<h3><strong class="">1. What is the Java Collection Framework?</strong></h3><p>The Java Collection Framework is a set of classes and interfaces that provide data structures (like lists, sets, maps) and algorithms (like sorting, searching). It is part of the <code>java.util</code> package and helps manage and manipulate collections of objects efficiently.</p><p><br></p><h3><strong class="">2. What are the main interfaces in the Java Collection Framework?</strong></h3><ul><li style="background-color: transparent"><strong class="">Collection</strong>: The root interface for all collections.    <ul><li style="background-color: transparent"><strong class="">List</strong>: Ordered collection (e.g., <strong>ArrayList, LinkedList).</strong></li><li style="background-color: transparent"><strong class="">Set</strong>: Unordered collection with no duplicate elements (e.g., <strong>HashSet, TreeSet).</strong></li><li style="background-color: transparent"><strong class="">Queue</strong>: Collection for holding elements prior to processing (e.g., <strong>PriorityQueue</strong>).</li></ul></li><li style="background-color: transparent"><strong class="">Map</strong>: Key-value pairs (e.g., <strong>HashMap, TreeMap).</strong></li></ul><h3><br></h3><h3>3. What is the difference between ArrayList and LinkedList?<br></h3><table class="se-table-layout-fixed"><thead><tr><th><div>Feature<br></div></th><th><div>ArrayList<br></div></th><th><div>LinkedList<br></div></th></tr></thead><tbody><tr><td><div>Implementation<br></div></td><td><div>Uses a dynamic array.<br></div></td><td><div>Uses a doubly linked list.<br></div></td></tr><tr><td><div>​<strong class="">Access Time</strong>​<br></div></td><td><div>O(1) for random access.<br></div></td><td><div>O(n) for random access.<br></div></td></tr><tr><td><div>​<strong class="">Insertion/Deletion</strong>​<br></div></td><td><div>Slow for large data (O(n)).<br></div></td><td><div>Faster for large data (O(1) at head/tail).<br></div></td></tr><tr><td><div>​<strong class="">Memory</strong>​<br></div></td><td><div>Less memory overhead.<br></div></td><td><div>More memory overhead (node pointers).<br></div></td></tr></tbody></table><p><br></p><h3>4. What is the difference between HashSet and TreeSet?<br></h3><table class="se-table-layout-fixed"><thead><tr><th><div>Feature<br></div></th><th><div>HashSet<br></div></th><th><div>TreeSet<br></div></th></tr></thead><tbody><tr><td><div>Order<br></div></td><td><div>Unordered.<br></div></td><td><div>Sorted in natural or custom order.<br></div></td></tr><tr><td><div>​<strong class="">Performance</strong>​<br></div></td><td><div>O(1) for add, remove, search.<br></div></td><td><div>O(log n) for add, remove, search.<br></div></td></tr><tr><td><div>​<strong class="">Null Values</strong>​<br></div></td><td><div>Allows one <code>null</code> value.​<br></div></td><td><div>Does not allow <code>null</code> values.​<br></div></td></tr></tbody></table><p><br></p><h3>5. How does HashMap work internally?<br></h3><ul><li style="background-color: transparent"><strong class="">Hashing</strong>: Keys are hashed using their <strong>hashCode()</strong> to determine the bucket.</li><li style="background-color: transparent"><strong class="">Collision Handling</strong>: If two keys hash to the same bucket:</li><li style="background-color: transparent"><strong class="">Key Lookup</strong>: HashMap first finds the bucket using the hash, then compares keys using <strong>equals()</strong>.    <ul><li style="background-color: transparent">Java 7: Uses a linked list.</li><li style="background-color: transparent">Java 8+: Uses a red-black tree for better performance when collisions are high.</li></ul></li></ul><p><br></p><h3>6. What is the difference between HashMap and Hashtable?<br></h3><table class="se-table-layout-fixed"><thead><tr><th><div>Feature<br></div></th><th><div>HashMap<br></div></th><th><div>Hashtable<br></div></th></tr></thead><tbody><tr><td><div>​<strong class="">Thread Safety</strong>​<br></div></td><td><div>Not synchronized.<br></div></td><td><div>Synchronized (thread-safe).<br></div></td></tr><tr><td><div>Null Keys/Values<br></div></td><td><div>Allows one <code>null</code> key and multiple <code>null</code> values.​<br></div></td><td><div>Does not allow <code>null</code> keys or values.​<br></div></td></tr><tr><td><div>Performance<br></div></td><td><div>Faster (no synchronization).<br></div></td><td><div>Slower (synchronization overhead).<br></div></td></tr></tbody></table><p><br></p><h3>7. What is the difference between ArrayList and Vector?