Java generics passing parameters

Question

Hope somebody can help me out of this confussion.

I made this method:

public static <T> void myMethod(Map<Class<T>, MyInterface<T>> map) {
}

Used paramter T in order to make sure that the class used as key is the same as the class used as parameter in MyInterface.

Now I want to pass a map which different classes as keys, of course, and corresponding implementations of MyInterface.

But it doesn't work, getting syntax errors because of type parameters. Here is the code, I hope is self explanatory.

    import java.util.HashMap;
    import java.util.Map;

    public class Test {

    public static void main(String[] args) {
        Map<Class<?>, MyInterface<?>> map = new HashMap<Class<?>, MyInterface<?>>();

    //      Map<Class<Object>, MyInterface<Object>> map = new HashMap<Class<Object>, MyInterface<Object>>();

        map.put(Object.class, new MyObjectImpl());

        //if I use Map<Class<Object>, MyInterface<Object>> I get a compiler error here
        //because map<String> is not map<Object> basically
        map.put(String.class, new MyStringImpl());

        //this would be possible using <?>, which is exactly what I don't want
    //      map.put(String.class, new MyIntegerImpl());

        //<?> generates anyways a compiler error
        myMethod(map);
    }

    //use T to make sure the class used as key is the same as the class of the parameter "object" in doSomething  
    public static <T> void myMethod(Map<Class<T>, MyInterface<T>> map) {

    }

    interface MyInterface<T> {
        void doSomething(T object);
    }

    static class MyObjectImpl implements MyInterface<Object> {
        @Override
        public void doSomething(Object object) {
            System.out.println("MyObjectImpl doSomething");
        }
    }

    static class MyStringImpl implements MyInterface<String> {
        @Override
        public void doSomething(String object) {
            System.out.println("MyStringImpl doSomething");
        }
    }

    static class MyIntegerImpl implements MyInterface<Integer> {
        @Override
        public void doSomething(Integer object) {
            System.out.println("MyIntegerImpl doSomething");
        }
    }
}

Answer 1

You can't do that, because there is no constraint defined in Map 's put() method between the key and the value . If you want to assure that your map is populated properly (ie create such constraint), hide the map behind some API that will check the correctness, for example:

public <T> void registerInterface(Class<T> clazz, MyInterface<T> intf) {
    map.put(clazz, intf);
}

Then, just call the registerInterface instead of manually populating the map.

Answer 2

As far as I know, you cannot declare a Map like you describe in Java. All you can do is performing type checking and/or add constraints.

Guava offers something that approaches your problem with ClassToInstanceMap . So one way to do this would be to use MapConstraints.constrainedMap (like the example below)

import java.text.ParseException;
import java.util.HashMap;
import java.util.Map;

import com.google.common.collect.MapConstraint;
import com.google.common.collect.MapConstraints;

public class Main {

    interface MyInterface<T> {
        void doSomething(T object);

        Class<T> getType();
    }

    static class MyObjectImpl implements MyInterface<Object> {
        @Override
        public void doSomething(Object object) {
            System.out.println("MyObjectImpl doSomething");
        }

        @Override
        public Class<Object> getType() {
            return Object.class;
        }
    }

    static class MyStringImpl implements MyInterface<String> {
        @Override
        public void doSomething(String object) {
            System.out.println("MyStringImpl doSomething");
        }

        @Override
        public Class<String> getType() {
            return String.class;
        }
    }

    static class MyIntegerImpl implements MyInterface<Integer> {
        @Override
        public void doSomething(Integer object) {
            System.out.println("MyIntegerImpl doSomething");
        }

        @Override
        public Class<Integer> getType() {
            return Integer.class;
        }
    }

    public static void main(String[] args) throws ParseException {

        Map<Class<?>, MyInterface<?>> map = MapConstraints.constrainedMap(new HashMap<Class<?>, Main.MyInterface<?>>(),
                new MapConstraint<Class<?>, MyInterface<?>>() {
                    @Override
                    public void checkKeyValue(Class<?> key, MyInterface<?> value) {
                        if (value == null) {
                            throw new NullPointerException("value cannot be null");
                        }
                        if (value.getType() != key) {
                            throw new IllegalArgumentException("Value is not of the correct type");
                        }
                    }
                });
        map.put(Integer.class, new MyIntegerImpl());
        map.put(String.class, new MyStringImpl());
        map.put(Object.class, new MyObjectImpl());
        map.put(Float.class, new MyIntegerImpl()); //<-- Here you will get an exception
    }
}

Answer 3

I do not think this is possible :

Class<T> only ever accepts T.class as value. Class<Object> will not accept String.class , even though Object is a superclass of String.

For this reason any map with Class<T> as key can have only one element, with T.class as key value, whatever the value of T .

The compiler will only ever accept a map with a definite value of T as parameter. You cannot write Map<Class<?>, MyInterface<?>> because each ? is assumed to be different : it does not match Map<Class<T>, MyInterface<T>> which requires T to have the same value.

That said, myMethod will only ever accept single-entry maps, which does not seem useful.

Answer 4

Change your method signature to

public static <T> void myMethod(Map<Class<? extends T>, MyInterface<? extends T>> map) {

}

now your declaration and invocation should work..

Map<Class<?>, MyInterface<?>> map = new HashMap<Class<?>, MyInterface<?>>();
    map.put(Integer.class, new MyIntegerImpl());
    map.put(String.class, new MyStringImpl());
    map.put(Object.class, new MyObjectImpl());
    myMethod(map);