What is the difference between a.getClass() and A.class in Java?

I wouldn't compare them in terms of pros/cons since they have different purposes and there's seldom a "choice" to make between the two.

• a.getClass() returns the runtime type of a. I.e., if you have A a = new B(); then a.getClass() will return the B class.

• A.class evaluates to the A class statically, and is used for other purposes often related to reflection.

In terms of performance, there may be a measurable difference, but I won't say anything about it because in the end it is JVM and/or compiler dependent.

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This post has been rewritten as an article here.

have a look at the examples below

a.getClass()!= A.class, i.e. a is not an instance of A but of an anonymous sub class of A

a.getClass() requires an instance of type A

They are actually different with regards to where you can use them. A.class works at compile time while a.getClass() requires an instance of type A and works at runtime.

There may be a performance difference as well. While A.class can be resolved by the compiler because it knows the actual type of A, a.getClass() is a virtual method call happening at runtime.

For reference, a compiler targeting bytecode typically emits the following instructions for Integer.getClass():

aload_1
invokevirtual   #3; //Method java/lang/Object.getClass:()Ljava/lang/Class;

and the following for Integer.class:

//const #3 = class  #16;    //  java/lang/Integer

ldc_w   #3; //class java/lang/Integer

The former would typically involve a virtual method dispatch and therefore presumably take longer time to execute. That is in the end JVM-dependent however.

Use a.getClass when you have an instance of class/type and you want to get exact type of it. while a.class is used when you have type available and you want to create instance of it.
Also getClass() returns runtime type of instance while .class is evaluated at compile time.
Considering performance of getClass() and.class ,.class has better performance than getClass() .
Example :

public class PerfomanceClass {

    public static void main(String[] args) {
        // TODO Auto-generated method stub

        long time=System.nanoTime();
        Class class1="String".getClass();
        class1="String".getClass();
        class1="String".getClass();
        class1="String".getClass();

        System.out.println("time (getClass()) :"+(System.nanoTime()-time)+" ns");     


        long time2=System.nanoTime();
        Class class2=String.class;
        class2=String.class;
        class2=String.class;
        class2=String.class;

        System.out.println("time (.class):"+(System.nanoTime()-time2)+" ns");
    }

}

Output :

time (getClass()) : 79410 ns
time (.class)     : 8032 ns