Predict the Output

concat() returns a new String. Without reassignment, the original is unchanged. Output: "Hello" (not "HelloWorld").

The parent's static method is called because static methods are resolved by reference type at compile time, not by object type at runtime.

false. 0.1 + 0.2 in double is 0.30000000000000004, not exactly 0.3. IEEE 754 cannot represent 0.1 exactly in binary.

x++ returns the old value then increments. ++x increments then returns the new value. Mixing them in one expression is confusing and should be avoided.

"Hello" + null = "Hellonull". The + operator converts null to the String "null". But null.concat() throws NPE because you are calling a method on null.

The second start() throws IllegalThreadStateException. A Thread can only be started once. After termination, create a new Thread to run the logic again.

null instanceof AnyType is always false. No exception. This makes instanceof safe as a null-guarded type check.

false. null instanceof AnyType is always false with no exception. See Q144 for the complete explanation.

The finally block's return wins. The try's return value is discarded. Never return from finally – it is a code smell.

System.exit(int) terminates the JVM immediately. Exit code 0 = success, non-zero = failure. Shutdown hooks run, but finally blocks in the calling thread do not.