Java HashMap类

Java HashMap类通过使用哈希表来实现map接口。它继承了AbstractMap类并实现Map接口。

要记住的要点

• Java HashMap类包含基于键的值。
• Java HashMap类仅包含唯一键。
• Java HashMap类可以具有一个null键和多个null值。
• Java HashMap类不同步。
• Java HashMap类不维护任何顺序。
• Java HashMap类的初始默认容量为16, 负载因子为0.75。

HashMap类的层次结构

如上图所示, HashMap类扩展了AbstractMap类并实现了Map接口。

HashMap类声明

我们来看一下java.util.HashMap类的声明。

public class HashMap<K, V> extends AbstractMap<K, V> implements Map<K, V>, Cloneable, Serializable

HashMap类参数

让我们看一下java.util.HashMap类的参数。

• K：这是此地图维护的键的类型。
• V：这是映射值的类型。

Java HashMap类的构造方法

Java HashMap类的方法

Java HashMap示例中的add()元素

在这里, 我们看到了插入元素的不同方法。

import java.util.*;
class HashMap1{
 public static void main(String args[]){
   HashMap<Integer, String> hm=new HashMap<Integer, String>();  
	System.out.println("Initial list of elements: "+hm);
	  hm.put(100, "Amit");  
	  hm.put(101, "Vijay");  
	  hm.put(102, "Rahul"); 
	 
	  System.out.println("After invoking put() method ");
	  for(Map.Entry m:hm.entrySet()){  
	   System.out.println(m.getKey()+" "+m.getValue());  
	  }
	  
	  hm.putIfAbsent(103, "Gaurav");
	  System.out.println("After invoking putIfAbsent() method ");
	  for(Map.Entry m:hm.entrySet()){  
		   System.out.println(m.getKey()+" "+m.getValue());  
		  }
	  HashMap<Integer, String> map=new HashMap<Integer, String>();
	  map.put(104, "Ravi");
	  map.putAll(hm);
	  System.out.println("After invoking putAll() method ");
	  for(Map.Entry m:map.entrySet()){  
		   System.out.println(m.getKey()+" "+m.getValue());  
		  }
 }
}

Initial list of elements: {}
After invoking put() method 
100 Amit
101 Vijay
102 Rahul
After invoking putIfAbsent() method 
100 Amit
101 Vijay
102 Rahul
103 Gaurav
After invoking putAll() method 
100 Amit
101 Vijay
102 Rahul
103 Gaurav
104 Ravi

Java HashMap示例, 以remove()元素

在这里, 我们看到了删除元素的不同方法。

import java.util.*;
public class HashMap2 {
   public static void main(String args[]) {
  	HashMap<Integer, String> map=new HashMap<Integer, String>();  	  
	  map.put(100, "Amit");  
	  map.put(101, "Vijay");  
	  map.put(102, "Rahul");
	  map.put(103, "Gaurav");
	System.out.println("Initial list of elements: "+map);
	//key-based removal
	map.remove(100);
	System.out.println("Updated list of elements: "+map);
	//value-based removal
	map.remove(101);
	System.out.println("Updated list of elements: "+map);
	//key-value pair based removal
	map.remove(102, "Rahul");
	System.out.println("Updated list of elements: "+map);
   }    
}

输出：

Initial list of elements: {100=Amit, 101=Vijay, 102=Rahul, 103=Gaurav}
Updated list of elements: {101=Vijay, 102=Rahul, 103=Gaurav}
Updated list of elements: {102=Rahul, 103=Gaurav}
Updated list of elements: {103=Gaurav}

Java HashMap示例替换()元素

在这里, 我们看到了替换元素的不同方法。

import java.util.*;
class HashMap3{
 public static void main(String args[]){
   HashMap<Integer, String> hm=new HashMap<Integer, String>();  
	  hm.put(100, "Amit");  
	  hm.put(101, "Vijay");  
	  hm.put(102, "Rahul"); 
	  System.out.println("Initial list of elements:");
	 for(Map.Entry m:hm.entrySet())
	 {
		System.out.println(m.getKey()+" "+m.getValue()); 
	 }
	 System.out.println("Updated list of elements:");
	 hm.replace(102, "Gaurav");
	 for(Map.Entry m:hm.entrySet())
	 {
		System.out.println(m.getKey()+" "+m.getValue()); 
	 }
	 System.out.println("Updated list of elements:");
	 hm.replace(101, "Vijay", "Ravi");
	 for(Map.Entry m:hm.entrySet())
	 {
		System.out.println(m.getKey()+" "+m.getValue()); 
	 } 
	 System.out.println("Updated list of elements:");
	 hm.replaceAll((k, v) -> "Ajay");
	 for(Map.Entry m:hm.entrySet())
	 {
		System.out.println(m.getKey()+" "+m.getValue()); 
	 }
 }
}

Initial list of elements:
100 Amit
101 Vijay
102 Rahul
Updated list of elements:
100 Amit
101 Vijay
102 Gaurav
Updated list of elements:
100 Amit
101 Ravi
102 Gaurav
Updated list of elements:
100 Ajay
101 Ajay
102 Ajay

HashSet和HashMap之间的区别

HashSet仅包含值, 而HashMap包含条目（键和值）。

Java HashMap示例：书籍

import java.util.*;  
class Book {  
int id;  
String name, author, publisher;  
int quantity;  
public Book(int id, String name, String author, String publisher, int quantity) {  
    this.id = id;  
    this.name = name;  
    this.author = author;  
    this.publisher = publisher;  
    this.quantity = quantity;  
}  
}  
public class MapExample {  
public static void main(String[] args) {  
    //Creating map of Books  
    Map<Integer, Book> map=new HashMap<Integer, Book>();  
    //Creating Books  
    Book b1=new Book(101, "Let us C", "Yashwant Kanetkar", "BPB", 8);  
    Book b2=new Book(102, "Data Communications & Networking", "Forouzan", "Mc Graw Hill", 4);  
    Book b3=new Book(103, "Operating System", "Galvin", "Wiley", 6);  
    //Adding Books to map 
    map.put(1, b1);
    map.put(2, b2);
    map.put(3, b3);
    
    //Traversing map
    for(Map.Entry<Integer, Book> entry:map.entrySet()){  
    	int key=entry.getKey();
    	Book b=entry.getValue();
        System.out.println(key+" Details:");
        System.out.println(b.id+" "+b.name+" "+b.author+" "+b.publisher+" "+b.quantity); 
    }  
}  
}

输出：

1 Details:
101 Let us C Yashwant Kanetkar BPB 8
2 Details:
102 Data Communications & Networking Forouzan Mc Graw Hill 4
3 Details:
103 Operating System Galvin Wiley 6