Java Interview Questions -14

Java Interview Questions -14

1. What is mutable object and immutable object?

If a object value is changeable then we can call it as Mutable object. (Ex., StringBuffer, …) If you are not allowed to change the value of an object, it is immutable object. (Ex., String, Integer, Float, …)

2. What is the purpose of assert keyword?

In order to validate certain expressions. It effectively replaces the if block and automatically throws the AssertionError on failure. This keyword should be used for the critical arguments. Meaning, without that the method does nothing.

3. What is reflection?

Reflection allows programmatic access to information about the fields, methods and constructors of loaded classes, and the use reflected fields, methods, and constructors to operate on their underlying counterparts on objects, within security restrictions.

4. What kind of thread is the Garbage collector thread?

It is a daemon thread. What is a daemon thread?
These are the threads which can run without user intervention. The JVM can exit when there are daemon thread by killing them abruptly.

5. Is JVM a compiler or an interpreter?


6. What is the finalize method do?

Before the invalid objects get garbage collected, the JVM give the user a chance to clean up some resources before it got garbage collected.

7. Differentiate between a Class and an Object?

A Class is only the definition or prototype of real life object. Whereas an object is an instance or living representation of real life object. Every object belongs to a class and every class contains one or more related objects.

8. What is a package?

To group set of classes into a single unit is known as packaging. Packages provides wide namespace ability.

9. What is JIT and its use?

Really, just a very fast compiler… In this incarnation, pretty much a one-pass compiler — no offline computations. So you can’t look at the whole method, rank the expressions according to which ones are re-used the most, and then generate code. In theory terms, it’s an on-line problem.

10. What is garbage collection? What is the process that is responsible for doing that in java?

Reclaiming the unused memory by the invalid objects. Garbage collector is responsible for this process

11. What is the base class for Error and Exception?


12. What do you understand by a variable?

The variables plays very important role in computer programming. Variables enable programmers to write flexible programs. It is a memory location that has been named so that it can be easily be referred in the program. The variable is used to hold the data and it can be changed changed during the course of the execution of the program.

13. What are the types of casting?

There are two types of casting in Java, these are Implicit casting and explicit casting.

14. What is Implicit casting?

Implicit casting is the process of simply assigning one entity to another without any transformation guidance to the compiler. This type of casting is not permitted in all kinds of transformations and may not workout for all application scenarios.
int i = 20;
long h = i; //Implicit casting

15. How will you get the platform dependent values like line separator, path separator, etc., ?

Using Sytem.getProperty(…) (line.separator, path.separator, …)

16. What is the basic difference between string and stringbuffer object?

String is an immutable object. StringBuffer is a mutable object.

17. What do you understand by casting in java language?

The process of converting one datatype to another in Java language is called Casting.

18. What is the purpose of Void class?

The Void class is an uninstantiable placeholder class to hold a reference to the Class object representing the primitive Java type void.

19. How will you invoke any external process in Java?


20. What do you understand by numeric promotion?

The Numeric promotion is the conversion of a smaller numeric type to a larger numeric type, so that integer and floating-point operations may take place. In the numerical promotion process the byte, char, and short values are converted to int values. The int values are also converted to long values, if necessary. The long and float values are converted to double values, as required

21. What is inner class?

If the methods of the inner class can only be accessed via the instance of the inner class, then it is called inner class.

22. What do you understand by downcasting?

The process of Downcasting refers to the casting from a general to a more specific type, i.e. casting down the hierarchy

23. What is explicit casting?

Explicit casting in the process in which the complier are specifically informed to about transforming the object.
long ln = 700.20;
t = (int) ln; //Explicit casting

24. What is the major difference between LinkedList and ArrayList?

LinkedList are meant for sequential accessing. ArrayList are meant for random accessing.

25. What are the methods in Object?

clone, equals, wait, finalize, getClass, hashCode, notify, notifyAll, toString

26. What is singleton?

It is one of the design pattern. This falls in the creational pattern of the design pattern. There will be only one instance for that entire JVM. You can achieve this by having the private constructor in the class. For eg., public class Singleton { private static final Singleton s = new Singleton(); private Singleton() { } public static Singleton getInstance() { return s; } // all non static methods … }

27. What is Class.forName() does and how it is useful?

It loads the class into the ClassLoader. It returns the Class. Using that you can get the instance ( “class-instance”.newInstance() ).

