Interview Q&A / Java

1. What is the difference between == and equals() in Java, and what is the String pool?

   == checks whether two references point to the same object, while equals() checks whether two objects have the same content (if the class overrides it, as String does). String literals are stored once in the String pool, which is why two identical literals can be == but new String("x") cannot.

   BeginnerIn-depth3 min read
2. What is the difference between checked and unchecked exceptions in Java?

   Checked exceptions (subclasses of Exception other than RuntimeException) must be caught or declared with throws, and the compiler enforces this. Unchecked exceptions (RuntimeException and its subclasses, plus Errors) are not checked at compile time and usually signal programming bugs.

   BeginnerIn-depth3 min read
3. What is the difference between final, finally and finalize in Java?

   final is a keyword that stops change (a variable cannot be reassigned, a method cannot be overridden, a class cannot be extended). finally is a block that runs after try/catch, usually for cleanup. finalize() was a method the garbage collector might call before removing an object; it is deprecated for removal and should not be used.

   BeginnerIn-depth3 min read
4. What is the difference between JDK, JRE and JVM?

   The JVM runs Java bytecode. The JRE is the JVM plus the standard libraries needed to run a program. The JDK is the JRE plus developer tools such as the javac compiler, so you need the JDK to write and build Java, and only a runtime to run it.

   BeginnerIn-depth3 min read
5. What is the difference between String, StringBuilder and StringBuffer?

   String is immutable, so every change creates a new object. StringBuilder and StringBuffer are mutable and change the same object in place. StringBuffer's methods are synchronized (thread-safe but slower), while StringBuilder is not synchronized and is the usual choice for building strings in a single thread.

   BeginnerIn-depth3 min read
6. What does the static keyword mean in Java?

   static means a member belongs to the class itself, not to any one object. A static variable has one shared copy, a static method can be called without creating an object, a static block runs once when the class is loaded, and a static nested class does not need an outer object.

   IntermediateIn-depth3 min read
7. What is the difference between ArrayList and LinkedList in Java?

   ArrayList is backed by a resizable array, so reading by index is O(1) but inserting or removing in the middle shifts elements. LinkedList is a doubly linked list, so adding or removing at the ends is O(1) but reaching an index means walking the nodes. In practice ArrayList is the better default almost every time.

   IntermediateIn-depth3 min read
8. How does HashMap work internally in Java?

   HashMap stores entries in an array of buckets. It hashes the key, picks a bucket from that hash, and stores the entry there; keys that land in the same bucket form a linked list, which since Java 8 turns into a red-black tree when it gets long. When the map is 75% full it doubles the array and redistributes entries.

   AdvancedIn-depth4 min read
9. Can a try block exist without a catch block in Java?

   Yes, as long as it has either a finally block or resources in try-with-resources. A bare try with neither is a compile error.

   BeginnerQuick answer1 min read
10. Does calling System.gc() force garbage collection in Java?

    No. System.gc() is only a hint, and the JVM may ignore it. Even when it runs, it often triggers an expensive full collection, so production code should not call it.

    BeginnerQuick answer1 min read
11. How do you create a custom exception in Java?

    Extend Exception for a checked custom exception or RuntimeException for an unchecked one, and add constructors that pass the message and optionally a cause to the parent. Use it to express a business rule clearly.

    BeginnerQuick answer1 min read
12. How does a HashSet avoid storing duplicates in Java?

    A HashSet is backed by a HashMap and stores each element as a key. add() uses hashCode to find the bucket and equals to check for an existing equal element; if one exists the add returns false and nothing is stored.

    BeginnerQuick answer1 min read
13. How many objects does new String("India") create in Java?

    Up to two. The literal "India" goes into the string pool if it is not already there, and new String(...) always creates a separate object on the heap. So two objects the first time, and one if the pool already had the literal.

    BeginnerQuick answer1 min read
14. Should you extend Thread or implement Runnable in Java?

    Implement Runnable in most cases. It separates the task from the thread, leaves your class free to extend something else, and works with thread pools. Extending Thread only makes sense if you are changing the thread behaviour itself.

