Computing a Bounding Box with Android Rect.union

Desired Outcome

Obtain a single Rect that encloses two given rectangles, useful for bounding‑box calculations in UI layout or collision detection.

Prerequisites

• Android Studio or any JVM/Android build environment.
• Project targeting API level 1 or higher (the Rect.union method exists since API 1).
• Two android.graphics.Rect instances (or the ability to create them).

Procedure

1. Create or obtain the rectangles. Ensure they are not null. If they come from UI components, call getGlobalVisibleRect or similar.
2. Normalize each rectangle (optional but recommended). If you suspect left > right or top > bottom, call rect.sort() to swap the coordinates so that left ≤ right and top ≤ bottom.
3. Decide whether to preserve the original rectangles.
   • If you can mutate one of the inputs, call rectA.union(rectB); directly. After the call, rectA holds the union.
   • If you need both originals unchanged, create a copy first:

     Rect unionRect = new Rect(rectA);
     unionRect.union(rectB);

     This allocates a new Rect and mutates the copy.
4. Use the result. Pass the mutated rectangle (either rectA or unionRect) to drawing APIs, layout calculations, or store it for later use.

Expected Checks

• Verify that the resulting rectangle's left equals Math.min(rectA.left, rectB.left) and similarly for top.
• Verify that right equals Math.max(rectA.right, rectB.right) and bottom equals Math.max(rectA.bottom, rectB.bottom).
• If either input rectangle is empty (isEmpty() returns true), the union should be identical to the non‑empty rectangle (or empty if both are empty).
• Log or assert these values in a unit test or debug output to confirm correctness.

Recovery Options

• Null argument. Guard the call: if (rectA == null || rectB == null) { /* handle error */ } else { /* union */ } to avoid NullPointerException.
• Non‑normalized rectangles. After union, inspect width() and height(). If either is negative, re‑normalize the inputs with sort() and repeat the union.
• Unexpected empty result. Confirm that at least one rectangle had area; if both are empty, the union will be empty, which may be the intended outcome.

Limitations

• The instance method union(Rect) mutates the receiver; it does not return a new Rect. You must assign the return value of a copy constructor if you need the merged bounds without altering the original.
• Union works only with axis‑aligned rectangles; it cannot represent rotated bounds.
• Because a new Rect is allocated only when you explicitly copy, frequent use in tight loops can be made allocation‑free by reusing a mutable Rect instance.