Check if a Class Object is subclass of another Class Object in Java

Question

I\'m playing around with Java\'s reflection API and trying to handle some fields. Now I\'m stuck with identifying the type of my fields. Strings are easy, just do myFi

Answer 1

//Inheritance

    class A {
      int i = 10;
      public String getVal() {
        return "I'm 'A'";
      }
    }

    class B extends A {
      int j = 20;
      public String getVal() {
        return "I'm 'B'";
      }
    }

    class C extends B {
        int k = 30;
        public String getVal() {
          return "I'm 'C'";
        }
    }

//Methods

    public static boolean isInheritedClass(Object parent, Object child) {
      if (parent == null || child == null) {
        return false;
      } else {
        return isInheritedClass(parent.getClass(), child.getClass());
      }
    }

    public static boolean isInheritedClass(Class<?> parent, Class<?> child) {
      if (parent == null || child == null) {
        return false;
      } else {
        if (parent.isAssignableFrom(child)) {
          // is child or same class
          return parent.isAssignableFrom(child.getSuperclass());
        } else {
          return false;
        }
      }
    }

// Test the code

    System.out.println("isInheritedClass(new A(), new B()):" + isInheritedClass(new A(), new B()));
    System.out.println("isInheritedClass(new A(), new C()):" + isInheritedClass(new A(), new C()));
    System.out.println("isInheritedClass(new A(), new A()):" + isInheritedClass(new A(), new A()));
    System.out.println("isInheritedClass(new B(), new A()):" + isInheritedClass(new B(), new A()));


    System.out.println("isInheritedClass(A.class, B.class):" + isInheritedClass(A.class, B.class));
    System.out.println("isInheritedClass(A.class, C.class):" + isInheritedClass(A.class, C.class));
    System.out.println("isInheritedClass(A.class, A.class):" + isInheritedClass(A.class, A.class));
    System.out.println("isInheritedClass(B.class, A.class):" + isInheritedClass(B.class, A.class));

//Result

    isInheritedClass(new A(), new B()):true
    isInheritedClass(new A(), new C()):true
    isInheritedClass(new A(), new A()):false
    isInheritedClass(new B(), new A()):false
    isInheritedClass(A.class, B.class):true
    isInheritedClass(A.class, C.class):true
    isInheritedClass(A.class, A.class):false
    isInheritedClass(B.class, A.class):false

Answer 2

In addition to @To-kra's answer. If someone doesn't like recurrence:

    public static boolean isSubClassOf(Class<?> clazz, Class<?> superClass) {
        if(Object.class.equals(superClass)) {
            return true;
        }

        for(; !Object.class.equals(clazz); clazz = clazz.getSuperclass()) {
            if(clazz.getSuperclass().equals(superClass)) {
                return true;
            }
        }

        return false;
    }

NOTE: no null checking for clarity.

Answer 3

instanceof works on instances, i.e. on Objects. Sometimes you want to work directly with classes. In this case you can use the asSubClass method of the Class class. Some examples:

1)

    Class o=Object.class;
    Class c=Class.forName("javax.swing.JFrame").asSubclass(o);

this will go through smoothly because JFrame is subclass of Object. c will contain a Class object representing the JFrame class.

2)

    Class o=JButton.class;
    Class c=Class.forName("javax.swing.JFrame").asSubclass(o);

this will launch a java.lang.ClassCastException because JFrame is NOT subclass of JButton. c will not be initialized.

3)

    Class o=Serializable.class;
    Class c=Class.forName("javax.swing.JFrame").asSubclass(o);

this will go through smoothly because JFrame implements the java.io.Serializable interface. c will contain a Class object representing the JFrame class.

Of course the needed imports have to be included.

Answer 4

Another option is instanceof:

Object o =...
if (o instanceof Number) {
  double d = ((Number)o).doubleValue(); //this cast is safe
}

Answer 5

You want this method:

boolean isList = List.class.isAssignableFrom(myClass);

where in general, List (above) should be replaced with superclass and myClass should be replaced with subclass

From the JavaDoc:

Determines if the class or interface represented by this Class object is either the same as, or is a superclass or superinterface of, the class or interface represented by the specified Class parameter. It returns true if so; otherwise it returns false. If this Class object represents a primitive type, this method returns true if the specified Class parameter is exactly this Class object; otherwise it returns false.

Reference:

• Class.isAssignableFrom(Class)

---

Related:

a) Check if an Object is an instance of a Class or Interface (including subclasses) you know at compile time:

boolean isInstance = someObject instanceof SomeTypeOrInterface;

Example:

assertTrue(Arrays.asList("a", "b", "c") instanceof List<?>);

b) Check if an Object is an instance of a Class or Interface (including subclasses) you only know at runtime:

Class<?> typeOrInterface = // acquire class somehow
boolean isInstance = typeOrInterface.isInstance(someObject);

Example:

public boolean checkForType(Object candidate, Class<?> type){
    return type.isInstance(candidate);
}

Answer 6

A recursive method to check if a Class<?> is a sub class of another Class<?>...

Improved version of @To Kra's answer:

protected boolean isSubclassOf(Class<?> clazz, Class<?> superClass) {
    if (superClass.equals(Object.class)) {
        // Every class is an Object.
        return true;
    }
    if (clazz.equals(superClass)) {
        return true;
    } else {
        clazz = clazz.getSuperclass();
        // every class is Object, but superClass is below Object
        if (clazz.equals(Object.class)) {
            // we've reached the top of the hierarchy, but superClass couldn't be found.
            return false;
        }
        // try the next level up the hierarchy.
        return isSubclassOf(clazz, superClass);
    }
}