What Happens in Memory

10 cards, about a minute. Read them the night before an interview, or right after the chapter to lock it in.

1. ◆what it is

   Each call gets a frame on the stack (parameters + locals). Every new object or array lives on the heap. Variables hold references: arrows to objects.

   #1
2. ◆what it is

   Java is always pass-by-value. For an object, the value copied is the reference: a method can change the object, never the caller's variable.

   #2
3. ↯use it when

   Use long when sums or products can pass ~2.1·10⁹: many big values, a * b above 46,340, prefix sums, mod 10⁹+7 math.

   #3
4. { }template

   Safe midpoint: int mid = lo + (hi - lo) / 2; or int mid = (lo + hi) >>> 1;

   #4
5. O( )cost

   Passing an array or list to a method is O(1): one reference is copied. Copying it (clone, new ArrayList<>(list)) is O(n).

   #5
6. O( )cost

   Recursion depth d uses O(d) stack. A default 1 MB stack holds roughly 10⁴ simple frames, then StackOverflowError.

   #6
7. !trap

   == on objects compares references. Use equals for String/Integer and Arrays.equals for arrays. Literals only look safe because they are pooled.

   #7
8. !trap

   result.add(path) stores the same list every time. Snapshot it: result.add(new ArrayList<>(path)).

   #8
9. !trap

   (long) (a * b) is too late: the int multiply already wrapped. Write (long) a * b.

   #9
10. ✦pro tip

    Garbage = unreachable. In your own array-backed stack, set items[size] = null after a pop so the GC can free the object.

    #10

Shaky on a card? Open the chapter or flip its recall cards.