<br></h3><table class="se-table-layout-fixed"><thead><tr><th><div>​<span style="background-color: rgb(243, 243, 243)">Feature</span>​<br></div></th><th><div>ArrayList<br></div></th><th><div>Vector<br></div></th></tr></thead><tbody><tr><td><div>​<strong class="">Thread Safety</strong>​<br></div></td><td><div>Not synchronized.<br></div></td><td><div>Synchronized (thread-safe).<br></div></td></tr><tr><td><div>​<strong class="">Performance</strong>​<br></div></td><td><div>Faster (no synchronization).<br></div></td><td><div>Slower (synchronization overhead).<br></div></td></tr><tr><td><div>​<strong class="">Growth</strong>​<br></div></td><td><div>Increases by 50% when full.<br></div></td><td><div>Doubles its size when full.<br></div></td></tr></tbody></table><p><br></p><h3>8. What is the difference between Comparable and Comparator?<br></h3><p>​</p><ul><li style="background-color: transparent"><strong class="">Comparable</strong>: Used to define natural ordering of objects. Implemented using the <strong>compareTo()</strong> method.</li><li style="background-color: transparent"><strong class="">Comparator</strong>: Used to define custom ordering. Implemented using the <strong>compare()</strong> method.</li></ul><p><strong class="">Example:</strong></p><pre>​  <span style="color: #569cd6">class </span><span style="color: #4ec9b0">Employee </span><span style="color: #569cd6">implements </span><span style="color: #4ec9b0">Comparable</span><span style="color: #cccccc">&lt;</span><span style="color: #4ec9b0">Employee</span><span style="color: #cccccc">&gt; {</span><br><span style="color: #569cd6">private </span><span style="color: #4ec9b0">int </span><span style="color: #9cdcfe">id</span><span style="color: #cccccc">;</span><br><span style="color: #569cd6">public </span><span style="color: #4ec9b0">int </span><span style="color: #dcdcaa">compareTo</span><span style="color: #cccccc">(</span><span style="color: #4ec9b0">Employee</span><span style="color: #9cdcfe">e</span><span style="color: #cccccc">) {</span><br><span style="color: #c586c0">return </span><span style="color: #569cd6">this</span><span style="color: #cccccc">.</span><span style="color: #9cdcfe">id</span><span style="color: #d4d4d4">-</span><span style="color: #9cdcfe">e</span><span style="color: #cccccc">.</span><span style="color: #9cdcfe">id</span><span style="color: #cccccc">; </span><span style="color: #6a9955">// Natural ordering by ID</span><br><span style="color: #cccccc">    }</span><br><span style="color: #cccccc">}<br></span><br></pre><p><br></p><h3>9. What is the difference between fail-fast and fail-safe iterators?</h3><ul><li style="background-color: transparent"><strong class="">Fail-Fast</strong>: Throws <strong>ConcurrentModificationException</strong> if the collection is modified during iteration (e.g., <strong>ArrayList, HashMap).</strong></li><li style="background-color: transparent"><strong class="">Fail-Safe</strong>: Does not throw an exception as it works on a copy of the collection (e.g., <strong>CopyOnWriteArrayList, ConcurrentHashMap).</strong></li></ul><p><br></p><h3>10. What is the difference between HashMap and TreeMap?<br></h3><table class="se-table-layout-fixed"><thead><tr><th><div>Feature<br></div></th><th><div>HashMap<br></div></th><th><div>TreeMap<br></div></th></tr></thead><tbody><tr><td><div>Order<br></div></td><td><div>Unordered.<br></div></td><td><div>Sorted by natural or custom order.<br></div></td></tr><tr><td><div>Performance<br></div></td><td><div>O(1) for add, remove, search.<br></div></td><td><div>O(log n) for add, remove, search.<br></div></td></tr><tr><td><div>​<strong class="">Null Keys</strong>​<br></div></td><td><div>Allows one <code>null</code> key.​<br></div></td><td><div>Does not allow <code>null</code> keys.​<br></div></td></tr></tbody></table><p><br></p><h3>11. What is the difference between Iterator and ListIterator?<br></h3><table class="se-table-layout-fixed"><thead><tr><th><div>Feature<br></div></th><th><div>Iterator<br></div></th><th><div>ListIterator<br></div></th></tr></thead><tbody><tr><td><div>​<strong class="">Traversal</strong>​<br></div></td><td><div>Forward only.<br></div></td><td><div>Forward and backward.<br></div></td></tr><tr><td><div>​<strong class="">Applicable</strong>​<br></div></td><td><div>All collections.<br></div></td><td><div>Only for <strong>List</strong> implementations.<br></div></td></tr><tr><td><div>​<strong class="">Add Elements</strong>​<br></div></td><td><div>Not supported.<br></div></td><td><div>Supported.