Does the reference variable in Java have any size?

Question

In C++ we used sizeof() operator, which function can we use in Java to check the size of an object?

My basic doubt is that whether the reference variable in java has any size or not. Consider the following example:

SampleClass obj = new SampleClass();

Here, will obj have any size? If yes, How can we check it in Java?

Answer 1

obj is a variable, not an object. The value of obj is a reference - which is likely to be 4 or 8 bytes, depending on the JVM.

The size of the object that the value refers to is also JVM-dependent. As several objects can refer to each other, it's generally tricky to talk about the size of an object in Java in any particularly useful way... what usually matters is how much more memory would be potentially available if a particular object became eligible for garbage collection, and that depends on what other objects the first object refers to - and whether there are other objects that refer to those, too.

Obviously the "raw" size of an object is somewhat important too, and at least somewhat easier to predict (to an approximation, at least), but it's still VM-specific and can't easily be requested at execution time. (You could create millions of objects, prevent them from being garbage collected, and measure memory differences, but that's the closest I know of, at least outside a debugger API.)