What are stack frames, references, and object headers in Java (high level)?

Stack, heap, references

Local variables and call frames live on the stack (per thread). Objects live on the heap; variables hold references (like pointers without arithmetic). Object header stores metadata (class pointer, hash, lock state).

stack frame: ref ──▶ heap object { header, fields... }

• Primitives on stack (usually); objects always on heap.
• Passing reference copies the pointer—mutations visible to caller.