<br></div></td></tr></tbody></table><p><br></p><h3>12. What is the difference between Queue and Deque?<br></h3><ul><li style="background-color: transparent"><strong class="">Queue</strong>: A collection that follows FIFO (First-In-First-Out) order (e.g., <strong>PriorityQueue</strong>).</li><li style="background-color: transparent"><strong class="">Deque</strong>: A double-ended queue that allows insertion and removal from both ends (e.g., <strong>ArrayDeque</strong>).</li></ul><p><br></p><h3>13. What is the difference between Set and List?<br></h3><table class="se-table-layout-fixed"><thead><tr><th><div>Feature<br></div></th><th><div>Set<br></div></th><th><div>List<br></div></th></tr></thead><tbody><tr><td><div>​<strong class="">Duplicates</strong>​<br></div></td><td><div>Does not allow duplicates.<br></div></td><td><div>Allows duplicates.<br></div></td></tr><tr><td><div>Order<br></div></td><td><div>Unordered (except <strong>LinkedHashSet</strong>).<br></div></td><td><div>Ordered.<br></div></td></tr></tbody></table><p><br></p><h3>14. What is the difference between ConcurrentHashMap and SynchronizedMap?<br></h3><ul><li style="background-color: transparent"><strong class="">ConcurrentHashMap</strong>: Allows concurrent access by dividing the map into segments. Better performance in multithreaded environments.</li><li style="background-color: transparent"><strong class="">SynchronizedMap</strong>: Synchronizes the entire map, leading to slower performance.</li></ul><p><br></p><h3>15. What is the difference between peek(), poll(), and remove() in a Queue?<br></h3><ol><li style="background-color: transparent"><strong>peek()</strong>: Retrieves the head of the queue without removing it. Returns <strong>null</strong> if the queue is empty.</li><li style="background-color: transparent"><strong>poll()</strong>: Retrieves and removes the head of the queue. Returns <strong>null</strong> if the queue is empty.</li><li style="background-color: transparent"><strong>remove()</strong>: Retrieves and removes the head of the queue. Throws <strong>NoSuchElementException</strong> if the queue is empty.</li></ol><p><br></p><h3>16. How does LinkedHashMap maintain insertion order?<br></h3><p><strong>LinkedHashMap</strong> uses a doubly linked list to maintain the order of elements as they are inserted. It also allows access-order iteration if configured.<br></p><p><br></p><h3>17. What is the difference between HashSet and LinkedHashSet?<br></h3><table class="se-table-layout-fixed"><thead><tr><th><div>Feature<br></div></th><th><div>HashSet<br></div></th><th><div>LinkedHashSet<br></div></th></tr></thead><tbody><tr><td><div>Order<br></div></td><td><div>Unordered<br></div></td><td><div>Maintains insertion order.<br></div></td></tr><tr><td><div>Performance<br></div></td><td><div>Slightly faster.<br></div></td><td><div>Slightly slower due to ordering.<br></div></td></tr></tbody></table><p><br></p><h3>18. What is the purpose of PriorityQueue?<br></h3><p><strong>PriorityQueue</strong> is a queue where elements are ordered based on their natural ordering or a custom comparator. The head of the queue is the smallest or highest-priority element.<br></p><p><br></p><h3>19. How do you sort a collection in Java?</h3><ul><li style="background-color: transparent">Use <strong>Collections.sort()</strong> for lists.</li><li style="background-color: transparent">Use <strong>TreeSet or TreeMap for sorted sets or maps.</strong></li><li style="background-color: transparent">Use <strong>Comparator</strong> for custom sorting.</li></ul><p><strong class="">Example:</strong></p><pre>​  <span style="color: #4ec9b0">List</span><span style="color: #cccccc">&lt;</span><span style="color: #4ec9b0">String</span><span style="color: #cccccc">&gt; </span><span style="color: #9cdcfe">names </span><span style="color: #d4d4d4">= </span><span style="color: #9cdcfe">Arrays</span><span style="color: #cccccc">.</span><span style="color: #dcdcaa">asList</span><span style="color: #cccccc">(</span><span style="color: #ce9178">"Alice"</span><span style="color: #cccccc">, </span><span style="color: #ce9178">"Bob"</span><span style="color: #cccccc">, </span><span style="color: #ce9178">"Charlie"</span><span style="color: #cccccc">);</span><br><span style="color: #9cdcfe">Collections</span><span style="color: #cccccc">.</span><span style="color: #dcdcaa">sort</span><span style="color: #cccccc">(names); </span><span style="color: #6a9955">// Natural order</span><br><span style="color: #9cdcfe">Collections</span><span style="color: #cccccc">.