Java Developer Freshers Top Interview Questions and Answers Part 1

Java Developer Freshers Top Interview Questions and Answers Part 1:

1. What is Java?
   • Java is a high-level, object-oriented programming language developed by Sun Microsystems, which is now owned by Oracle Corporation. It is designed to be platform-independent and follows the principle of “Write Once, Run Anywhere” (WORA).
2. What are the key features of Java?
   • Key features of Java include platform independence, object-oriented programming, automatic memory management (garbage collection), strong type checking, and a rich set of libraries.
3. Explain the difference between JDK, JRE, and JVM.
   • JDK (Java Development Kit) is a software development kit used to develop Java applications. JRE (Java Runtime Environment) is a part of the JDK package that contains the Java Virtual Machine (JVM) and other libraries required to run Java applications. JVM (Java Virtual Machine) is an abstract machine that provides a runtime environment in which Java bytecode can be executed.
4. What is the main difference between == and .equals() method in Java?
   • The == operator in Java is used to compare references (memory addresses) of objects, whereas the .equals() method is used to compare the contents (values) of objects.
5. What is the difference between ArrayList and LinkedList in Java?
   • ArrayList is implemented as a resizable array, whereas LinkedList is implemented as a doubly linked list. ArrayList provides fast random access but slow insertion and deletion, while LinkedList provides fast insertion and deletion but slow random access.
6. What is a constructor in Java?
   • A constructor is a special type of method in Java that is used to initialize objects of a class. It has the same name as the class and does not have a return type.
7. What is method overloading?
   • Method overloading is a feature in Java that allows a class to have multiple methods with the same name but different parameters.
8. What is method overriding?
   • Method overriding is a feature in Java that allows a subclass to provide a specific implementation of a method that is already defined in its superclass.
9. What is the difference between method overloading and method overriding?
   • Method overloading occurs in the same class with different signatures, while method overriding occurs in different classes with the same signature.
10. What is inheritance in Java?
    • Inheritance is a mechanism in Java by which one class can inherit properties and behavior from another class.
11. What is encapsulation in Java?
    • Encapsulation is a mechanism in Java that binds the data (variables) and methods (functions) that operate on the data into a single unit called a class. It helps in data hiding and abstraction.
12. What is abstraction in Java?
    • Abstraction is a concept in Java that refers to the process of hiding the implementation details and showing only the essential features of an object.
13. What is a package in Java?
    • A package in Java is a mechanism for organizing classes and interfaces into a namespace.
14. What is a static method in Java?
    • A static method in Java belongs to the class rather than any instance of the class. It can be called without creating an instance of the class.
15. What is a static variable in Java?
    • A static variable in Java is a class-level variable that is shared among all instances of the class.
16. What is a final variable in Java?
    • A final variable in Java is a variable whose value cannot be changed once it is assigned.
17. What is a final class in Java?
    • A final class in Java is a class that cannot be subclassed.
18. What is the difference between abstract class and interface in Java?
    • An abstract class in Java can have both abstract and concrete methods, whereas an interface can only have abstract methods. A class can implement multiple interfaces but can extend only one abstract class.
19. What is a synchronized method in Java?
    • A synchronized method in Java is a method that is synchronized (thread-safe) and can be accessed by only one thread at a time.
20. What is a thread in Java?
    • A thread in Java is a lightweight process that executes a series of instructions independently of other threads.
21. How do you create a thread in Java?
    • There are two ways to create a thread in Java: by extending the Thread class or by implementing the Runnable interface and passing it to the Thread constructor.
22. What is the difference between start() and run() methods of a Thread class?
    • The start() method is used to start a new thread, whereas the run() method contains the code that will be executed by the thread.
23. What is a deadlock in Java?
    • A deadlock in Java occurs when two or more threads are waiting for each other to release resources that they need to proceed.
24. What is exception handling in Java?
    • Exception handling in Java is a mechanism for handling runtime errors (exceptions) that occur during the execution of a program.
25. What is the difference between checked and unchecked exceptions in Java?
    • Checked exceptions are checked at compile-time, whereas unchecked exceptions are not checked at compile-time.
26. What is the difference between throw and throws in Java?
    • throw is used to explicitly throw an exception, whereas throws is used to declare exceptions that a method might throw.
27. What is the finally block in Java?
    • The finally block in Java is used to execute important code such as closing resources or releasing locks, regardless of whether an exception is thrown or not.
28. What is the try-with-resources statement in Java?
    • The try-with-resources statement in Java is used to automatically close resources such as files, streams, or sockets after they are no longer needed.
29. What is a lambda expression in Java?
    • A lambda expression in Java is a concise way to represent an anonymous function (a function without a name).
30. What is a functional interface in Java?
    • A functional interface in Java is an interface that contains only one abstract method. It is used to implement lambda expressions.
31. What is the difference between == and .equals() method for comparing strings in Java?
    • The == operator in Java compares references (memory addresses) of strings, whereas the .equals() method compares the contents (values) of strings.
32. What is the StringBuilder class in Java?
    • The StringBuilder class in Java is used to create mutable (modifiable) sequences of characters.
33. What is the StringBuffer class in Java?
    • The StringBuffer class in Java is similar to the StringBuilder class but is synchronized (thread-safe).
34. What is the difference between StringBuilder and StringBuffer classes?
    • StringBuilder is not synchronized (not thread-safe) and is faster than StringBuffer, whereas StringBuffer is synchronized (thread-safe) but slower than StringBuilder.

