what is the size of empty class in C++,java?

Short Answer for C++:

The C++ standard explicitly says that a class can not have zero size.

Long Answer for C++:

Because each object needs to have a unique address (also defined in the standard) you can't really have zero sized objects.
Imagine an array of zero sized objects. Because they have zero size they would all line up on the same address location. So it is easier to say that objects can not have zero size.

Note:

Even though an object has a non zero size, if it actually takes up zero room it does not need to increase the size of derived class:

Example:

#include <iostream>

class A {};
class B {};
class C: public A, B {};

int main()
{
     std::cout << sizeof(A) << "\n";
     std::cout << sizeof(B) << "\n";
     std::cout << sizeof(C) << "\n";  // Result is not 3 as intuitively expected.
}

g++ ty.cpp
./a.out
1
1
1

In the Java case:

• There is no simple way to find out how much memory an object occupies in Java; i.e. there is no sizeof operator.
• There are a few ways (e.g. using Instrumentation or 3rd party libraries) that will give you a number, but the meaning is nuanced¹; see In Java, what is the best way to determine the size of an object?
• The size of an object (empty or non-empty) is platform specific.

The size of an instance of an "empty class" (i.e. java.lang.Object) is not zero because the instance has implicit state associated with it. For instance, state is needed:

• so that the object can function as a primitive lock,
• to represent its identity hashcode,
• to indicate if the object has been finalized,
• to refer to the object's runtime class,
• to hold the object's GC mark bits,
• and so on.

Current Hotspot JVMs use clever tricks to represent the state in an object header that occupies two 32 bit words. (This expands in some circumstances; e.g. when a primitive lock is actually used, or after identityHashCode() is called.)

^{1 - For example, does the size of the string object created by new String("hello") include the size of that backing array that holds the characters? From the JVM perspective, that array is a separate object!}

Because every C++ object needs to have a separate address, it isn't possible to have a class with zero size (other than some special cases related to base classes). There is more information in C++: What is the size of an object of an empty class? .

Because an object has to have an address in memory, and to have an address in memory, it has to occupy "some" memory. So, it is usually, in C++, the smallest possible amount, i.e. 1 char (but that might depend on the compiler). In Java, I wouldn't be so sure.. it might have some default data (more than just a placeholder like in C++), but it would be surprising if it was much more than in C++.