</span><span style="color: #dcdcaa">sort</span><span style="color: #cccccc">(names, </span><span style="color: #9cdcfe">Comparator</span><span style="color: #cccccc">.</span><span style="color: #dcdcaa">reverseOrder</span><span style="color: #cccccc">()); </span><span style="color: #6a9955">// Custom order<br></span><br></pre><h3><br></h3><h3>20. What is the difference between CopyOnWriteArrayList and ArrayList?<br></h3><ul><li style="background-color: transparent"><strong class="">CopyOnWriteArrayList</strong>: Thread-safe; creates a new copy of the list on every modification. Suitable for read-heavy operations.</li><li style="background-color: transparent"><strong class="">ArrayList</strong>: Not thread-safe; faster for single-threaded operations.</li></ul><p><br></p>

Java Collection Framework Interview Question

<div class="se-component se-image-container __se__float-none" style="width: 100%"><figure style="width: 100%;"><img src="https://www.api.cstechie.com/upload/editor-image/JAVA-INTERVIEW-BANNER_1746949464451.png" alt="Java Interview Image" data-rotate="" data-proportion="true" data-size="100%," data-align="none" data-percentage="100," data-file-name="JAVA-INTERVIEW-BANNER.png" data-file-size="528346" data-origin="," style="width: 100%;"></figure></div><p><span style="font-size: 18px">​<br></span></p><p><span style="font-size: 18px">In this interview preparation roadmap, we are going to cover below modules:</span></p><ul><li><a href="https://cstechie.com/interview/java/core-java-interview-question"><span style="font-size: 18px">Core Java Interview Question</span></a><span style="font-size: 18px">​</span></li><li style="font-size: 18px">​<a href="https://cstechie.com/interview/java/java-collection-framework-interview-question">Java Collection Framework Interview Question</a>​</li><li><span style="font-size: 18px">​</span><a href="https://cstechie.com/interview/java/java-new-features-inteview-questions"><span style="font-size: 18px">Java New Features Inteview Questions</span></a>​<br></li></ul>

Java Interview Questions

<h3><span style="font-size: 19px">1. What are the main features of Java?</span></h3><ol><li style="background-color: transparent"><strong class="">Object-Oriented</strong>: Everything in Java is an object.</li><li style="background-color: transparent"><strong class="">Platform-Independent</strong>: Java code is compiled into bytecode, which can run on any platform with a JVM.</li><li style="background-color: transparent"><strong class="">Robust</strong>: Java has strong memory management, exception handling, and garbage collection.</li><li style="background-color: transparent"><strong class="">Secure</strong>: Java provides a secure runtime environment with features like bytecode verification and no explicit pointer usage.</li><li style="background-color: transparent"><strong class="">Multithreaded</strong>: Java supports concurrent execution of threads.</li><li style="background-color: transparent"><strong class="">High Performance</strong>: Java uses Just-In-Time (JIT) compilers for better performance.</li></ol><h3><br></h3><h3>2. What is the difference between JDK, JRE, and JVM?<br></h3><ol><li style="background-color: transparent"><strong class="">JDK (Java Development Kit)</strong>: A development environment that includes tools like the compiler (<code>javac</code>), debugger, and JRE.</li><li style="background-color: transparent"><strong class="">JRE (Java Runtime Environment)</strong>: Provides the libraries and JVM required to run Java applications.</li><li style="background-color: transparent"><strong class="">JVM (Java Virtual Machine)</strong>: Executes Java bytecode and provides platform independence.</li></ol><h3><br></h3><h3>3. What is the difference between == and equals() in Java?</h3><ul><li style="background-color: transparent">== compares references (memory addresses) for objects and values for primitives.</li><li style="background-color: transparent">equals() compares the content of objects. By default, it behaves like ==, but it can be overridden in classes like String.</li></ul><p><strong class="">Example:</strong></p><pre><strong class="">​​<span style="color: #4ec9b0">String</span><span style="color: #9cdcfe">s1</span><span style="color: #d4d4d4">=</span><span style="color: #c586c0">new</span><span style="color: #dcdcaa">String</span><span style="color: #cccccc">(</span><span style="color: #ce9178">"Hello"</span><span style="color: #cccccc">);</span><br><span style="color: #4ec9b0">String</span><span style="color: #9cdcfe">s2</span><span style="color: #d4d4d4">=</span><span style="color: #c586c0">new</span><span style="color: #dcdcaa">String</span><span style="color: #cccccc">(</span><span style="color: #ce9178">"Hello"</span><span style="color: #cccccc">);</span><br><span style="color: #9cdcfe">System</span><span style="color: #cccccc">.