Top 50 Most Asked Java Interview Questions

Find the process id with jps or ps, then use jcmd, which is the recommended tool. jps -l jcmd <pid> Thread.print > threads.txt jcmd <pid> GC.heap_dump /tmp/heap.hprof Older tools jstack and jmap do the same. Take three thread dumps a few seconds apart to spot stuck threads, and analyse heap dumps in Eclipse MAT.

Use groupingBy with a summing collector. Map<String, Double> totals = orders.stream() .collect(Collectors.groupingBy( Order::city, TreeMap::new, Collectors.summingDouble(Order::amount))); TreeMap keeps cities sorted. For money, store amounts as long paise or BigDecimal and use Collectors.reducing with BigDecimal::add.

A typical command is: java -Xms4g -Xmx4g \ -XX:+UseG1GC -XX:MaxGCPauseMillis=200 \ -XX:+HeapDumpOnOutOfMemoryError \ -XX:HeapDumpPath=/var/log/app \ -Xlog:gc*:file=/var/log/app/gc.log \ -jar app.jar In containers prefer -XX:MaxRAMPercentage=75 so the heap follows the container memory limit.

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()); } }

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.

Batching sends many rows in one round trip. String sql = "INSERT INTO product(name, price) VALUES (?, ?)"; try (Connection c = ds.getConnection(); PreparedStatement ps = c.prepareStatement(sql)) { c.setAutoCommit(false); int n = 0; for (Product p : products) { ps.setString(1, p.name()); ps.setLong(2, p.price()); ps.addBatch(); if (++n % 500 == 0) ps.executeBatch(); } ps.executeBatch(); c.commit(); }

Take the last digit with % 10 and build the result, checking for overflow. public static int reverse(int x) { long rev = 0; while (x != 0) { rev = rev * 10 + x % 10; x /= 10; } if (rev > Integer.MAX_VALUE || rev < Integer.MIN_VALUE) return 0; return (int) rev; } Negative numbers work because % keeps the sign in Java.

The simplest safe options are an enum or the holder idiom. public class Config { private Config() {} private static class Holder { static final Config INSTANCE = new Config(); } public static Config getInstance() { return Holder.INSTANCE; } } The JVM loads Holder lazily and class initialisation is thread safe. An enum singleton also resists reflection and serialization attacks.

Access order plus removeEldestEntry gives LRU in a few lines. class LruCache<K, V> extends LinkedHashMap<K, V> { private final int capacity; LruCache(int capacity) { super(16, 0.75f, true); this.capacity = capacity; } @Override protected boolean removeEldestEntry(Map.Entry<K, V> e) { return size() > capacity; } } get and put are O(1).

A builder avoids long constructors. public class User { private final String name; private final int age; private User(Builder b) { name = b.name; age = b.age; } public static class Builder { private String name; private int age; public Builder name(String n) { name = n; return this; } public Builder age(int a) { age = a; return this; } public User build() { return new User(this); } } } Usage: new User.Builder().name("Asha").age(25).build(). Lombok @Builder generates this.