28. What is nested class?

If all the methods of a inner class is static then it is a nested class.

29. What do you understand by downcasting?

The process of Downcasting refers to the casting from a general to a more specific type, i.e. casting down the hierarchy

30. What do you understand by final value?

FINAL for a variable: value is constant. FINAL for a method: cannot be overridden. FINAL for a class: cannot be derived

31. What is the final keyword denotes?

final keyword denotes that it is the final implementation for that method or variable or class. You can’t override that method/variable/class any more.

32. What do you achieve through good class and interface design?

Loosely coupled classes, objects, and components enabling your application to easily grow and adapt to changes without being rigid or fragile.
Less complex and reusable code that increases maintainability, extendability and testability.

33. What are the 3 main concepts of OOP?

Encapsulation, polymorphism, and inheritance are the 3 main concepts or pillars of an object oriented programming. Abstraction is another important concept that can be applied to both object oriented and non object oriented programming. [Remember: a pie ? abstraction, polymorphism, inheritance, and encapsulation.]

34.What problem(s) does abstraction and encapsulation solve?

Both abstraction and encapsulation solve same problem of complexity in different dimensions. Encapsulation exposes only the required details of an object to the caller by forbidding access to certain members, whereas an abstraction not only hides the implementation details, but also provides a basis for your application to grow and change over a period of time. For example, if you abstract out the make and model of a vehicle as class attributes as opposed to as individual classes like Toyota, ToyotaCamry, ToyotaCorolla, etc, you can easily incorporate new types of cars at runtime by creating a new car object with the relevant make and model as arguments as opposed to having to declare a new set of classes.

35.What is the difference between private, protected, and public?

These keywords are for allowing privileges to components such as java methods and variables.
Public: accessible to all classes
Private: accessible only to the class to which they belong
Protected: accessible to the class to which they belong and any subclasses.
Access specifiers are keywords that determines the type of access to the member of a class. These are:
* Public
* Protected
* Private
* Defaults

36.What's the difference between an interface and an abstract class? Also discuss the similarities. (Very Important)

Abstract class is a class which contain one or more abstract methods, which has to be implemented by sub classes. Interface is a Java Object containing method declaration and doesn't contain implementation. The classes which have implementing the Interfaces must provide the method definition for all the methods
Abstract class is a Class prefix with a abstract keyword followed by Class definition. Interface is a Interface which starts with interface keyword.
Abstract class contains one or more abstract methods. where as Interface contains all abstract methods and final declarations
Abstract classes are useful in a situation that Some general methods should be implemented and specialization behavior should be implemented by child classes. Interfaces are useful in a situation that all properties should be implemented.
Differences are as follows:
* Interfaces provide a form of multiple inheritance. A class can extend only one other class.
* Interfaces are limited to public methods and constants with no implementation. Abstract classes can have a partial implementation, protected parts, static methods, etc.
* A Class may implement several interfaces. But in case of abstract class, a class may extend only one abstract class.
* Interfaces are slow as it requires extra indirection to to find corresponding method in in the actual class. Abstract classes are fast.


* Neither Abstract classes or Interface can be instantiated.

37.How to define an Abstract class?

A class containing abstract method is called Abstract class. An Abstract class can't be instantiated.
Example of Abstract class:

abstract class testAbstractClass {
    protected String myString;
    public String getMyString() {
    return myString;
public abstract string anyAbstractFunction();

38. Can you call one constructor from another if a class has multiple constructors

Yes. Use this() to call a constructor from an other constructor.

39. Explain the usage of Java packages.

This is a way to organize files when a project consists of multiple modules. It also helps resolve naming conflicts when different packages have classes with the same names. Packages access level also allows you to protect data from being used by the non-authorized classes.

40. Explain in your own words the "bottom line" benefits of the use of an interface.

The interface makes it possible for a method in one class to invoke methods on objects of other classes, without the requirement to know the true class of those objects, provided that those objects are all instantiated from classes that implement one or more specified interfaces. In other words, objects of classes that implement specified interfaces can be passed into methods of other objects as the generic type Object, and the methods of the other objects can invoke methods on the incoming objects by first casting them as the interface type.

