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Program: How to eliminate duplicate user defined objects as a key from HashMap?


Description:

Below example shows how to avoid duplicate user defined objects as a key from HashMap. You can achieve this by implementing equals and hashcode methods at the user defined objects.


Code:
package com.java2novice.hashmap;

import java.util.HashMap;
import java.util.Set;

public class MyDuplicateKeyEx {

	public static void main(String a[]){
		
		HashMap<Price, String> hm = new HashMap<Price, String>();
		hm.put(new Price("Banana", 20), "Banana");
		hm.put(new Price("Apple", 40), "Apple");
		hm.put(new Price("Orange", 30), "Orange");
		printMap(hm);
		Price key = new Price("Banana", 20);
		System.out.println("Adding duplicate key...");
		hm.put(key, "Grape");
		System.out.println("After adding dulicate key:");
		printMap(hm);
	}
	
	public static void printMap(HashMap<Price, String> map){
		
		Set<Price> keys = map.keySet();
		for(Price p:keys){
			System.out.println(p+"==>"+map.get(p));
		}
	}
}

class Price{
	
	private String item;
	private int price;
	
	public Price(String itm, int pr){
		this.item = itm;
		this.price = pr;
	}
	
	public int hashCode(){
		int hashcode = 0;
		hashcode = price*20;
		hashcode += item.hashCode();
		return hashcode;
	}
	
	public boolean equals(Object obj){
		if (obj instanceof Price) {
			Price pp = (Price) obj;
			return (pp.item.equals(this.item) && pp.price == this.price);
		} else {
			return false;
		}
	}
	
	public String getItem() {
		return item;
	}
	public void setItem(String item) {
		this.item = item;
	}
	public int getPrice() {
		return price;
	}
	public void setPrice(int price) {
		this.price = price;
	}
	
	public String toString(){
		return "item: "+item+"  price: "+price;
	}
}

Output:
item: Apple  price: 40==>Apple
item: Orange  price: 30==>Orange
item: Banana  price: 20==>Banana
Adding duplicate key...
After adding dulicate key:
item: Apple  price: 40==>Apple
item: Orange  price: 30==>Orange
item: Banana  price: 20==>Grape
<< Previous Program | Next Program >>

List Of All HashMap Sample Programs:

  1. Basic HashMap Operations.
  2. How to iterate through HashMap?
  3. How to copy Map content to another HashMap?
  4. How to search a key in HashMap?
  5. How to search a value in HashMap?
  6. How to get all keys from HashMap?
  7. How to get entry set from HashMap?
  8. How to delete all elements from HashMap?
  9. How to eliminate duplicate user defined objects as a key from HashMap?
  10. How to find user defined objects as a key from HashMap?
  11. How to delete user defined objects as a key from HashMap?
Knowledge Centre
What is abstract class or abstract method?
We cannot create instance for an abstract class. We can able to create instance for its subclass only. By specifying abstract keyword just before class, we can make a class as abstract class.

public abstract class MyAbstractClass{

}

Abstract class may or may not contains abstract methods. Abstract method is just method signature, it does not containes any implementation. Its subclass must provide implementation for abstract methods. Abstract methods are looks like as given below:

public abstract int getLength();
Famous Quotations
Insanity: doing the same thing over and over again and expecting different results.
-- Albert Einstein

About Author

I'm Nataraja Gootooru, programmer by profession and passionate about technologies. All examples given here are as simple as possible to help beginners. The source code is compiled and tested in my dev environment.

If you come across any mistakes or bugs, please email me to [email protected].

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Reference: Java™ Platform Standard Ed. 7 - API Specification | Java™ Platform Standard Ed. 8 - API Specification | Java is registered trademark of Oracle.
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