Java interview questions

How to answer

Start with a one-sentence definition, give a tiny example, then one practical consequence. Stop and let the interviewer ask follow-ups. If you do not know, say so and reason from what you do know; confident wrong answers hurt more.

The most asked questions, with crisp answers

Output-prediction favourites

String a = "hi", b = new String("hi");
System.out.println(a == b);              // false
System.out.println(a.equals(b));         // true
Integer x = 127, y = 127, p = 128, q = 128;
System.out.println((x == y) + " " + (p == q));   // true false (Integer cache)
System.out.println(1 + 2 + "3" + 4 + 5);         // 3345
System.out.println(10 / 4 + " " + 10 % 4 + " " + 10 / 4.0);   // 2 2 2.5
List<Integer> l = new ArrayList<>(List.of(1, 2, 3));
l.remove(1);                              // removes index 1 → [1, 3]
System.out.println(l);

Coding round expectations

• Strings: reverse words, palindrome, anagram, first non-repeating character (HashMap or LinkedHashMap).
• Arrays: two-sum (HashMap), max subarray sum, rotate, remove duplicates from a sorted array.
• Collections and streams: group employees by department, top-N by salary, frequency counts.
• OOP design: parking lot, library management, ATM, with classes, interfaces and a short explanation.
• Spring Boot: design a REST endpoint, explain layers, @Transactional and exception handling.