41. Question: How you can force the garbage collection?

Garbage collection automatic process and can't be forced. You could request it by calling System.gc(). JVM does not guarantee that GC will be started immediately.
Garbage collection is one of the most important feature of Java, Garbage collection is also called automatic memory management as JVM automatically removes the unused variables/objects (value is null) from the memory. User program can't directly free the object from memory, instead it is the job of the garbage collector to automatically free the objects that are no longer referenced by a program. Every class inherits finalize() method from java.lang.Object, the finalize() method is called by garbage collector when it determines no more references to the object exists. In Java, it is good idea to explicitly assign null into a variable when no more in use. I Java on calling System.gc() and Runtime.gc(), JVM tries to recycle the unused objects, but there is no guarantee when all the objects will garbage collected.

42. What's the difference between constructors and normal methods?

Constructors must have the same name as the class and can not return a value. They are only called once while regular methods could be called many times and it can return a value or can be void.

43. Explain the usage of the keyword transient?

Transient keyword indicates that the value of this member variable does not have to be serialized with the object. When the class will be de-serialized, this variable will be initialized with a default value of its data type (i.e. zero for integers).

44. What's the difference between the methods sleep() and wait()

The code sleep(1000); puts thread aside for exactly one second. The code wait(1000), causes a wait of up to one second. A thread could stop waiting earlier if it receives the notify() or notifyAll() call. The method wait() is defined in the class Object and the method sleep() is defined in the class Thread.

45. What would you use to compare two String variables - the operator == or the method equals()?

I'd use the method equals() to compare the values of the Strings and the == to check if two variables point at the same instance of a String object.

46. Why would you use a synchronized block vs. synchronized method?

Synchronized blocks place locks for shorter periods than synchronized methods.

47. What are some advantages and disadvantages of Java Sockets?

Some advantages of Java Sockets:
Sockets are flexible and sufficient. Efficient socket based programming can be easily implemented for general communications. Sockets cause low network traffic. Unlike HTML forms and CGI scripts that generate and transfer whole web pages for each new request, Java applets can send only necessary updated information.
Some disadvantages of Java Sockets:
Security restrictions are sometimes overbearing because a Java applet running in a Web browser is only able to establish connections to the machine where it came from, and to nowhere else on the network   Despite all of the useful and helpful Java features, Socket based communications allows only to send packets of raw data between applications. Both the client-side and server-side have to provide mechanisms to make the data useful in any way.

48. What access level do you need to specify in the class declaration to ensure that only classes from the same directory can access it?

You do not need to specify any access level, and Java will use a default package access level.

49. Can an inner class declared inside of a method access local variables of this method?

It's possible if these variables are final.

50. What can go wrong if you replace && with & in the following code:

String a=null; if (a!=null && a.length()>10) {...}
A single ampersand here would lead to a NullPointerException.

51.What's the main difference between a Vector and an ArrayList?

Java Vector class is internally synchronized and ArrayList is not synchronized.

52. How could Java classes direct program messages to the system console, but error messages, say to a file?

The class System has a variable out that represents the standard output, and the variable err that represents the standard error device. By default, they both point at the system console. This how the standard output could be re-directed:
Stream st = new Stream(new FileOutputStream("output.txt")); System.setErr(st); System.setOut(st);

53. If a class is located in a package, what do you need to change in the OS environment to be able to use it?

You need to add a directory or a jar file that contains the package directories to the CLASSPATH environment variable. Let's say a class Employee belongs to a package; and is located in the file c:\dev\com\xyz\hr\Employee.javIn this case, you'd need to add c:\dev to the variable CLASSPATH. If this class contains the method main(), you could test it from a command prompt window as follows:

54. What's the difference between J2SDK 1.5 and J2SDK 5.0?

There's no difference, Sun Microsystems just re-branded this version.

55. Does it matter in what order catch statements for FileNotFoundException and IOExceptipon are written?

Yes, it does. The FileNoFoundException is inherited from the IOException. Exception's subclasses have to be caught first.

56. How do you know if an explicit object casting is needed?

If you assign a superclass object to a variable of a subclass's data type, you need to do explicit casting. For example:
Object a; Customer b; b = (Customer) a;

57. When you assign a subclass to a variable having a supeclass type, the casting is performed automatically. Can you write a Java class that could be used both as an applet as well as an application?

Yes. Add a main() method to the applet.

58. When should the method invokeLater()be used?

This method is used to ensure that Swing components are updated through the event-dispatching thread.

