Comparing Class Types in Java

Question

I want to compare the class type in Java.

I thought I could do this:

class MyObject_1 {}
class MyObject_2 extends MyObject_1 {}

public boolean funct         


        

Answer 1

If you had two Strings and compared them using == by calling the getClass() method on them, it would return true. What you get is a reference on the same object.

This is because they are both references on the same class object. This is true for all classes in a java application. Java only loads the class once, so you have only one instance of a given class at a given time.

String hello  = "Hello";
String world = "world";

if (hello.getClass() == world.getClass()) {
    System.out.println("true");
} // prints true

Answer 2

It prints true on my machine. And it should, otherwise nothing in Java would work as expected. (This is explained in the JLS: 4.3.4 When Reference Types Are the Same)

Do you have multiple classloaders in place?

---

Ah, and in response to this comment:

I realise I have a typo in my question. I should be like this:

MyImplementedObject obj = new MyImplementedObject ();
if(obj.getClass() == MyObjectInterface.class) System.out.println("true");

MyImplementedObject implements MyObjectInterface So in other words, I am comparing it with its implemented objects.

OK, if you want to check that you can do either:

if(MyObjectInterface.class.isAssignableFrom(obj.getClass()))

or the much more concise

if(obj instanceof MyobjectInterface)

Answer 3

Hmmm... Keep in mind that Class may or may not implement equals() -- that is not required by the spec. For instance, HP Fortify will flag myClass.equals(myOtherClass).

Answer 4

Check Class.java source code for equals()

public boolean equals(Object obj) {
  return this == obj;
}

Answer 5

Try this:

MyObject obj = new MyObject();
if(obj instanceof MyObject){System.out.println("true");} //true

Because of inheritance this is valid for interfaces, too:

class Animal {}
class Dog extends Animal {}    

Dog obj = new Dog();
Animal animal = new Dog();
if(obj instanceof Animal){System.out.println("true");} //true
if(animal instanceof Animal){System.out.println("true");} //true
if(animal instanceof Dog){System.out.println("true");} //true

For further reading on instanceof: http://mindprod.com/jgloss/instanceof.html

Answer 6

Comparing an object with a class using instanceOf or ... is already answered.

If you have two objects and you want to compare their types with each other, you can use:

if (obj1.getClass() == obj2.getClass()) {
   // Both have the same type
}