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JavaBeginner#jvm#fundamentals
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Java Interview Questions
148 questions · Core Java, OOP, collections, multithreading, Spring and JVM interview questions.
148 results
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JavaBeginner#jvm#jdk
Difference between JDK, JRE and JVM?
3
JavaBeginner#operators#object
What is the difference between == and equals() in Java?
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JavaBeginner#oop
Explain OOP principles in Java.
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JavaIntermediate#oop#interface
What is the difference between abstract class and interface?
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JavaBeginner#oop#polymorphism
What is method overloading vs overriding?
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JavaBeginner#keywords
What is the final keyword?
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JavaIntermediate#memory#jvm
Explain Java memory model: heap vs stack.
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JavaIntermediate#gc#memory
What is garbage collection?
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JavaIntermediate#exceptions
What are checked vs unchecked exceptions?
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JavaBeginner#strings
What is the difference between String, StringBuilder, and StringBuffer?
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JavaIntermediate#strings
Why are Strings immutable in Java?
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JavaIntermediate#collections
Explain the Collections framework hierarchy.
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JavaBeginner#collections#list
ArrayList vs LinkedList?
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JavaIntermediate#collections#concurrency
HashMap vs Hashtable vs ConcurrentHashMap?
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JavaAdvanced#collections#hashmap
How does HashMap work internally?
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JavaAdvanced#collections
What is the difference between fail-fast and fail-safe iterators?
18
JavaIntermediate#java8#streams
Explain Java 8 Streams.
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JavaIntermediate#java8#lambda
What are lambda expressions?
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JavaIntermediate#java8
What is a functional interface?
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JavaIntermediate#java8
What is Optional?
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JavaIntermediate#concurrency
Explain multithreading and the Thread lifecycle.
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JavaAdvanced#concurrency
Difference between synchronized and volatile?
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JavaAdvanced#concurrency
What is the Executor framework?
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JavaIntermediate#concurrency
What is the difference between Callable and Runnable?
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JavaAdvanced#concurrency
Explain wait(), notify() and notifyAll().
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JavaAdvanced#design-patterns
What is the Singleton pattern? How to implement it thread-safely?
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JavaIntermediate#design-patterns#spring
What is dependency injection?
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JavaBeginner#exceptions
What is the difference between throw and throws?
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JavaIntermediate#exceptions
Explain try-with-resources.
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JavaIntermediate#fundamentals
What is autoboxing and unboxing?
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JavaIntermediate#collections
What is the difference between Comparable and Comparator?
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JavaAdvanced#jvm
What is reflection in Java?
34
JavaIntermediate#generics
What are Java generics?
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JavaAdvanced#generics
Difference between '? extends T' and '? super T'?
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JavaAdvanced#concurrency#jvm
What is the Java Memory Model (JMM)?
37
JavaAdvanced#spring
Explain Spring Bean lifecycle.
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JavaIntermediate#spring
What is the difference between Spring Boot and Spring Framework?
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JavaIntermediate#jpa
What is JPA vs Hibernate?
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JavaIntermediate#fundamentals
What is the difference between == and .equals() for Integer in Java?
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JavaBeginner#fundamentals
What are Java's primitive data types?
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JavaIntermediate#fundamentals
What is the difference between pass-by-value and pass-by-reference in Java?
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JavaBeginner#fundamentals
What is the difference between static and instance methods?
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JavaBeginner#oop
What is a constructor and how does constructor overloading work?
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JavaIntermediate#oop
What is the difference between instance initializer blocks and constructors?
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JavaBeginner#object#fundamentals
What is the difference between == and equals() when comparing objects that don't override equals()?
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JavaIntermediate#object#hashmap
What is the purpose of the hashCode() method and its contract with equals()?
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JavaBeginner#fundamentals
What is the instanceof operator?
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JavaBeginner#fundamentals#java10
What is the difference between var and explicit typing in Java 10+?
50
JavaIntermediate#inner-classes
What is the difference between a static nested class and a non-static inner class?
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JavaIntermediate#inner-classes
What are anonymous inner classes used for?
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JavaIntermediate#inner-classes
What is a local class in Java?
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JavaBeginner#fundamentals
What is the difference between an enum and a class with constants?
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JavaIntermediate#fundamentals#java14
What are records in Java (Java 14+)?
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JavaAdvanced#fundamentals#java17
What is the difference between sealed classes and final classes (Java 17)?
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JavaIntermediate#fundamentals#java14
What is the difference between switch statement and switch expression (Java 14+)?