59. How can a subclass call a method or a constructor defined in a superclass?

Use the following syntax: super.myMethod(); To call a constructor of the superclass, just write super(); in the first line of the subclass's constructor.

60. Name the containers which uses Border Layout as their default layout?

Containers which uses Border Layout as their default are: window, Frame and Dialog classes.

61. You are planning to do an indexed search in a list of objects. Which of the two Java collections should you use:

ArrayList or LinkedList?

62. What do you understand by Synchronization?

Synchronization is a process of controlling the access of shared resources by the multiple threads in such a manner that only one thread can access one resource at a time. In non synchronized multithreaded application, it is possible for one thread to modify a shared object while another thread is in the process of using or updating the object's value. Synchronization prevents such type of data corruption.
E.g. Synchronizing a function:
public synchronized void Method1 () {
    // Appropriate method-related code.
E.g. Synchronizing a block of code inside a function:
public myFunction (){
    synchronized (this) {
    // Synchronized code here.

63. What's the difference between a queue and a stack?

Stacks works by last-in-first-out rule (LIFO), while queues use the FIFO rule

64. You can create an abstract class that contains only abstract methods. On the other hand, you can create an interface that declares the same methods. So can you use abstract classes instead of interfaces?

Sometimes. But your class may be a descendent of another class and in this case the interface is your only option.

65. If you're overriding the method equals() of an object, which other method you might also consider?


66. What is Collection API?

The Collection API is a set of classes and interfaces that support operation on collections of objects. These classes and interfaces are more flexible, more powerful, and more regular than the vectors, arrays, and hashtables if effectively replaces.
Example of classes: HashSet, HashMap, ArrayList, LinkedList, TreeSet and TreeMap.
Example of interfaces: Collection, Set, List and Map.

67. How would you make a copy of an entire Java object with its state?

Have this class implement Cloneable interface and call its method clone().

68. How can you minimize the need of garbage collection and make the memory use more effective?

Use object pooling and weak object references.

69. There are two classes: A and B. The class B need to inform a class A when some important event has happened. What Java technique would you use to implement it?

If these classes are threads I'd consider notify() or notifyAll(). For regular classes you can use the Observer interface.

70. Explain the Encapsulation principle.

Encapsulation is a process of binding or wrapping the data and the codes that operates on the data into a single entity. This keeps the data safe from outside interface and misuse. One way to think about encapsulation is as a protective wrapper that prevents code and data from being arbitrarily accessed by other code defined outside the wrapper.

71. Explain the Inheritance principle.

Inheritance is the process by which one object acquires the properties of another object.

72. Explain the Polymorphism principle.

The meaning of Polymorphism is something like one name many forms. Polymorphism enables one entity to be used as as general category for different types of actions. The specific action is determined by the exact nature of the situation. The concept of polymorphism can be explained as "one interface, multiple methods".
From a practical programming viewpoint, polymorphism exists in three distinct forms in Java:
* Method overloading
* Method overriding through inheritance
* Method overriding through the Java interface

73. Is Iterator a Class or Interface? What is its use?

Iterator is an interface which is used to step through the elements of a Collection.

74. Explain the user defined Exceptions?

User defined Exceptions are the separate Exception classes defined by the user for specific purposed. An user defined can created by simply sub-classing it to the Exception class. This allows custom exceptions to be generated (using throw) and caught in the same way as normal exceptions.

class myCustomException extends Exception {
     / The class simply has to exist to be an exception

75.How does Java acheive platform independence?

A Java source file on compilation produces an intermediary .class rather than a executable file. This .class file is interpreted by the JVM. Since JVM acts as an intermediary layer.

76.What is the difference between a JDK and a JVM?

JDK is Java Development Kit which is for development purpose and it includes execution environment also. But JVM is purely a run time environment and hence you will not be able to compile your source files using a JVM.

77.What is a pointer and does Java support pointers?

Pointer is a reference handle to a memory location. Improper handling of pointers leads to memory leaks and reliability issues hence Java doesn't support the usage of pointers.

78.How is an object reference different from a pointer?

Both object reference and pointer point to the memory location of the object. You can manipulate pointers but not object references.

79.What is a JVM?

JVM is Java Virtual Machine which is a run time environment for the compiled java class files.

80.Are JVM's platform independent?

