Where a Value Lives While Your Method Runs

Two failures that look alike from the outside: a program that dies after four seconds of recursion, and one that dies after four hours of steady work. They exhaust different things, and telling them apart starts with knowing there are two.

The Java Virtual Machine — the JVM — divides the memory it uses into two regions with different rules.

The two regions, what lives in each, and what clears them
Compared on	The stack	The heap
Holds	One frame per method call	Every object created with `new`
A frame holds	Parameters, local variables, the return address	—
Cleared	When the method returns	By the garbage collector, when nothing can reach it
Per thread	One stack each	Shared by all threads
Size	Fixed, small	Configurable, large

Every object is on the heap, without exception. A local variable holding a List sits in the stack frame; the list itself is on the heap, and the variable holds its address.

So a local int disappears the moment its method returns, and an object outlives the method that made it for as long as anything still points at it. That difference is what the rest of this topic is about.