai");
	map.put(6, "Amit");
	//Traversing Map
	Set set=map.entrySet();//Converting to Set so that we can traverse
	Iterator itr=set.iterator();
	while(itr.hasNext()){
		//Converting to Map.Entry so that we can get key and value separately
		Map.Entry entry=(Map.Entry)itr.next();
		System.out.println(entry.getKey()+" "+entry.getValue());
	}
}
}

输出：

1 Amit
2 Jai
5 Rahul
6 Amit

Java Map示例：通用（新样式）

import java.util.*;
class MapExample2{
 public static void main(String args[]){
  Map<Integer, String> map=new HashMap<Integer, String>();
  map.put(100, "Amit");
  map.put(101, "Vijay");
  map.put(102, "Rahul");
  //Elements can traverse in any order
  for(Map.Entry m:map.entrySet()){
   System.out.println(m.getKey()+" "+m.getValue());
  }
 }
}

输出：

102 Rahul
100 Amit
101 Vijay

Java Map示例：compareByKey()

import java.util.*;
class MapExample3{
 public static void main(String args[]){
Map<Integer, String> map=new HashMap<Integer, String>();  	  
	  map.put(100, "Amit");  
	  map.put(101, "Vijay");  
	  map.put(102, "Rahul"); 
      //Returns a Set view of the mappings contained in this map  	  
      map.entrySet()
	  //Returns a sequential Stream with this collection as its source
	  .stream()
	  //Sorted according to the provided Comparator
	  .sorted(Map.Entry.comparingByKey())
	  //Performs an action for each element of this stream
	  .forEach(System.out::println);
 }
}

输出：

100=Amit
101=Vijay
102=Rahul

Java Map示例：降序比对

import java.util.*;
class MapExample4{
 public static void main(String args[]){
Map<Integer, String> map=new HashMap<Integer, String>();  	  
	  map.put(100, "Amit");  
	  map.put(101, "Vijay");  
	  map.put(102, "Rahul");  
      //Returns a Set view of the mappings contained in this map  
	  map.entrySet()
	  //Returns a sequential Stream with this collection as its source
	  .stream()
	  //Sorted according to the provided Comparator
	  .sorted(Map.Entry.comparingByKey(Comparator.reverseOrder()))
	  //Performs an action for each element of this stream
	  .forEach(System.out::println);
 }
}

输出：

102=Rahul
101=Vijay
100=Amit

Java Map示例：compareByValue()

import java.util.*;
class MapExample5{
 public static void main(String args[]){
Map<Integer, String> map=new HashMap<Integer, String>();  	  
	  map.put(100, "Amit");  
	  map.put(101, "Vijay");  
	  map.put(102, "Rahul");  
      //Returns a Set view of the mappings contained in this map  
	  map.entrySet()
	  //Returns a sequential Stream with this collection as its source
	  .stream()
	  //Sorted according to the provided Comparator
	  .sorted(Map.Entry.comparingByValue())
	  //Performs an action for each element of this stream
	  .forEach(System.out::println);
 }
}

输出：

100=Amit
102=Rahul
101=Vijay

Java Map示例：降序比较

import java.util.*;
class MapExample6{
 public static void main(String args[]){
Map<Integer, String> map=new HashMap<Integer, String>();  	  
	  map.put(100, "Amit");  
	  map.put(101, "Vijay");  
	  map.put(102, "Rahul");  
     //Returns a Set view of the mappings contained in this map  
	 map.entrySet()
	 //Returns a sequential Stream with this collection as its source
	 .stream()
	 //Sorted according to the provided Comparator
	 .sorted(Map.Entry.comparingByValue(Comparator.reverseOrder()))
	 //Performs an action for each element of this stream
	 .forEach(System.out::println);
 }
}

输出：

101=Vijay
102=Rahul
100=Amit