Why will a StackOverflowError occur if there are no local variables? What will the memory fill with?

A StackOverflowError occurs when the call stack overflows, usually due to infinite recursion or excessively deep call nesting.

Even if there are no local variables in methods, the stack still fills up with call records (stack frames), which contain return addresses, parameters, and other administrative information.

Thus, even without local variables, the stack is filled with function call information. In the case of infinite recursion or very deep calls, the stack eventually overflows, leading to a StackOverflowError.