Java: how do I get a class literal from a generic type?

Question

Typically, I\'ve seen people use the class literal like this:

Class cls = Foo.class;

But what if the type is generic, e.g. List?

Answer 1

Well as we all know that it gets erased. But it can be known under some circumstances where the type is explicitly mentioned in the class hierarchy:

import java.lang.reflect.*;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.LinkedHashMap;
import java.util.Map;
import java.util.stream.Collectors;

public abstract class CaptureType {
    /**
     * {@link java.lang.reflect.Type} object of the corresponding generic type. This method is useful to obtain every kind of information (including annotations) of the generic type.
     *
     * @return Type object. null if type could not be obtained (This happens in case of generic type whose information cant be obtained using Reflection). Please refer documentation of {@link com.types.CaptureType}
     */
    public Type getTypeParam() {
        Class bottom = getClass();
        Map, Type> reifyMap = new LinkedHashMap<>();

        for (; ; ) {
            Type genericSuper = bottom.getGenericSuperclass();
            if (!(genericSuper instanceof Class)) {
                ParameterizedType generic = (ParameterizedType) genericSuper;
                Class actualClaz = (Class) generic.getRawType();
                TypeVariable>[] typeParameters = actualClaz.getTypeParameters();
                Type[] reified = generic.getActualTypeArguments();
                assert (typeParameters.length != 0);
                for (int i = 0; i < typeParameters.length; i++) {
                    reifyMap.put(typeParameters[i], reified[i]);
                }
            }

            if (bottom.getSuperclass().equals(CaptureType.class)) {
                bottom = bottom.getSuperclass();
                break;
            }
            bottom = bottom.getSuperclass();
        }

        TypeVariable var = bottom.getTypeParameters()[0];
        while (true) {
            Type type = reifyMap.get(var);
            if (type instanceof TypeVariable) {
                var = (TypeVariable) type;
            } else {
                return type;
            }
        }
    }

    /**
     * Returns the raw type of the generic type.
     * 
For example in case of {@code CaptureType}, it would return {@code Class}
     * For more comprehensive examples, go through javadocs of {@link com.types.CaptureType}
     *
     * @return Class object
     * @throws java.lang.RuntimeException If the type information cant be obtained. Refer documentation of {@link com.types.CaptureType}
     * @see com.types.CaptureType
     */
    public Class getRawType() {
        Type typeParam = getTypeParam();
        if (typeParam != null)
            return getClass(typeParam);
        else throw new RuntimeException("Could not obtain type information");
    }


    /**
     * Gets the {@link java.lang.Class} object of the argument type.
     * 
If the type is an {@link java.lang.reflect.ParameterizedType}, then it returns its {@link java.lang.reflect.ParameterizedType#getRawType()}
     *
     * @param type The type
     * @param   type of class object expected
     * @return The Class object of the type
     * @throws java.lang.RuntimeException If the type is a {@link java.lang.reflect.TypeVariable}. In such cases, it is impossible to obtain the Class object
     */
    public static  Class getClass(Type type) {
        if (type instanceof GenericArrayType) {
            Type componentType = ((GenericArrayType) type).getGenericComponentType();
            Class componentClass = getClass(componentType);
            if (componentClass != null) {
                return (Class) Array.newInstance(componentClass, 0).getClass();
            } else throw new UnsupportedOperationException("Unknown class: " + type.getClass());
        } else if (type instanceof Class) {
            Class claz = (Class) type;
            return claz;
        } else if (type instanceof ParameterizedType) {
            return getClass(((ParameterizedType) type).getRawType());
        } else if (type instanceof TypeVariable) {
            throw new RuntimeException("The type signature is erased. The type class cant be known by using reflection");
        } else throw new UnsupportedOperationException("Unknown class: " + type.getClass());
    }

