search perticular object element from user defined hashmap key in java

Question

I want to search element in user defined key in java hashmap for example

It is possible to search in this example like hm.containsKey(new Price("Banana", 20)), but in this example i want to search by specific data member like price or item wise.

public class MyObjectKeySearch {
 
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("Does key available? "+hm.containsKey(key));
    }
     
    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(){
        System.out.println("In hashcode");
        int hashcode = 0;
        hashcode = price*20;
        hashcode += item.hashCode();
        return hashcode;
    }
     
    public boolean equals(Object obj){
        System.out.println("In equals");
        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;
    }
}

I have posted my code in this example i can search by new Price(item, price), but i want to search by items or price only — aman sikarwar
– aman sikarwar, Commented Nov 25, 2015 at 14:51

user2390182 · Accepted Answer · 2015-11-25 17:13:41Z

1

Yes:

hm.containsKey(new Price("Banana", 20))

is possible and behaves the way you would expect.

And No: You cannot do any contains-check based on members of your key objects that does not involve iterating over the key set of the HashMap. containsKey(paramKey) returns true if there is a key in the appropriate "hash bucket" identified by key.hashCode() that is equal to paramKey. The value of hashCode() obscures member values. You could use a more complex data structure like a nested HashMap:

HashMap<String, HashMap<Integer, String>> hm;

Where you use item Strings as keys in the outer Map and price Integers as keys in the inner Map. Then you can query like:

(hm.containsKey("Banana") && hm.get("Banana").containsKey(20))

which will be much more efficient than iterating over a large set of Price objects. This, however, does still not allow you to query efficiently for all inner entries with a specific price (Integer) without iterating over the outer key set. You can reverse inner and outer key types accordingly though:

HashMap<Integer, HashMap<String, String>> hm;

You can, of course, create your own class that encapsulates the functionality of both:

/* Provides constant time (O(1)) Price lookup by item and price */
class PriceMap {
    private HashMap<String, Set<Price>> stringMap;
    private HashMap<Integer, Set<Price>> intMap;

    public PriceMap() {
        stringMap = new HashMap<String, Set<Price>>();
        intMap = new HashMap<Integer, Set<Price>>();
    }

    public void add(Price price) {
        String item = price.getItem()
        if (!stringMap.containsKey(item))
            stringMap.put(item, new HashSet<Price>());
        stringMap.get(item).add(Price);
        int price = price.getPrice()
        if (!intMap.containsKey(price))
            intMap.put(price, new HashSet<Price>());
        intMap.get(price).add(Price);
    }

    public Set<Price> prices(int price) {
        return intMap.containsKey(price) ? intMap.get(price) : new HashSet<Price>(); 
    }

    public boolean containsKey(int price) {
        return intMap.containsKey(price); 
    }

    public Set<Price> prices(String item) {
        return stringMap.containsKey(item) ? stringMap.get(item) : new HashSet<Price>(); 
    }

    public boolean containsKey(String item) {
        return stringMap.containsKey(item); 
    }

}

edited Nov 25, 2015 at 17:13

answered Nov 25, 2015 at 15:02

user2390182

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4 Comments

aman sikarwar Over a year ago

thanks ,can you please give me some source or links for this to explaining more in details

user2390182 Over a year ago

In this thread, there is some explanation: stackoverflow.com/questions/5056708/…, otherwise general java API documentation: docs.oracle.com/javase/7/docs/api/java/util/HashMap.html

aman sikarwar Over a year ago

i dont want to use nested hashmap, i just want to explaination of you're menstioning "NO section" in your answer

user2390182 Over a year ago

Keys in a HashMap are looked up by their hashCode() function which returns an int value that obscures the key's members. Thus, finding a key with a particular member value requires you to look at each key and observe that member. I will edit my answer to specify that...

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