StackOverflowError Against OutOfMemoryError

Two errors, both fatal, both with the word memory somewhere near them. Which one you got tells you which half of the program to look at, and they have almost nothing in common.

The two memory failures, on what ran out and what to look for
Compared on	`StackOverflowError`	`OutOfMemoryError`
What ran out	One thread's stack	The heap
Caused by	Too many nested calls, usually unbounded recursion	Too many reachable objects
Time to failure	Seconds — the stack is small and fixed	Hours or days of steady growth
The stack trace	Thousands of identical frames	Ordinary, and at whatever line asked for memory
First thing to check	The base case of a recursive method	What is holding references it no longer needs

java

static int countdown(int n) {
    return countdown(n - 1);
}

no base case, so every call adds a frame and none ever returns

The decision rule for reading one. Look at the trace. Thousands of repeating frames means the stack, and the fix is in the recursion. A trace that looks like any other, on a line that happens to allocate, means the heap — and the line in the trace is where the last straw landed rather than where the problem is.