36. What is polymorphism in Java?

• Polymorphism in Java is the ability of an object to take on many forms. It allows objects of different types to be treated as objects of a common superclass.

37. Explain method hiding in Java.

• Method hiding occurs when a subclass defines a static method with the same signature as a static method in its superclass. The subclass method hides the superclass method.

38. What is method chaining in Java?

• Method chaining is a technique in Java where multiple method calls are chained together in a single statement, typically by returning the object itself from each method call.

39. What is the this keyword in Java?

• The this keyword in Java refers to the current instance of the class. It is used to differentiate between instance variables and local variables with the same name.

40. What is method recursion in Java?

• Method recursion occurs when a method calls itself repeatedly until a base case is reached. It is a common technique used for solving problems that can be broken down into smaller, similar subproblems.

41. What is the purpose of the super keyword in Java?

• The super keyword in Java is used to refer to the superclass of the current object. It is used to access superclass methods and constructors.

42. What is a constructor chaining in Java?

• Constructor chaining occurs when one constructor calls another constructor in the same class or its superclass using the this() or super() keyword.

43. Explain the concept of method parameter passing in Java.

• In Java, method parameters can be passed by value or by reference. When passed by value, a copy of the parameter’s value is passed to the method. When passed by reference, a reference to the original object is passed.

44. What is a wrapper class in Java?

• A wrapper class in Java is a class that wraps (encapsulates) primitive data types into objects. It provides utility methods for converting between primitive data types and objects.

45. What is autoboxing and unboxing in Java?

• Autoboxing is the automatic conversion of primitive data types to their corresponding wrapper classes, and unboxing is the automatic conversion of wrapper classes to their corresponding primitive data types.

46. What is the Comparable interface in Java?

• The Comparable interface in Java is used to define a natural ordering of objects of a class. It contains a single method, compareTo(), which compares two objects for ordering.

47. What is the Comparator interface in Java?

• The Comparator interface in Java is used to define custom ordering of objects of a class. It contains a single method, compare(), which compares two objects for ordering.

48. What is the purpose of the assert statement in Java?

• The assert statement in Java is used for debugging purposes to check if a given condition is true. If the condition is false, an AssertionError is thrown.

49. What is the enum keyword in Java?

• The enum keyword in Java is used to define a special type called an enumeration, which consists of a fixed set of named constants.

50. What is serialization in Java?

• Serialization in Java is the process of converting an object into a stream of bytes so that it can be stored in a file or transmitted over a network. Deserialization is the reverse process of converting a stream of bytes back into an object.