</span><span style="color: #9cdcfe">out</span><span style="color: #cccccc">.</span><span style="color: #dcdcaa">println</span><span style="color: #cccccc">(s1 </span><span style="color: #d4d4d4">==</span><span style="color: #cccccc"> s2); </span><span style="color: #6a9955">// false (different references)</span><br><span style="color: #9cdcfe">System</span><span style="color: #cccccc">.</span><span style="color: #9cdcfe">out</span><span style="color: #cccccc">.</span><span style="color: #dcdcaa">println</span><span style="color: #cccccc">(</span><span style="color: #9cdcfe">s1</span><span style="color: #cccccc">.</span><span style="color: #dcdcaa">equals</span><span style="color: #cccccc">(s2)); </span><span style="color: #6a9955">// true (same content)<br></span><br></strong></pre><h3><br></h3><h3>4. What is the difference between String, StringBuilder, and StringBuffer?<br></h3><ul><li style="background-color: transparent"><strong class="">String</strong>: Immutable; every modification creates a new object.</li><li style="background-color: transparent"><strong class="">StringBuilder</strong>: Mutable and not synchronized; faster for single-threaded operations.</li><li style="background-color: transparent"><strong class="">StringBuffer</strong>: Mutable and synchronized; thread-safe but slower than StringBuilder.</li></ul><h3><br></h3><h3>5. What are the access modifiers in Java?<br></h3><ol><li style="background-color: transparent"><strong class="">Public</strong>: Accessible from anywhere.</li><li style="background-color: transparent"><strong class="">Protected</strong>: Accessible within the same package and subclasses.</li><li style="background-color: transparent"><strong class="">Default (no modifier)</strong>: Accessible within the same package.</li><li style="background-color: transparent"><strong class="">Private</strong>: Accessible only within the same class.</li></ol><h3><br></h3><h3>6. What is the difference between final, finally, and finalize()?<br></h3><ul><li style="background-color: transparent"><strong>final: </strong>A keyword used to declare constants, prevent method overriding, or inheritance.</li><li style="background-color: transparent"><strong>finally: </strong>A block in exception handling that always executes, regardless of whether an exception is thrown.</li><li style="background-color: transparent"><strong>finalize():</strong> A method called by the garbage collector before an object is destroyed (deprecated in modern Java).</li></ul><h3><br></h3><h3>7. What is the difference between Array and ArrayList?<br></h3><ul><li style="background-color: transparent"><strong class="">Array</strong>: Fixed size, can store primitives and objects.</li><li style="background-color: transparent"><strong class="">ArrayList</strong>: Dynamic size, can only store objects, and provides utility methods like add(), remove(), etc.</li></ul><h3><br></h3><h3>8. What is the difference between HashMap and Hashtable?<br></h3><ul><li style="background-color: transparent"><strong class="">HashMap</strong>: Not synchronized, allows one null key and multiple null values.</li><li style="background-color: transparent"><strong class="">Hashtable</strong>: Synchronized, does not allow null keys or values.</li></ul><h3><br></h3><h3>9. What is the difference between Abstract Class and Interface?<br></h3><ul><li style="background-color: transparent"><strong class="">Abstract Class</strong>:<br><ul><li style="background-color: transparent">Can have abstract and concrete methods.</li><li style="background-color: transparent">Supports constructors.</li><li style="background-color: transparent">Can have instance variables.</li></ul></li><li style="background-color: transparent"></li><li style="background-color: transparent"><strong class="">Interface</strong>:<ul><li style="background-color: transparent">All methods are abstract (until Java 8, which introduced default and static methods).</li><li style="background-color: transparent">Cannot have constructors.</li><li style="background-color: transparent">Only constants (static final variables) are allowed.</li></ul><br></li></ul><h3>10. What is the difference between throw and throws?<br></h3><ul><li style="background-color: transparent"><strong>throw</strong>: Used to explicitly throw an exception.