Objects, statics, and mutation

Object and static state. Instance fields belong to an object; static fields belong to the class — one copy shared by every instance and every call. Traces go wrong when the two mix:

public class Counter {
    private static int created = 0;
    private int value;
    public Counter(int v) { value = v; created++; }
    public int bump() { value++; return value; }
}

After new Counter(5), new Counter(7), and c.bump() on the first: value lives per-object (5→6), created lives on the class (2). A question asking "what is created after these calls" is testing whether you keep the two ledgers separate.

ArrayList mutation adds its own trap: remove(i) shifts everything after i left. A loop that removes while iterating forward skips the element that slid into position i — the classic trace answer is "one element survived that you believe was removed."

Pass-by-value of references: a method can mutate the object its parameter points at (visible to the caller) but can never make the caller's variable point somewhere else. Traced precisely:

public static void scramble(int[] a, int[] b) {
    a[0] = 100;      // visible to caller
    b = new int[9];  // local rebinding only
    b[0] = 7;        // lost
}