HashMap.containsKey not working for some reason

Question

I wonder why the logic if (!map.containsKey("Africa")) neither go to if block (XX) nor else block (YY)

public class HashMapWithListTest {

public static void main (String args[]) {
    HashMap<String,List<String>> map=new HashMap<String,List<String>>();

    //to put data firs time
    String country="USA";
    //create list for cities
    List<String> cityList=new ArrayList<String>();
    //then fill list
    cityList.add("New York");
    cityList.add("Los Angeles ");
    cityList.add("Chicago");

    //lets put this data to map
    map.put(country, cityList);

    //same thind with other data
    country="Pakistan";
    cityList=new ArrayList<String>();
    cityList.add("Lahore");
    cityList.add("Karachi");
    map.put(country, cityList);

    country="Malaysia";
    cityList=new ArrayList<String>();
    cityList.add("Kuala Lumpur");
    cityList.add("Johor Baru");
    map.put(country, cityList);

    country="Indonesia";
    cityList=new ArrayList<String>();
    cityList.add("Jakarta");
    cityList.add("Bandung");
    map.put(country, cityList);

    //now lets check what is in map
    System.out.println(map);

    //to add city in USA
    //you need to get List of cities and add new one 
    map.get("USA").add("Washington");

    //to get all values from USA
    System.out.println("city in USA:");
    List<String> tmp=map.get("USA");
    for (String city:tmp)
        System.out.println(city);

    System.out.println("x------------------x");
    map.remove("USA");
    System.out.println(map);
    System.out.println("x------------------x");
    map.get("Indonesia").add("Lembang");
    System.out.println(map.get("Indonesia"));
    System.out.println("x------------------x");
    country="Indonesia";
    cityList=new ArrayList<String>();
    cityList.add("Semarang");
    cityList.add("Bali");
    map.putIfAbsent(country, cityList);

    if (!map.containsKey("Africa")) {
        System.out.println(map.get("XX"));
    } else {
        System.out.println(map.get("YY"));
    }
}

}

What makes you think it doesn't execute the if statement? It should print "null" regardless of the condition, since neither of they keys XX and YY appear in your map. — Eran, Mar 23 '16 at 10:08
I suggest you step through the code in your debugger to get a better understanding of what it is doing. — Peter Lawrey, Mar 23 '16 at 10:17

Cootri · Accepted Answer · 2016-03-23T10:12:35.827

0

map.containsKey("Africa") returns false in your code (because there is no such key in your HashMap) and then map.get("XX") produces null value (there is no value associated with "XX" key in your HashMap), which is printed by System.out.println.

Code is correct, possibly there is a semantics issue

edited Mar 23 '16 at 10:12

answered Mar 23 '16 at 10:09

Cootri

3,806
20
30

1

no problem, mate. You're welcome :) I recommend you to use debugger - it is very easy to find any mistakes with its help – Cootri Mar 23 '16 at 10:12

score 0 · Answer 2 · answered Mar 23 '16 at 10:14

0

Best way to check existence of a key:

Myobject value = map.get(key);
if (value != null) {
...
} else {
if (map.containsKey(key)) {
   // key exists but it's null
} else {
   // No such key
}
}

Because this is the way HashMap.containsKey works:

@Override
public boolean containsKey(Object key) {
Entry<K, V> m = getEntry(key);
return m != null;
}

answered Mar 23 '16 at 10:14

Stef_ro89

81
1
10

So the best way is to duplicate the behavior of `containsKey`? – Tom Mar 23 '16 at 10:15
I just pointed how the containsKey works...The first code is just for better understanding and debugging of the issue. – Stef_ro89 Mar 23 '16 at 10:20

score 0 · Answer 3 · answered Mar 23 '16 at 10:18

0

it is going in if loop..if you want to check, repace System.out.println(map.get("XX")); with System.out.println("XX");

answered Mar 23 '16 at 10:18

NikhilP

1,508
14
23

HashMap.containsKey not working for some reason

3 Answers3