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Strings and Exception Handling Interview Questions for Java

25 Strings and Exception Handling questions with answers for java roles — 11 easy, 12 medium, 2 hard.

About Strings and Exception Handling

Strings appear in almost every coding round, and exception handling in every design discussion. Learn string immutability, the string pool, StringBuilder, text blocks and classic string problems, then checked and unchecked exceptions, try-with-resources, custom exceptions, chaining and how to design clean error handling for REST APIs.

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01

Why are String objects immutable in Java?

Strings and Exception HandlingEasy
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Strings are immutable so they can be safely shared from the string pool, used as HashMap keys with a cached hash code, passed between threads without locking and trusted in security checks such as file paths and class names. Any change, like concat or replace, creates a new String. For heavy modification use StringBuilder, which changes its internal buffer in place.
02

What is the String constant pool?

Strings and Exception HandlingEasy
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The string pool is a special area in the heap where string literals are stored once and reused. String a = "hi" and String b = "hi" point to the same object. new String("hi") creates a separate heap object, and intern() returns the pooled copy. Pooling saves memory because many strings in apps repeat, such as status codes and keys.
03

Compare String, StringBuilder and StringBuffer.

Strings and Exception HandlingEasy
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String is immutable and thread safe by design. StringBuilder is mutable and not synchronized, so it is the fastest choice for building strings in a single thread, such as in loops. StringBuffer is mutable and synchronized, so it is thread safe but slower, and is rarely needed today. The compiler already optimises simple + concatenation outside loops.
04

What is the difference between checked and unchecked exceptions?

Strings and Exception HandlingEasy
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Checked exceptions extend Exception (but not RuntimeException) and must be caught or declared with throws, for example IOException and SQLException. Unchecked exceptions extend RuntimeException, such as NullPointerException and IllegalArgumentException, and need no declaration. Use checked exceptions for recoverable conditions callers must handle and unchecked ones for programming errors. Errors like OutOfMemoryError should not be caught.
05

What is try-with-resources?

Strings and Exception HandlingEasy
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Try-with-resources, from Java 7, automatically closes any object that implements AutoCloseable when the try block ends, even if an exception is thrown. Resources close in reverse order of opening. If both the body and close() throw, the close exception is added as a suppressed exception. It replaces long finally blocks for files, JDBC connections and streams.
06

What is the difference between throw and throws?

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throw is a statement used inside a method to actually raise an exception object, for example throw new IllegalStateException("closed"). throws is used in a method signature to declare which checked exceptions the method may pass to its caller, for example void read() throws IOException. A method can declare many exceptions with throws but throws one object at a time.
07

What are text blocks in Java?

Strings and Exception HandlingEasy
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Text blocks, final in Java 15, are multi-line string literals written between triple double quotes. They keep line breaks, remove common leading indentation and need far fewer escape characters, which makes embedded JSON, SQL and HTML readable. Methods like formatted() can insert values. They are still normal String objects in the pool.
08

Write a Java method to check whether a string is a palindrome, ignoring case and non-letters.

Strings and Exception HandlingEasy
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Use two pointers from both ends. public static boolean isPalindrome(String s) { int i = 0, j = s.length() - 1; while (i < j) { char a = s.charAt(i), b = s.charAt(j); if (!Character.isLetterOrDigit(a)) { i++; continue; } if (!Character.isLetterOrDigit(b)) { j--; continue; } if (Character.toLowerCase(a) != Character.toLowerCase(b)) return false; i++; j--; } return true; } Time O(n), space O(1).
09

Count the frequency of each character in a string using Java.

Strings and Exception HandlingEasy
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A map with merge() is simple; streams also work. Map<Character, Integer> freq = new LinkedHashMap<>(); for (char c : s.toCharArray()) { freq.merge(c, 1, Integer::sum); } // Streams version Map<Character, Long> f = s.chars() .mapToObj(c -> (char) c) .collect(Collectors.groupingBy(c -> c, LinkedHashMap::new, Collectors.counting())); LinkedHashMap keeps first-seen order.
10

Write code to check if two strings are anagrams.

Strings and Exception HandlingEasy
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Count characters with an array for lowercase letters. public static boolean isAnagram(String a, String b) { if (a.length() != b.length()) return false; int[] count = new int[26]; for (int i = 0; i < a.length(); i++) { count[a.charAt(i) - 'a']++; count[b.charAt(i) - 'a']--; } for (int c : count) if (c != 0) return false; return true; } O(n) time; use a HashMap for Unicode input.
11

How do you handle disagreement with a reviewer about your code?

Strings and Exception HandlingEasy
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Situation: a senior reviewer asked me to replace my checked exceptions with unchecked ones. Task: decide the right approach without conflict. Action: I asked for the reasons, read our codebase conventions and wrote a small comparison, then agreed because unchecked exceptions fit our global handler. Result: the change merged the same day and I learned to check team conventions before designing error handling.
12

Does the finally block always execute?

Strings and Exception HandlingMedium
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finally runs after try and catch in almost every case, including when the try block returns or throws. It does not run if the JVM exits through System.exit(), the process is killed or the thread dies abruptly. Avoid returning from finally, because that silently overrides the try block's return value and swallows exceptions.
13

How do you create a custom exception and when should you?

