Output from the HashMap containsKey() query not returning the correct value

Question

The program will print the incorrect key and value based on the If statement. Can someone explain why?

E.g. Key = Uncle tom + Value = 02086542222 Key = Harry + Value = 020826262

Query = Uncle tom

Returns = Key = Harry + Value = 00826262

Quote from the documentation states below:

"More formally, returns true if and only if this map contains a mapping for a key k such that (key==null ? k==null : key.equals(k))"

So I was under the impression that if(Contacts.containsKey(query)) would compare the input query against the key using key.equals(k)

import java.util.HashMap;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Scanner;

public class HRHashMap {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        Scanner scan = new Scanner(System.in);

        Map<String, Integer> Contacts = new HashMap<String, Integer>();//Specify HashMap of type String

        int numOfContacts = scan.nextInt();
        scan.nextLine();

        //Add contacts
        for (int i = 0; i < numOfContacts; i++) {
            String contactName = scan.nextLine();
            int contactNumber = scan.nextInt();
            scan.nextLine();
            Contacts.put(contactName, contactNumber);
        }

        //Iterate over the Map
        for (Entry<String, Integer> entry : Contacts.entrySet()) {
            String query = scan.nextLine();
            if (Contacts.containsKey(query)) {
                //System.out.println(Contacts.get(query));
                System.out.println(entry.getKey() + "=" + entry.getValue());
            } else {
                System.out.println("Not found");
            }
        }

    }
}

Answer 1

Your program iterates over every entry in the map, asking for some input (query) for each entry, then checks if query is a key in the map and if it is prints the currently visited entry (which is completely unrelated to query).

So the output seems "correct": The map does contain "Uncle Tom", so it proceeds to print the first entry ("Harry"). Note that "first" is a muddy concept in a HashMap, the iteration order of entries is unspecified.

I don't quite understand why you loop over all entries, but the line you commented out (which prints the entry matching query) may work better:

System.out.println(Contacts.get(query));

Answer 2

try to use the entry variable declared in your for-loop as such

Scanner sc = new Scanner(System.in);

        Map<String, Integer> Contacts = new HashMap<>();

        while (sc.hasNext()) {
        String contactName = sc.nextLine();
        int contactNumber = sc.nextInt();
    sc.nextLine();
    Contacts.put(contactName, contactNumber);
    }

 for (Entry<String, Integer> entry : Contacts.entrySet()) {
while(sc.hasNext()) {
                if (entry.containsKey(sc.nextLine())) {
                    //System.out.println(Contacts.get(query));
                    System.out.println(entry.getKey() + "=" + entry.getValue());
                } else {
                    System.out.println("Not found");
                }
}
            }

Hope that helps. Use the entry variable on the for-loop and since it's a small program, you can use a while loop on the Scanner.