</li><li style="background-color: transparent"><strong>throws</strong>: Declares exceptions that a method might throw.</li></ul><p>Example:<br></p><pre><span style="color: #4ec9b0">void</span><span style="color: #dcdcaa">method</span><span style="color: #cccccc">() throws IOException {</span><br><span style="color: #c586c0">throw</span><span style="color: #c586c0">new</span><span style="color: #dcdcaa">IOException</span><span style="color: #cccccc">(</span><span style="color: #ce9178">"Exception thrown"</span><span style="color: #cccccc">);</span><br><span style="color: #cccccc">}<br></span><br></pre><h3><br></h3><h3>11. What is the difference between checked and unchecked exceptions?<br></h3><ul><li style="background-color: transparent"><strong>Checked Exceptions:</strong> Must be handled or declared using throws (e.g., IOException, SQLException).</li><li style="background-color: transparent"><strong>Unchecked Exceptions:</strong> Runtime exceptions that do not need to be declared or handled (e.g., NullPointerException, ArithmeticException).</li></ul><h3><br></h3><h3>12. What is the difference between static and instance variables?<br></h3><ul><li style="background-color: transparent"><strong class="">Static Variables</strong>: Shared across all instances of a class; belong to the class.</li><li style="background-color: transparent"><strong class="">Instance Variables</strong>: Unique to each object; belong to the instance.</li></ul><h3><br></h3><h3>13. What is the difference between overloading and overriding?<br></h3><ul><li style="background-color: transparent"><strong class="">Overloading</strong>: Same method name, different parameter lists (compile-time polymorphism).</li><li style="background-color: transparent"><strong class="">Overriding</strong>: Subclass provides a specific implementation of a method in the parent class (runtime polymorphism).</li></ul><h3><br></h3><h3>14. What is the purpose of the transient keyword?<br></h3><p>The <strong>transient</strong> keyword is used to mark a variable that should not be serialized.<br></p><p><br></p><h3>15. What is the purpose of the synchronized keyword?<br></h3><p>​The <strong>synchronized</strong> keyword ensures that only one thread can access a block of code or method at a time, preventing race conditions.<br></p><p><br></p><h3>16. What is the difference between wait(), notify(), and notifyAll()?<br></h3><ol><li style="background-color: transparent"><strong>wait():</strong> Causes the current thread to wait until another thread calls <strong>notify() </strong>or <strong>notifyAll() </strong>on the same object.</li><li style="background-color: transparent"><strong>notify()</strong>: Wakes up one thread waiting on the object's monitor.</li><li style="background-color: transparent"><strong>notifyAll()</strong>: Wakes up all threads waiting on the object's monitor.</li></ol><h3><br></h3><h3>17. What is the difference between Comparable and Comparator?<br></h3><ol><li style="background-color: transparent"><strong class="">Comparable</strong>: Used to define natural ordering of objects. Implemented using <strong>compareTo()</strong>.</li><li style="background-color: transparent"><strong class="">Comparator</strong>: Used to define custom ordering. Implemented using <strong>compare()</strong>.</li></ol><h3><br></h3><h3>18. What is the purpose of the volatile keyword?<br></h3><p>The <strong>volatile</strong> keyword ensures that changes to a variable are immediately visible to all threads. It prevents caching of the variable by threads.<br></p><p><br></p><h3>19. What is the difference between HashSet and TreeSet?<br></h3><ul><li style="background-color: transparent"><strong class="">HashSet</strong>: Unordered, allows <strong>null</strong>, and uses a hash table for storage.</li><li style="background-color: transparent"><strong class="">TreeSet</strong>: Ordered (sorted), does not allow <strong>null</strong>, and uses a red-black tree for storage.</li></ul><h3><br></h3><h3>20. What is the purpose of the default keyword in interfaces (Java 8)?<br></h3><p>The <strong>default</strong> keyword allows interfaces to have concrete methods with a default implementation. This helps in adding new methods to interfaces without breaking existing implementations.</p>

Core Java Interview Question

<p>​<a href="https://cstechie.com/interview/java/core-java-interview-question"><span style="font-size: 18px">Core Java Interview Question</span></a>​<br></p>

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