Generics.java

/**
 * 
 */
package org.minnal.utils.reflection;

import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
import java.lang.reflect.TypeVariable;

/**
 * Helper methods for class type parameters.
 * @see <a href="http://gafter.blogspot.com/2006/12/super-type-tokens.html">Super Type Tokens</a>
 */
public final class Generics {

    private Generics() { /* singleton */ }

    /**
     * Finds the type parameter for the given class.
     *
     * @param klass    a parameterized class
     * @return the class's type parameter
     */
    public static Class<?> getTypeParameter(Class<?> klass) {
        return getTypeParameter(klass, Object.class);
    }

    /**
     * Finds the type parameter for the given class which is assignable to the bound class.
     *
     * @param klass    a parameterized class
     * @param bound    the type bound
     * @param <T>      the type bound
     * @return the class's type parameter
     */
    public static <T> Class<T> getTypeParameter(Class<?> klass, Class<? super T> bound) {
        Type t = klass;
        while (t instanceof Class<?>) {
    		for (Type type : ((Class<?>) t).getGenericInterfaces()) {
    			if (! (type instanceof Class<?>)) {
    				t = type;
        			break;
    			}
    		}
    		if (t instanceof Class<?>) {
    			t = ((Class<?>) t).getGenericSuperclass();
    		}
        }
        
        if (t == null) {
        	t = klass;
        	
        }
        /* This is not guaranteed to work for all cases with convoluted piping
         * of type parameters: but it can at least resolve straight-forward
         * extension with single type parameter (as per [Issue-89]).
         * And when it fails to do that, will indicate with specific exception.
         */
        if (t instanceof ParameterizedType) {
            // should typically have one of type parameters (first one) that matches:
            for (Type param : ((ParameterizedType) t).getActualTypeArguments()) {
                if (param instanceof Class<?>) {
                    final Class<T> cls = determineClass(bound, param);
                    if (cls != null) { return cls; }
                }
                else if (param instanceof TypeVariable) {
                    for (Type paramBound : ((TypeVariable<?>) param).getBounds()) {
                        if (paramBound instanceof Class<?>) {
                            final Class<T> cls = determineClass(bound, paramBound);
                            if (cls != null) { return cls; }
                        }
                    }
                }
            }
        }
        throw new IllegalStateException("Cannot figure out type parameterization for " + klass.getName());
    }

    @SuppressWarnings("unchecked")
    private static <T> Class<T> determineClass(Class<? super T> bound, Type candidate) {
        if (candidate instanceof Class<?>) {
            final Class<?> cls = (Class<?>) candidate;
            if (bound.isAssignableFrom(cls)) {
                return (Class<T>) cls;
            }
        }

        return null;
    }
}