这个java泛型方法语法有什么问题

Question

I've the following classes

KeyValue.java

package test;

public class KeyValue<T> {
    private String key;

    private T value;

    public String getKey() {
        return key;
    }

    public void setKey(String key) {
        this.key = key;
    }

    public T getValue() {
        return value;
    }

    public void setValue(T value) {
        this.value = value;
    }

}

Reader.java

package test;

public interface Reader<T> {
    <S extends T> S read(Class<S> clazz);
}

Test.java

package test;

import java.util.List;

public class Test {

    public static void main(String[] args) {
        List<KeyValue<Object>> list = find(KeyValue.class, new Reader<KeyValue<Object>>() {

            @Override
            public <S extends KeyValue<Object>> S read(Class<S> clazz) {
                return null;
            }
        });
    }

    public static <T> List<T> find(Class<T> targetClass, Reader<T> reader) {
        return null;
    }

}

Here the method call find(......) is failing at compile time with error message

The method find(Class, Reader) in the type Test is not applicable for the arguments (Class, new Reader>(){}).

This method has to return object of type List<KeyValue<Object>> .

What is wrong with this design and how to fix this.

Thank you.

Answer 1

find defines T and T (in first and second arg) to be of same type - your call to find uses the type Object in the first arg and the Type KeyValue<Object> in the second arg.

Either use two different type identifiers (eg T and X, ie public static <T, X extends T> List<T> find(Class<T> targetClass, List<X> reader) ) in your find declaration or repair your call to find .

Answer 2

you want to get a list of your class KeyValue from your method find but u defined it as List note that it is a list of T not KeyValue change ur method declaration to be as follows

private static <T> List<KeyValue<T> > find(Class<KeyValue> aClass, Reader<KeyValue<T> > reader) {
    throw new UnsupportedOperationException("Not yet implemented");
}

i think this is what u want

Answer 3

Try to declare Test as

public class Test<T> { .