Java – how can I say “these two generic types are the same” in the map?

I have a method toString (object), which delegates the transformation to the handler The handler is defined as follows:

public interface IToStringService<T> {
    public String toString( T value );
}

The code is as follows:

// (1) How can I say that these two wildcards must in fact be the same type?
private Map<Class<?>,IToStringService<?>> specialHandlers = Maps.newHashMap();

// Generic method,must accept Object (any type really)
@Override
public String toString( Object value ) {

    if( null == value ) {
        return "null";
    }

    Class<?> type = value.getClass();
    if( type.isArray() ) {
        return arrayToString( value );
    }

    // (2) How can I get rid of this SuppressWarnings?
    @SuppressWarnings( "unchecked" )
    IToStringService<Object> handler = (IToStringService<Object>) specialHandlers.get( type );

    if( null != handler ) {
        return handler.toString( value );
    }

    return value.toString();
}

public <T> void addSpecialHandler( Class<T> type,IToStringService<T> handler ) {
    specialHandlers.put( type,handler );
}

An implementation looks like this:

@SuppressWarnings( "rawtypes" ) // Can't add generics to "Class" or I get compile errors when calling DefaultToStringService.addSpecialHandler() :-(
public class ClassToStringService implements IToStringService<Class> {

    @Override
    public String toString( Class value ) {
        return value == null ? "null" : value.getName();
    }
}

I have a few questions:

>How can I say that the handler in the specialhandlers map must match the type used as the key? > How can I use the same information in methods to avoid casts and @ suppresswarnings? > When I change classtostringservice to implement itostringservice < class > When calling addspecialhandler, a compilation error (class. Class, new classtostringservice()) will be encountered; How can I solve this problem?

Solution

You can't do this in Java I'll explain why? Extensions is called existential type in type theory, but Java has a limited form? Extends x means that y exists, so that y extends X. we can rewrite map < class , IToStringService<？>> For example:

Map<Class<(exists T1 extends Object)>,IToStringService<(exist T2 extends Object)>>

But we want to:

Map<(exists T extends Object Class<T>,IToStringService<T>>

We can't express this in Java because we can't explicitly specify the existence quantifier in Java However, you can do this in scala