Do we need the method getClass() from java.lang.Object?

Question

I know getClass() method in java.lang.Object. But In my application, I know all of my classes. So, What is the actual purpose of using this method?

Answer 1

But In my application, I know all of my classes. So, What is the actual purpose of using this method?

So if you write this, do you still know what the real class of things[0] is?

Object[] things = new Object[]{
        "Hi mum", new Integer(42), /* and a whole bunch more */};
shuffle(things);

In theory, you could work out the class of things[0] using a long series on instanceof expressions, but that is painful. This solves the problem.

Class cls = things[0].getClass();

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Now, I'll grant you that you don't often need to do this sort of thing. But when you do, the functionality of Object.getClass() is pretty much indispensable.

And as other answers point out, the getClass() method is very important for applications that make use of reflection to get around the limitations of static typing.

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I guess, the other question is whether the existence of the Object.getClass() method has any impact on JVM performance. The answer is No. The method is only exposing information that has to be there anyway in order to implement the instanceof operator. And this information is typically used by the garbage collector.

Answer 2

You're wading into the wonderful world of Reflection. Using reflection, you can programmatically inspect classes, set their fields, call their methods, etc. You can do some really powerful stuff with it, and most frameworks that you use on a daily basis (e.g. Hibernate, Spring, Guice, etc.) depend very heavily on it to make things happen.

Answer 3

getClass() is a public method on Object, so it's inherited for all instances of all classes. If you are dealing with a super type of some sort (a super class or interface etc) and you want to know the implementation class, getClass() will tell you the class of any object. You can then inspect the class to find out more information.

A simple example:

public static void doSomething(Object obj) {
    if (obj.getClass().isArray()) {
        System.out.println("It's an array");
    } else {
        System.out.println("It's not an array");
    }
}

btw, getClass() itself isn't part of reflection - it's a straightforward method that returns a straightforward (class) object. However, the class object returned from getClass() can be used with reflection (see the java.lang.reflect package) to find out lots of interesting things about the class.

Answer 4

You can use Class objects for reflection, or you can use them for other things, such as keys into a map for keeping data about things at a class level.

For instance, you may have a class with static state & methods that do things like instantiate objects and gather statistics. In fact, you may have several such classes that all do the same thing and all seem to operate in the same way, and thus have possibilities for convergence. Since the class is static, you've naturally got a mapping from a class to some state and operations on that state.

Thus, you could extract the common code from your static classes and put them into a class that provides all the methods, with the addition of a Class argument to specify the state that should be used during following operations.

But In my application, I know all of my classes. So, What is the actual purpose of using this method?

Now that you've got a system that operates on Class objects, you can give it objects that you don't own, such as 3rd-party libraries that you decide to use in your application, and you can do it at runtime!

Answer 5

The JavaDoc states:

The returned Class object is the object that is locked by static synchronized methods of the represented class.

It means, if your classes has a static synchronized method, the JVM uses the getClass() to put a lock on the method.

Answer 6

Sometimes we need the Class instance for an object. Like we program against interfaces but need to now the actual type of an implementation:

List<?> someListImplementation = geListMagically();
System.out.println(someListImplementation.getClass().getName());

Here's trivial example why we need getClass() on java.lang.Object:

Object obj = "A String";
System.out.println(obj.getClass().getName()); // calls Object#getClass()
                                              // and prints java.lang.String :)

Answer 7

You may use to :

• get the constructors of the class :

 Constructor[] constructors = this.getClass().getConstructors()

• get the fields of the class (reflection):

   Field[] fields = this.getClass().getFields();

• use it instead of instanceof in equals method to perserve the symetry contract of the equal method (as explained here )

if (obj == null)
  return false;
if (getClass() != obj.getClass())
  return false;