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JavaBeginner#strings#java15
What is text block syntax in Java (Java 15+)?
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JavaIntermediate#strings
What is the String pool (intern pool)?
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JavaBeginner#strings
How do you reverse a String in Java?
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JavaBeginner#strings
How do you check if a String is a palindrome?
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JavaBeginner#strings
What is the difference between String.format and concatenation?
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JavaIntermediate#strings
How do you split a String and handle edge cases with delimiters that are regex special characters?
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JavaIntermediate#collections
What is the difference between Set implementations HashSet, LinkedHashSet, and TreeSet?
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JavaIntermediate#collections
What is the difference between Iterator and ListIterator?
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JavaIntermediate#collections
What is the difference between Deque and Queue?
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JavaBeginner#collections
How do you make a collection immutable?
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JavaBeginner#collections
What is the difference between Collection and Collections?
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JavaIntermediate#generics
What is a bounded type parameter in generics?
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JavaBeginner#generics
Can you use primitive types with generics?
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JavaAdvanced#generics
What is type erasure in Java generics?
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JavaIntermediate#collections
How do you sort a List of custom objects by multiple fields?
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JavaBeginner#generics
What is the diamond operator in generics?
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JavaBeginner#exceptions
What is the exception hierarchy in Java?
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JavaBeginner#exceptions
What is a custom exception and how do you create one?
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JavaIntermediate#exceptions
What is exception chaining (cause)?
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JavaBeginner#exceptions
What is the order of execution for try, catch, and finally?
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JavaAdvanced#exceptions
What happens if an exception is thrown in a finally block?
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JavaIntermediate#exceptions
What is multi-catch in Java 7+?
79
JavaIntermediate#concurrency
What is a race condition and how do you prevent it?
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JavaAdvanced#concurrency
What is a deadlock and how can it be avoided?
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JavaIntermediate#concurrency
What is the difference between synchronized methods and synchronized blocks?
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JavaIntermediate#concurrency
What are atomic classes like AtomicInteger used for?
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JavaAdvanced#concurrency
What is a CountDownLatch?
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JavaAdvanced#concurrency
What is a ThreadLocal and when would you use it?
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JavaIntermediate#concurrency
What is the difference between Thread.sleep() and Object.wait()?
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JavaAdvanced#concurrency
What is the difference between ReentrantLock and synchronized?
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JavaAdvanced#concurrency#java8
What is a CompletableFuture and how does it improve async programming?
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JavaIntermediate#concurrency
What are the different thread pool types provided by Executors?
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JavaAdvanced#concurrency
What is thread starvation?
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JavaIntermediate#java8#streams
What is the difference between map() and flatMap() in streams?
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JavaBeginner#streams
What is the difference between Collectors.toList() and Collectors.toSet()?
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JavaIntermediate#streams
How do you group elements using Collectors.groupingBy?
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JavaIntermediate#streams
What is the difference between findFirst() and findAny() in streams?
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JavaBeginner#java8
What is the difference between Optional.of() and Optional.ofNullable()?
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JavaIntermediate#java8#lambda
What is a method reference and what are its types?
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JavaIntermediate#streams
What is the difference between Stream.reduce() with and without an identity value?
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JavaAdvanced#streams
What is a parallel stream and when should you avoid it?
98
JavaAdvanced#memory#gc
What are the different generations in the JVM heap?
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JavaAdvanced#memory#gc
What causes a memory leak in Java despite having garbage collection?
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JavaAdvanced#memory#gc
What is the difference between strong, weak, soft, and phantom references?
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JavaAdvanced#gc#jvm
What is the difference between G1, CMS, and ZGC garbage collectors?
102
JavaIntermediate#gc#memory
What is the purpose of the finalize() method and why is it discouraged?
103
JavaIntermediate#serialization
What is serialization in Java?
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JavaIntermediate#serialization
What is the purpose of serialVersionUID?
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JavaIntermediate#serialization
What happens to static and transient fields during serialization?
106
JavaAdvanced#serialization
How can you customize the serialization process?
107
JavaIntermediate#design-patterns
What is the Factory design pattern?
108
JavaIntermediate#design-patterns
What is the Builder pattern and when should you use it?
109
JavaIntermediate#design-patterns
What is the Observer pattern?
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JavaIntermediate#design-patterns
What is the Strategy pattern?
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JavaAdvanced#design-patterns
What is the difference between the Adapter and Decorator patterns?
