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

Question

In Java what pros/cons exist surrounding the choice to use a.getClass() or A.class? Either can be used wherever a Class<?> is expected, but I imagine that there would be performance or other subtle benefits to using both in different circumstances (just like there are with Class.forName() and ClassLoader.loadClass().

Answer 1

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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Answer 2

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.