How HashMap works in Java


In this example, we will learn two ways to traverse a HashMap object, getting the key and getting the mapping relationship, as shown below.

Source Code

1)

package com.beginner.examples;

import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.Set;

public class CreateHashMapAndIterate {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		
		HashMap hMap=
					new HashMap();
		//add key,value
		hMap.put("Name", "Tom");
		
		hMap.put("Age", "20");
		
		hMap.put("Sex", "man");
		
		hMap.put("Hobby", "play basketball");
		
		//The first traversal method,The first traversal method stores 
					//the keys of this collection into the Set Set and traverses the Set Set
		
		Set keys=hMap.keySet();
		
		Iterator iterator=keys.iterator();
		while(iterator.hasNext())
		{
			String key=iterator.next();
			String value=hMap.get(key);
			System.out.println(key+"------"+value);
			
		}
		//Second, get the mapping class for this collection of hashmaps
		System.out.println("----------------------");
		 
		Set<Map.Entry> entry=hMap.entrySet();
		
		Iterator<Map.Entry> it1=entry.iterator();
		
		while(it1.hasNext())
		{
			Map.Entry e=it1.next();
			
			
			
			String key=e.getKey();
			String value=e.getValue();
			
			System.out.println(key+":::::::::"+value);
			
		}
		
		

	}

}

Output:

Name------Tom
Age------20
Hobby------play basketball
Sex------man
----------------------
Name:::::::::Tom
Age:::::::::20
Hobby:::::::::play basketball
Sex:::::::::man

2)


package com.beginner.examples;

import java.util.HashMap;

public class DeleteAndChange {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		
		HashMap asiccMap=new HashMap();
		//add 
		asiccMap.put('a', 90);
		asiccMap.put('b', 98);
		asiccMap.put('c',99);
		asiccMap.put('d', 100);
		
		//alter value  returns the replaced value
		
		System.out.println(asiccMap.put('a', 97)+"   alter after: "+asiccMap.get('a'));
		
		//Delect       Returns the deleted value
		
		System.out.println(asiccMap.remove('d')+" remove after:  "+asiccMap.get('d'));
		
		
	}

}

Output:

90   alter after: 97
100 remove after:  null

References

Imported packages in Java documentation:

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