[java] java: Class.isInstance vs Class.isAssignableFrom

Let clazz be some Class and obj be some Object.

Is

clazz.isAssignableFrom(obj.getClass())

always the same as

clazz.isInstance(obj)

?

If not, what are the differences?

I think the result for those two should always be the same. The difference is that you need an instance of the class to use isInstance but just the Class object to use isAssignableFrom.

Both answers are in the ballpark but neither is a complete answer.

MyClass.class.isInstance(obj) is for checking an instance. It returns true when the parameter obj is non-null and can be cast to MyClass without raising a ClassCastException. In other words, obj is an instance of MyClass or its subclasses.

MyClass.class.isAssignableFrom(Other.class) will return true if MyClass is the same as, or a superclass or superinterface of, Other. Other can be a class or an interface. It answers true if Other can be converted to a MyClass.

A little code to demonstrate:

public class NewMain
{
    public static void main(String[] args)
    {
        NewMain nm = new NewMain();
        nm.doit();
    }

    class A { }

    class B extends A { }

    public void doit()
    {
        A myA = new A();
        B myB = new B();
        A[] aArr = new A[0];
        B[] bArr = new B[0];

        System.out.println("b instanceof a: " + (myB instanceof A)); // true
        System.out.println("b isInstance a: " + A.class.isInstance(myB)); //true
        System.out.println("a isInstance b: " + B.class.isInstance(myA)); //false
        System.out.println("b isAssignableFrom a: " + A.class.isAssignableFrom(B.class)); //true
        System.out.println("a isAssignableFrom b: " + B.class.isAssignableFrom(A.class)); //false
        System.out.println("bArr isInstance A: " + A.class.isInstance(bArr)); //false
        System.out.println("bArr isInstance aArr: " + aArr.getClass().isInstance(bArr)); //true
        System.out.println("bArr isAssignableFrom aArr: " + aArr.getClass().isAssignableFrom(bArr.getClass())); //true
    }
}

For brevity, we can understand these two APIs like below:

1. X.class.isAssignableFrom(Y.class)

If X and Y are the same class, or X is Y's super class or super interface, return true, otherwise, false.

2. X.class.isInstance(y)

Say y is an instance of class Y, if X and Y are the same class, or X is Y's super class or super interface, return true, otherwise, false.