Specific class instead of generics in Java

Question

How can I extend HashMap with a specific class.

public class TestMap<K, V> extends HashMap<K, V>

I want V to be an Integer. Writing Integer instead of V overrides Integer class and causes bugs(Integer i = 1 does not work) when integer is used. How can I fix this?n

Integer i = 1 does work, can you show whats not working for you? — Sleiman Jneidi
– Sleiman Jneidi, Commented Nov 15, 2014 at 20:12
@SleimanJneidi Presumably they were trying to do TestMap<K, Integer> ... which hides java.lang.Integer. — Radiodef
– Radiodef, Commented Nov 15, 2014 at 20:13

Radiodef · Accepted Answer · 2014-11-15 21:02:50Z

4

Parameterize the extended class and only declare a type for the key:

class IntegerMap<K> extends HashMap<K, Integer> {}

Then you can do:

IntegerMap<String> integerByString = new IntegerMap<String>();
integerByString.put("0", 0);
integerByString.put("1", 1);

As a side note, if you are actually extending a JDK collection class, it is generally not considered a good style.

Normally you would write a class that controls the Map from the outside:

class MapController<K> {
    Map<K, Integer> theMap;
}

Or perhaps a method that prepares it in some way:

static <K> Map<K, Integer> prepMap() {
    Map<K, Integer> theMap = new HashMap<>();
    return theMap;
}

edited Nov 15, 2014 at 21:02

answered Nov 15, 2014 at 20:13

Radiodef

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2 Comments

Oliver Charlesworth Over a year ago

This is probably a bad idea. See e.g. ibm.com/developerworks/library/j-jtp02216

WVrock Over a year ago

It works. Thanks. Now I can get rid of those pesky (Integer) casts and what not.

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