Java concurrency and synchronization scope: predict the output and determinism of this program.\n\nclass Happy extends Thread {\n final StringBuffer sb1 = new StringBuffer();\n final StringBuffer sb2 = new StringBuffer();\n\n public static void main(String args[]) {\n final Happy h = new Happy();\n\n new Thread() {\n public void run() {\n synchronized (this) {\n h.sb1.append("A");\n h.sb2.append("B");\n System.out.println(h.sb1);\n System.out.println(h.sb2);\n }\n }\n }.start();\n\n new Thread() {\n public void run() {\n synchronized (this) {\n h.sb1.append("D");\n h.sb2.append("C");\n System.out.println(h.sb2);\n System.out.println(h.sb1);\n }\n }\n }.start();\n }\n}

Introduction / Context:
This question checks your understanding of Java threading, synchronization scopes, and why using synchronized(this) on different objects does not coordinate access across threads. It also touches on StringBuffer being thread-safe for individual operations but not making multi-object sequences atomic.

Given Data / Assumptions:

• Two anonymous Thread instances mutate shared fields h.sb1 and h.sb2.
• Each thread uses synchronized(this) where this refers to the anonymous Thread object, not the shared Happy instance.
• Each thread appends to sb1 and sb2 and then prints them in different orders.

Concept / Approach:
Synchronization must use the same monitor to enforce mutual exclusion. Here, each synchronized block locks a different monitor (the respective Thread instance), so there is no inter-thread coordination. Although StringBuffer methods are synchronized, they only serialize access per buffer method call, not across the two-buffer sequence. Therefore, the order of appends and prints can interleave in many ways.

Step-by-Step Solution:

Verification / Alternative check:
Replace synchronized(this) with synchronized(h) (or a dedicated common lock) to serialize both threads around the complete sequence; then you would see a predictable grouping of prints.

Why Other Options Are Wrong:

• AB… fixed-line options assume a deterministic schedule which is not enforced.
• The code does compile; syntax and imports are fine.

Common Pitfalls:
Assuming synchronized anywhere implies global serialization; confusing StringBuffer’s internal synchronization with atomic multi-step logic; locking on the wrong monitor.

Final Answer:
Output is non-deterministic due to concurrent interleaving