HashMap with String and Arraylist - Error Message

You have an HashMap that has Strings for keys and ArrayLists for values:

HashMap<String,ArrayList> map = new HashMap<String,ArrayList>();

You then try and see if it contains a value which is a String

System.out.println(map.containsValue("value2"));

That's very suspicious. Because it can't ever have that.

You can try this example,

import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashMap;
import java.util.List;

public class Main {
    public static void main(String[] a) {
        HashMap<String, List<String>> map = new HashMap<String, List<String>>();
        map.put("key1", new ArrayList());
        map.get("key1").add("value2");

        // System.out.println(map.containsKey("key1"));
        System.out.println(map.containsValue(Arrays.asList(new String[]{"value2"})));
    }
}

public class Main {

    public static void main(String[] a) {
        HashMap<String, List<String>> map = new HashMap<String, List<String>>();
        map.put("key1", new ArrayList());
        map.get("key1").add("value2");

        for (List<String> value : map.values()) {
            System.out.println(value.contains("value2"));
        }

    }
}

The message is pretty clear. The values of your map are of type ArrayList. You pass a String to containsValue() instead of an ArrayList. So you get an error.

BTW, a good habit to get into is to define the types for variables in the least restrictive way possible, say declare map as type Map instead of HashMap

Map<String,List> map = new HashMap<String,ArrayList>();

That way you can substitute different implementations as needed with minimal ripple effect. In this case, by declaring map as just a Map, you are free to switch to TreeMap in the future.

As you are a self-professed "not strong" programmer, I think it would be helpful to point out a couple of other things.

1. Even in the best of circumstances, this is going to involve testing (on average) half of the values in the hash table. If the value is not there, then it will test all of them. If there are many entries in the map, this will be expensive.

2. Bear in mind that equality of Lists is defined to be based on pair-wise testing the list elements using the Object.equals(Object).

3. If you are actually trying to find "value2" in one of the lists, then this approach won't work. You need a nested loop; e.g. @kingdavies solution. But the point is that that will be even more expensive.

4. There is a hint of "code smell" about this code snippet. I can't be sure without looking at the real application, but a mapping from a String to a list of Objects smells of "object denial"; i.e. trying to use generic collection types to avoid creating a custom class.