HashMap containsKey（）查询的输出未返回正确的值

Question

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

Eg 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.