Strings and Exception HandlingMedium
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Extend RuntimeException for unchecked or Exception for checked, and provide constructors that accept a message and a cause so the root stack trace is kept. Create one when a domain error needs specific handling, such as InsufficientBalanceException, which a Spring @ControllerAdvice can map to an HTTP 422 response. Do not create exceptions for normal control flow.
14

What is exception chaining?

Strings and Exception HandlingMedium
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Exception chaining wraps a low level exception in a higher level one while keeping the original as the cause, for example throw new OrderException("save failed", sqlEx). The full stack trace shows both, with Caused by lines. It lets each layer speak its own language without losing the root cause, which is essential when debugging production logs.
15

Is it good practice to catch Exception or Throwable?

Strings and Exception HandlingMedium
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Usually no. Catching Throwable also catches Errors like OutOfMemoryError that the app cannot recover from, and catching Exception hides bugs by treating all failures the same. Catch the most specific exceptions you can handle. A broad catch is acceptable only at a top boundary, such as a global handler or thread entry point, where you log and return a safe error.
16

A log file shows a NullPointerException only for some users. How do you debug it?

Strings and Exception HandlingMedium
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Start with the stack trace; since Java 14 helpful NPE messages name exactly which variable was null. Reproduce with the failing user's data, which often has an optional field missing. Fix the root cause by validating input at the boundary, using Optional for values that may be absent and annotating with @NonNull. Add a unit test for that data case so it does not return.
17

Code builds a large CSV report with String += inside a loop and it is very slow. Why and what is the fix?

Strings and Exception HandlingMedium
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Each += creates a new String and copies all previous characters, so building n rows is O(n squared) in time and creates huge garbage. Use a single StringBuilder with append() inside the loop, ideally with an initial capacity, or write rows directly to a BufferedWriter for very large output. This turns minutes of work into milliseconds.
18

Exceptions are being swallowed with empty catch blocks in your codebase. How would you fix this?

Strings and Exception HandlingMedium
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Empty catches hide failures and make production issues invisible. I would find them with static analysis like SonarQube, then decide per case: handle the error properly, rethrow wrapped with context, or at least log with the stack trace. I would add a global exception handler for APIs and a team rule in code review and lint config to block new empty catch blocks.
19

Your REST API returns stack traces to clients when errors happen. How do you design proper error handling?

Strings and Exception HandlingMedium
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Never expose stack traces; they leak internals. In Spring Boot use a @RestControllerAdvice with @ExceptionHandler methods that map domain exceptions to status codes, such as 404 for not found and 400 for validation. Return a consistent body, ideally RFC 7807 ProblemDetail, with a code, message and trace id. Log full details on the server with the same trace id.
20

Write a custom unchecked exception and a Spring handler that returns HTTP 404.

Strings and Exception HandlingMedium
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Define the exception, then map it centrally. public class NotFoundException extends RuntimeException { public NotFoundException(String msg) { super(msg); } } @RestControllerAdvice public class ApiErrors { @ExceptionHandler(NotFoundException.class) public ProblemDetail notFound(NotFoundException e) { return ProblemDetail.forStatusAndDetail( HttpStatus.NOT_FOUND, e.getMessage()); } }
21

Find the first non-repeating character in a string.

Strings and Exception HandlingMedium
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Count in one pass, then scan again in order. public static char firstUnique(String s) { int[] cnt = new int[128]; for (char c : s.toCharArray()) cnt[c]++; for (char c : s.toCharArray()) { if (cnt[c] == 1) return c; } return '_'; } Two passes, O(n) time and O(1) extra space for ASCII input.
22

Read a large text file line by line efficiently and count lines containing ERROR.

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Stream the file lazily so it never loads fully into memory. Path p = Path.of("app.log"); try (Stream<String> lines = Files.lines(p)) { long errors = lines .filter(l -> l.contains("ERROR")) .count(); System.out.println(errors); } try-with-resources closes the file handle. BufferedReader.readLine() in a loop is an equally good option.
23

Tell me about a time a production incident happened because of your code.

Strings and Exception HandlingMedium
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Situation: a release I made caused failed payments for some users. Task: restore service and find the cause fast. Action: I rolled back within ten minutes, then found I had not handled a null bank code from one partner, which threw an exception. I fixed it, added validation and a test with that partner's data and shared a short write-up. Result: no repeat and the team added null checks to our review checklist.
24

A file handle leak causes Too many open files errors after a day. What is the likely cause?

Strings and Exception HandlingHard
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Streams, readers or connections are opened but not closed on all paths, especially when an exception is thrown before close(). Check with lsof or jcmd which files are open. Fix by wrapping every resource in try-with-resources so it always closes, and make sure Files.lines() and Files.list() streams are closed too, since they hold file handles.
25

Should a service layer throw checked or unchecked exceptions? Defend your choice.

Strings and Exception HandlingHard
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I prefer unchecked domain exceptions in the service layer. Checked exceptions force every caller to catch or declare them, leak low level details through layers and do not work well with lambdas. Unchecked exceptions with clear names, handled centrally by a controller advice, keep code clean. Spring follows the same idea by translating SQLException into its unchecked DataAccessException family.

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How important is Strings and Exception Handling in Java interviews?

Strings appear in almost every coding round, and exception handling in every design discussion. Learn string immutability, the string pool, StringBuilder, text blocks and classic string problems, then checked and unchecked exceptions, try-with-resources, custom exceptions, chaining and how to design clean error handling for REST APIs.

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