    /**
     * This method is the preferred method of usage in case of complex generic types.
     * 
It returns {@link com.types.TypeADT} object which contains nested information of the type parameters
     *
     * @return TypeADT object
     * @throws java.lang.RuntimeException If the type information cant be obtained. Refer documentation of {@link com.types.CaptureType}
     */
    public TypeADT getParamADT() {
        return recursiveADT(getTypeParam());
    }

    private TypeADT recursiveADT(Type type) {
        if (type instanceof Class) {
            return new TypeADT((Class) type, null);
        } else if (type instanceof ParameterizedType) {
            ArrayList generic = new ArrayList<>();
            ParameterizedType type1 = (ParameterizedType) type;
            return new TypeADT((Class) type1.getRawType(),
                    Arrays.stream(type1.getActualTypeArguments()).map(x -> recursiveADT(x)).collect(Collectors.toList()));
        } else throw new UnsupportedOperationException();
    }

}

public class TypeADT {
    private final Class reify;
    private final List parametrized;

    TypeADT(Class reify, List parametrized) {
        this.reify = reify;
        this.parametrized = parametrized;
    }

    public Class getRawType() {
        return reify;
    }

    public List getParameters() {
        return parametrized;
    }
}

And now you can do things like:

static void test1() {
        CaptureType t1 = new CaptureType() {
        };
        equals(t1.getRawType(), String.class);
    }

    static void test2() {
        CaptureType> t1 = new CaptureType>() {
        };
        equals(t1.getRawType(), List.class);
        equals(t1.getParamADT().getParameters().get(0).getRawType(), String.class);
    }


    private static void test3() {
            CaptureType>> t1 = new CaptureType>>() {
            };
            equals(t1.getParamADT().getRawType(), List.class);
        equals(t1.getParamADT().getParameters().get(0).getRawType(), List.class);
    }

    static class Test4 extends CaptureType> {
    }

    static void test4() {
        Test4 test4 = new Test4();
        equals(test4.getParamADT().getRawType(), List.class);
    }

    static class PreTest5 extends CaptureType {
    }

    static class Test5 extends PreTest5 {
    }

    static void test5() {
        Test5 test5 = new Test5();
        equals(test5.getTypeParam(), Integer.class);
    }

    static class PreTest6 extends CaptureType {
    }

    static class Test6 extends PreTest6 {
    }

    static void test6() {
        Test6 test6 = new Test6();
        equals(test6.getTypeParam(), Integer.class);
    }



    class X extends CaptureType {
    }

    class Y extends X {
    }

    class Z extends Y>>>> {
    }

    void test7(){
        Z z = new Z<>();
        TypeADT param = z.getParamADT();
        equals(param.getRawType(), Map.class);
        List parameters = param.getParameters();
        equals(parameters.get(0).getRawType(), Integer.class);
        equals(parameters.get(1).getRawType(), List.class);
        equals(parameters.get(1).getParameters().get(0).getRawType(), List.class);
        equals(parameters.get(1).getParameters().get(0).getParameters().get(0).getRawType(), List.class);
        equals(parameters.get(1).getParameters().get(0).getParameters().get(0).getParameters().get(0).getRawType(), Integer.class);
    }




    static void test8() throws IllegalAccessException, InstantiationException {
        CaptureType type = new CaptureType() {
        };
        equals(type.getRawType(), int[].class);
    }

    static void test9(){
        CaptureType type = new CaptureType() {
        };
        equals(type.getRawType(), String[].class);
    }

    static class SomeClass extends CaptureType{}
    static void test10(){
        SomeClass claz = new SomeClass<>();
        try{
            claz.getRawType();
            throw new RuntimeException("Shouldnt come here");
        }catch (RuntimeException ex){

        }
    }

    static void equals(Object a, Object b) {
        if (!a.equals(b)) {
            throw new RuntimeException("Test failed. " + a + " != " + b);
        }
    }

More info here. But again, it is almost impossible to retrieve for:

class SomeClass extends CaptureType{}
SomeClass claz = new SomeClass<>();

where it gets erased.