JVM's are not platform independent. JVM's are platform specific run time implementation provided by the vendor. A Windows JVM cannot be installed in Linux.

81.Who provides the JVM?

Any software vendor can provide a JVM but it should comply to the Java langauge specification.

82.What are the most important features of Java?

Java is object oriented, platform independent,secure,robust,simple,etc

83.Which one of them do you consider the best feature of Java?

Platform independence.

84.Is Java a pure object oriented language?

Java is a pure object oriented language. Except for the primitives everything else are objects in Java.

85.What is the difference between Path and Classpath?

Path and Classpath are operating system level environment variales. Path is used define where the system can find the executables(.exe) files and classpath is used to specify the location .class files.

86.Why does Java not support operator overloading?

Operator overloading makes the code very difficult to read and maintain. To maintain code simplicity, Java doesn't support operator overloading.

87.What are keywords?

Keywords cannot be used as identifiers.

88.What are reserved keywords? And can you name few reserved keywords?

Few of the words are reserved as keywords for future usage. Compiler forces the developer not to use a reserved keyword. const and

89.What is the overhead of introducing a new keyword?

When a new keyword is used in a new version of the JDK, there is high chances that has been used by developers as identifiers. This make tougher for the code base to migrate to new version since it requires code change, recompilation,testing and release cycle.

90.What do you mean by platform independence?

Platform independence means that we can write and compile the java code in one platform (eg Windows) and can execute the class in any other supported platform eg (Linux,Solaris,etc).

91.What is byte code?

Byte code is a set of instructions generated by the compiler. JVM executes the byte code.

92.Are primitive data types objects in Java?

Primitive data types are not objects.

93.What are all the number data types? And are they signed or unsigned?

Excpet boolean and char, others are number data types and they are all signed which means that they can hold both positive and negative values.

94.What are the possible values that a boolean data type can hold?

The boolean is the simplest data type which can hold true or false.

95.What are default values?

Values which are defaulted during object initialisation are called default values. Each data type has a default value.

96.What are identifiers?

Identifiers are names given to a variable, method, class and interface. Identifiers must conform to the following rules:a. The identifiers can contain a to z, A to Z,0 to 9,_ and $.b. Special characters other than _ and $ cannot be used in identifiers.c. Identifiers cannot start with numbers. d. keywords cannot be used as identifiers.

97.What is meant by naming conventions? And what are the naming conventions followed in Java?

Naming conventions are part of coding standards which are prescribed for better readability and maintenance. The following are simple conventions followed in Java:1. Instance and local Variables should start with a lowercase and subsequent word should start with a capital letter. Examples: int quantity; double unitPrice; 2. Class level variables ie constants should be in capital letters and _ is used word seprator final static double PI = 3.14; final static int MAX_PRIORITY = 10;3. Method names should start with small case and subsequent word should be capital letter. public double getAvailableBalance(String accountNo) throws InvalidAccountException{}4. Classes and Interfaces should start with a capital letter and subsequent words should also be capital letters. public class HelloWorld{}

98.What are literals?

Literals are source code representation of primitive data types.

99.Name the eight data types which are available in Java?

boolean, byte, short, int, long, float, double and char.

100.Does Java support multiple inheritance?

Java doesn't support multiple inheritance.

101.If you dont follow coding standards, will it result in compilation error?

No. These are standards. Each company or fot that matter each software unit might have its own coding standards. These are not enforced by the compiler.

102.How to make sure that all programmers are following the coding standards?

The best and simple way is to do peer reviews and code walkthroughs. You can use external plugins to your IDE (integrated development environment) to enforce during coding itself.

103.Why Java doesn't support multiple inheritance?

When a class inherits from more than class, it will lead to the diamond problem - say A is the super class of B and C & D is a subclass of both B and C. D inherits properties of A from two different inheritance paths ie via both B & C. This leads to ambiguity and related problems, so multiple inheritance is not allowed in Java.

104.Have you come across difficulties due to introduction of a new keyword?

Yes. enum keyword was used extensively as identifier in one of our project - we had to change the code in a lot of places to migrate it to newer v ersion.

105. How to define an Interface?

In Java Interface defines the methods but does not implement them. Interface can include constants. A class that implements the interfaces is bound to implement all the methods defined in Interface.
Example of Interface:

public interface sampleInterface {
    public void functionOne();
    public long CONSTANT_ONE = 1000;

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