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Java Reflection - Class.asSubclass() Examples

Java Reflection Java Java API 

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Class:

java.lang.Class

java.lang.Objectjava.lang.Objectjava.lang.Classjava.lang.Classjava.io.SerializableSerializablejava.lang.reflect.GenericDeclarationGenericDeclarationjava.lang.reflect.TypeTypejava.lang.reflect.AnnotatedElementAnnotatedElementLogicBig

Method:

public <U> Class<? extends U> asSubclass(Class<U> clazz)

This method casts this Class to return a subclass of the class represented by the specified clazz object. If cast is not valid, it throws a ClassCastException .

This method is useful when a client needs to "narrow" the type of a Class object to pass it to an API that restricts the Class objects that it is willing to accept.

Type Parameters:
U - the type to cast this class object to
Parameters:
clazz - the class of the type to cast this class object to
Returns:
this Class object, cast to represent a subclass of the specified class object.


Examples


package com.logicbig.example.clazz;

import java.util.HashSet;
import java.util.LinkedHashSet;

public class AsSubclassExample {

    public static void main(String... args) throws IllegalAccessException, InstantiationException {
        Class<LinkedHashSet> lhs = LinkedHashSet.class;
        Class<? extends HashSet> hs = lhs.asSubclass(HashSet.class);
        System.out.println(hs);
        HashSet hashSet = hs.newInstance();
        System.out.println(hashSet.getClass());
    }
}

Output

class java.util.LinkedHashSet
class java.util.LinkedHashSet




The method safeAsSubClass in following example is ClassCastException safe: 

package com.logicbig.example.clazz;

public class AsSubclassExample2 {

    public static void main(String... args) {
        Class<? extends Shape> r = safeAsSubClass(Circle.class, Shape.class);
        System.out.println("Circle as Shape: " + r);

        Class<? extends Shape> r2 = safeAsSubClass(Shape.class, Circle.class);
        System.out.println("Shape as Circle: " + r2);

        Class<?> r3 = safeAsSubClass(Circle.class, Object.class);
        System.out.println("Circle as Object: " + r3);

        Class<?> r4 = safeAsSubClass(Object.class, Circle.class);
        System.out.println("Object as Circle:" + r4);
    }

    public static <T> Class<? extends T> safeAsSubClass(Class<?> from, Class<T> as) {
        if (as.isAssignableFrom(from)) {
            return from.asSubclass(as);
        }
        return null;
    }

    public static class Shape {
    }

    public static class Circle extends Shape {
    }
}

Output

Circle as Shape: class com.logicbig.example.clazz.AsSubclassExample2$Circle
Shape as Circle: null
Circle as Object: class com.logicbig.example.clazz.AsSubclassExample2$Circle
Object as Circle:null




package com.logicbig.example.clazz;

public class AsSubclassExample3 {

    public static void main(String... args) {
        Class<Circle> cc = Circle.class;
        //now we need to call method workWithShapeClassUtil
        Class<? extends Shape> cs = cc.asSubclass(Shape.class);
        workWithShapeClassUtil(cs);
    }

    public static void workWithShapeClassUtil(Class<? extends Shape> cs) {
        System.out.println("working with shape instance: " + cs);
    }

    public static class Shape {
    }

    public static class Circle extends Shape {
    }
}

Output

working with shape instance: class com.logicbig.example.clazz.AsSubclassExample3$Circle

See Also