112
JavaAdvanced#design-patterns#oop
What is the difference between composition and inheritance, and why is 'favor composition over inheritance' recommended?
113
JavaAdvanced#design-patterns
What is the Template Method pattern?
114
JavaBeginner#jdbc
What is JDBC and what are its main components?
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JavaBeginner#jdbc
What is the difference between Statement and PreparedStatement?
116
JavaIntermediate#jdbc
What is a CallableStatement used for?
117
JavaIntermediate#jdbc
What is connection pooling and why is it important?
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JavaIntermediate#jdbc
How do you manage transactions manually in JDBC?
119
JavaAdvanced#jdbc
What is batch processing in JDBC?
120
JavaBeginner#spring
What is Inversion of Control (IoC) in Spring?
121
JavaIntermediate#spring
What are the different types of dependency injection in Spring?
122
JavaBeginner#spring
What is the difference between @Component, @Service, @Repository, and @Controller?
123
JavaIntermediate#spring
What is Spring Boot auto-configuration?
124
JavaIntermediate#spring
What is the purpose of @RestController vs @Controller in Spring MVC?
125
JavaAdvanced#spring
What is Spring AOP and what is it used for?
126
JavaAdvanced#spring
What is @Transactional and how does it work in Spring?
127
JavaBeginner#spring
What is the difference between application.properties and application.yml in Spring Boot?
128
JavaBeginner#spring
What is the purpose of Spring Boot starters?
129
JavaIntermediate#spring
What is the difference between @RequestParam, @PathVariable, and @RequestBody?
130
JavaIntermediate#spring
What are Spring bean scopes?
131
JavaIntermediate#puzzle#operators
What does this print? int x = 5; System.out.println(x++ + ++x);
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JavaBeginner#puzzle#strings
What is the output of this code? String s1 = "abc"; String s2 = "abc"; String s3 = new String("abc"); System.out.println(s1 == s2); System.out.println(s1 == s3);
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JavaAdvanced#puzzle#exceptions
What does this code output and why? try { return 1; } finally { return 2; }
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JavaAdvanced#puzzle#collections
What is the output? List<Integer> list = new ArrayList<>(); list.add(1); list.add(2); for (Integer i : list) { if (i == 1) list.remove(i); }
135
JavaIntermediate#puzzle#oop
What is the output of this code involving static and instance initialization order? class A { static { System.out.println("static A"); } { System.out.println("instance A"); } A() { System.out.println("constructor A"); } } new A(); new A();
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JavaIntermediate#puzzle#fundamentals
What does this print? Integer a = 127; Integer b = 127; Integer c = 128; Integer d = 128; System.out.println(a == b); System.out.println(c == d);
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JavaBeginner#puzzle#operators
What is the output? public class Test { public static void main(String[] args) { System.out.println(10 / 3); System.out.println(10.0 / 3); System.out.println(10 % 3); } }
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JavaAdvanced#puzzle#oop
What happens when you compile and run this code? class Parent { static void greet() { System.out.println("Parent"); } } class Child extends Parent { static void greet() { System.out.println("Child"); } } Parent p = new Child(); p.greet();
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JavaIntermediate#puzzle#arrays
What is the output of this array comparison? int[] a = {1, 2, 3}; int[] b = {1, 2, 3}; System.out.println(a == b); System.out.println(a.equals(b)); System.out.println(Arrays.equals(a, b));
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JavaAdvanced#puzzle#oop
What does this output? class Base { int x = 10; int getX() { return x; } } class Derived extends Base { int x = 20; int getX() { return x; } } Base obj = new Derived(); System.out.println(obj.x); System.out.println(obj.getX());
141
JavaAdvanced#jvm
What is the difference between a class loader's parent delegation model levels: Bootstrap, Extension/Platform, and Application?
142
JavaAdvanced#jvm
What is JIT compilation?
143
JavaIntermediate#oop#fundamentals
What is the difference between shallow copy and deep copy in Java?
144
JavaAdvanced#oop#interface
What is the diamond problem and how does Java avoid it with interfaces?
145
JavaIntermediate#serialization#keywords
What is the purpose of the transient keyword outside of serialization context — does it affect anything else?
146
JavaIntermediate#interface#java8
What is the difference between an interface's default method and a static method?
147
JavaAdvanced#streams#exceptions
What is the difference between checked exception wrapping in streams' lambdas — why can't you throw checked exceptions inside a lambda directly used in a stream?
148
JavaAdvanced#collections#oop