Java Interview Questions and Answers: StringBuilder, Null Checks, Integer Caching, Collections, File I/O, Float Precision, Multithreading, Loops, and Finally

1. StringBuilder replaces String concatenation, often asked in interviews

Difference between String, StringBuilder, and StringBuffer: String is immutable and concatenation creates many temporary objects, leading to low efficiency; use StringBuilder for mutable strings in single‑threaded contexts and StringBuffer when synchronization is required.

2. Not checking for null before calling equals can cause NPE

Always place the constant or a non‑null reference on the left side of the equals call to avoid a NullPointerException.

3. Integer caching and equality comparison

Integers between -128 and 127 are cached; comparing two Integer objects with == returns true for values in this range but false for larger values because they are distinct objects.

4. Passing a variable as a method argument and modifying it

Reassigning a primitive parameter does not affect the original variable, while modifying an object's fields does; therefore the shown total variable remains 0.

5. Converting an array to a list

The resulting list is a fixed‑size view backed by the original array, so adding or removing elements throws an UnsupportedOperationException.

6. Removing an element from a list during iteration

Removing an element while iterating with an index can cause the loop to exceed the new list size, leading to an IndexOutOfBoundsException.

7. Creating directories with mkdir() vs mkdirs()

mkdir() creates only a single directory level, whereas mkdirs() creates all nonexistent parent directories, enabling multi‑level directory creation.

8. Float precision loss in monetary calculations

Subtracting two float values can lose precision; use BigDecimal or scale the amounts to integers before performing arithmetic.

9. Ways to implement multithreading in Java

Common approaches are extending the Thread class and implementing the Runnable interface; a third option is implementing Callable to obtain a result from the thread.

10. Looping three times and integer overflow

The issue arises from using Integer.MAX_VALUE and adding 1, which causes overflow and unexpected loop behavior.

11. Finally block execution

The finally block runs regardless of a return statement in the try block, ensuring cleanup code (e.g., closing streams) is always executed; thus the example returns false.