    BeginnerQuick answer1 min read
15. What are the four access levels in Java and who can see each one?

    private is visible only inside the class. Default (no keyword) adds the same package. protected adds subclasses in other packages. public is visible everywhere.

    BeginnerQuick answer1 min read
16. What are the four pillars of object-oriented programming in Java?

    Encapsulation hides data behind methods, abstraction exposes only what matters, inheritance reuses a parent class, and polymorphism lets one call behave differently depending on the actual object.

    BeginnerQuick answer1 min read
17. What causes ConcurrentModificationException and how do you avoid it?

    It is thrown when a collection is structurally modified while you iterate over it, for example calling list.remove() inside a for-each loop. Remove through the Iterator, use removeIf, or iterate over a copy.

    BeginnerQuick answer1 min read
18. What does the transient keyword do in Java?

    transient marks a field to be skipped during serialization. When the object is deserialized, that field gets its default value (null, 0 or false), which is useful for passwords, caches and values that can be recomputed.

    BeginnerQuick answer1 min read
19. What happens if you call run() instead of start() on a Java thread?

    start() creates a new thread of execution and calls run() on it. Calling run() directly is an ordinary method call on the current thread, so no concurrency happens. Calling start() twice throws IllegalThreadStateException.

    BeginnerQuick answer1 min read
20. What is a functional interface in Java and which built-in ones should you know?

    A functional interface has exactly one abstract method, so a lambda can implement it. Core ones are Predicate (test), Function (apply), Consumer (accept) and Supplier (get). @FunctionalInterface makes the compiler check the rule.

    BeginnerQuick answer1 min read
21. What is a lambda expression in Java and where can you use it?

    A lambda is a short anonymous function written as (parameters) -> body. It can be used wherever a functional interface, an interface with exactly one abstract method, is expected, such as Runnable, Comparator or Predicate.

    BeginnerQuick answer1 min read
22. What is a method reference in Java and when would you use it instead of a lambda?

    A method reference is a shorthand like String::length for a lambda that only calls one existing method. It makes code shorter and clearer when the lambda adds nothing beyond that call.

    BeginnerQuick answer1 min read
23. What is constructor chaining in Java and are constructors inherited?

    Constructors are not inherited. Every constructor starts by calling another one, either this(...) in the same class or super(...) in the parent, and Java inserts super() automatically if you write neither.

    BeginnerQuick answer1 min read
24. What is the difference between Comparable and Comparator in Java?

    Comparable defines one natural order inside the class itself through compareTo. Comparator is a separate object with a compare method, so you can define many different orderings without changing the class.

    BeginnerQuick answer1 min read
25. What is the difference between encapsulation and abstraction in Java?

    Abstraction hides complexity by showing only what a user needs, usually through interfaces or abstract classes. Encapsulation hides data by keeping fields private and exposing controlled methods. One is about design, the other about protection.

    BeginnerQuick answer1 min read
26. What is the difference between Error and Exception in Java?

    Both extend Throwable. An Exception represents a problem a program can reasonably catch and handle, while an Error such as OutOfMemoryError signals a serious JVM-level failure that applications normally should not try to recover from.

    BeginnerQuick answer1 min read
27. What is the difference between HashMap, LinkedHashMap and TreeMap?

    HashMap has no guaranteed order and gives O(1) operations. LinkedHashMap keeps insertion order, also in O(1). TreeMap keeps keys sorted and costs O(log n). Pick by the order you need.

    BeginnerQuick answer1 min read
28. What is the difference between heap and stack memory in Java?

    The stack holds method call frames with local primitives and references, and each thread has its own. The heap holds all objects and is shared by every thread and cleaned by the garbage collector.

    BeginnerQuick answer1 min read
29. What is the difference between method overloading and method overriding in Java?

    Overloading is several methods with the same name but different parameters in one class, resolved at compile time. Overriding is a subclass replacing a parent method with the same signature, resolved at runtime.

    BeginnerQuick answer1 min read
30. What is the difference between OutOfMemoryError and StackOverflowError?

    StackOverflowError happens when a thread call stack is exhausted, usually by endless recursion. OutOfMemoryError happens when the heap, or another memory area, cannot supply memory even after garbage collection.

    BeginnerQuick answer1 min read