Issue using Generic Maps in Java

Question

I have a class which I am trying to use to test HashMap and TreeMap, shown below;

public class TestMap<KeyType, ValueType>
{
private Map<KeyType, ValueType> internalMap;

/*
 * Entry point for the application
 */
public static void main( String [ ] args )
{
    TestMap<String, Integer> testHashMap = new TestMap<String,Integer>(new HashMap<String, Integer>());
    testHashMap.test();

    TestMap<String, Integer> testTreeMap = new TestMap<String,Integer>(new TreeMap<String, Integer>());
    testTreeMap.test();
}    

/*
 * Constructor which accepts a generic Map for testing
 */
public TestMap(Map<KeyType, ValueType> m)
{
    this.internalMap = m;
}   

public void test()
{
    try
    {
        //put some values into the Map
        this.internalMap.put("Pittsburgh Steelers", 6);

        this.printMap("Tested Map", this.internalMap);  
    }
    catch (Exception ex)
    {

    }
}

}

In the attempted call to the put() method I am receiving the below error message;

The method put(KeyType, ValueType) in the type Map is not applicable for the arguments (String, int)

I am not receiving any other warnings, and I don't understand why I am getting this? Isn't this the entire point of generics? To define generically and implement concretely?

Thanks for any assistance!

As an aside, generic type parameters are normally restricted to being single capital letter eg. public class MyClass<T> {. Normally E or T is used, and for Maps K and V are used. — Dunes
– Dunes, Commented Oct 7, 2012 at 20:56

andersschuller · Accepted Answer · 2012-10-07 20:55:12Z

4

The test() method is part of the TestMap class. In any of the TestMap methods, you can only reference the generic type, not any specific type (because this depends on the individual instance). However, you can do this:

public static void main( String [ ] args )
{
    TestMap<String, Integer> testHashMap = new TestMap<String,Integer>(new HashMap<String, Integer>());
    testHashMap.internalMap.put("Pittsburgh Steelers", 6);

    TestMap<String, Integer> testTreeMap = new TestMap<String,Integer>(new TreeMap<String, Integer>());
    testTreeMap.internalMap.put("Pittsburgh Steelers", 6);
}

answered Oct 7, 2012 at 20:55

andersschuller

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Comments

CrazyCasta · Accepted Answer · 2012-10-07 20:51:18Z

3

The problem is that your overall class is generic, but you are trying to test it with a specific set of types. Try moving the test method out of the object and using it on a TestMap<String, int>.

answered Oct 7, 2012 at 20:51

CrazyCasta

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Comments

Nathan Villaescusa · Accepted Answer · 2012-10-07 21:09:39Z

1

Another option is to remove the generics from your TestMap object. They don't currently seem to be doing anything.

public class TestMap {
    private Map<String, Integer> internalMap;

    /*
     * Entry point for the application
     */
    public static void main( String [ ] args )
    {
        TestMap testHashMap = new TestMap(new HashMap<String, Integer>());
        testHashMap.test();

        TestMap testTreeMap = new TestMap(new TreeMap<String, Integer>());
        testTreeMap.test();
    }

    /*
     * Constructor which accepts a generic Map for testing
     */
    public TestMap(Map<String, Integer> m)
    {
        this.internalMap = m;
    }   

    public void test()
    {
        try
        {
            //put some values into the Map
            this.internalMap.put("Pittsburgh Steelers", 6);

            this.printMap("Tested Map", this.internalMap);  
        }
        catch (Exception ex)
        {

        }
    }
}

edited Oct 7, 2012 at 21:09

answered Oct 7, 2012 at 20:57

Nathan Villaescusa

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2 Comments

Chris Corbin Over a year ago

That was my thought as well... but according to my professor and grader, TestMap should be "Generic", and I should be able to test both a HashMap and TreeMap by only instantiating an instance of TestMap with each, and then calling test().

Nathan Villaescusa Over a year ago

In that case your test method could create test variables depending on what class KeyType and ValueType are.

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