Java

Question 1

List any five features of Java?

Answer 1

Some features include Object Oriented, Platform Independent, Robust, Interpreted, Multi-threaded

Question 2

Why is Java Architectural Neutral?

Answer 2

It’s compiler generates an architecture-neutral object file format, which makes the compiled code to be executable on many processors, with the presence of Java runtime system.

Question 3

How Java enabled High Performance?

Answer 3

Java uses Just-In-Time compiler to enable high performance. Just-In-Time compiler is a program that turns Java bytecode, which is a program that contains instructions that must be interpreted into instructions that can be sent directly to the processor.

Question 4

Why Java is considered dynamic?

Answer 4

It is designed to adapt to an evolving environment. Java programs can carry extensive amount of run-time information that can be used to verify and resolve accesses to objects on run-time.

Question 5

What is Java Virtual Machine and how it is considered in context of Java’s platform independent feature?

Answer 5

When Java is compiled, it is not compiled into platform specific machine, rather into platform independent byte code. This byte code is distributed over the web and interpreted by virtual Machine (JVM) on whichever platform it is being run.

Question 6

What is Singleton class?

Answer 6

Singleton class control object creation, limiting the number to one but allowing the flexibility to create more objects if the situation changes.

Question 7

List the three steps for creating an Object for a class?

Answer 7

An Object is first declared, then instantiated and then it is initialized.

Question 8

When a byte datatype is used?

Answer 8

This data type is used to save space in large arrays, mainly in place of integers, since a byte is four times smaller than an int.

Question 9

Variables used in a switch statement can be used with which datatypes?

Answer 9

Variables used in a switch statement can only be a string, enum, byte, short, int, or char.

Question 10

When parseInt() method can be used?

Answer 10

This method is used to get the primitive data type of a certain String.

Question 11

Why is String class considered immutable?

Answer 11

The String class is immutable, so that once it is created a String object cannot be changed. Since String is immutable it can safely be shared between many threads ,which is considered very important for multithreaded programming.

Question 12

Why is StringBuffer called mutable?

Answer 12

The String class is considered as immutable, so that once it is created a String object cannot be changed. If there is a necessity to make alot of modifications to Strings of characters then StringBuffer should be used.

Question 13

What is the difference between StringBuffer and StringBuilder class?

Answer 13

Use StringBuilder whenever possible because it is faster than StringBuffer. But, if thread safety is necessary then use StringBuffer objects.

Question 14

What is finalize() method?

Answer 14

It is possible to define a method that will be called just before an object’s final destruction by the garbage collector. This method is called finalize( ), and it can be used to ensure that an object terminates cleanly.

Question 15

Explain Runtime Exceptions?

Answer 15

It is an exception that occurs that probably could have been avoided by the programmer. As opposed to checked exceptions, runtime exceptions are ignored at the time of compilation.

Question 16

Which are the two subclasses under Exception class?

Answer 16

The Exception class has two main subclasses : IOException class and RuntimeException Class.

Question 17

When throws keyword is used?

Answer 17

If a method does not handle a checked exception, the method must declare it using the throws keyword. The throws keyword appears at the end of a method’s signature.

Question 18

When throw keyword is used?

Answer 18

An exception can be thrown, either a newly instantiated one or an exception that you just caught, by using throw keyword.

Question 19

How finally used under Exception Handling?

Answer 19

The finally keyword is used to create a block of code that follows a try block. A finally block of code always executes, whether or not an exception has occurred.

Question 20

What things should be kept in mind while creating your own exceptions in Java?

Answer 20

While creating your own exception −

- All exceptions must be a child of Throwable.

- If you want to write a checked exception that is automatically enforced by the Handle or Declare Rule, you need to extend 
  the Exception class.

- You want to write a runtime exception, you need to extend the RuntimeException class.

Question 21

Define Inheritance?

Answer 21

It is the process where one object acquires the properties of another. With the use of inheritance the information is made manageable in a hierarchical order.

Question 22

When super keyword is used?

Answer 22

If the method overrides one of its superclass’s methods, overridden method can be invoked through the use of the keyword super. It can be also used to refer to a hidden field.

Question 23

What is Polymorphism?

Answer 23

Polymorphism is the ability of an object to take on many forms. The most common use of polymorphism in OOP occurs when a parent class reference is used to refer to a child class object.

Question 24

What is Abstraction?

Answer 24

It refers to the ability to make a class abstract in OOP. It helps to reduce the complexity and also improves the maintainability of the system.

Question 25

What is Abstract class?

Answer 25

These classes cannot be instantiated and are either partially implemented or not at all implemented. This class contains one or more abstract methods which are simply method declarations without a body.

Question 26

When Abstract methods are used?

Answer 26

If you want a class to contain a particular method but you want the actual implementation of that method to be determined by child classes, you can declare the method in the parent class as abstract.

Question 27

What is Encapsulation?

Answer 27

It is the technique of making the fields in a class private and providing access to the fields via public methods. If a field is declared private, it cannot be accessed by anyone outside the class, thereby hiding the fields within the class. Therefore encapsulation is also referred to as data hiding.

Question 28

What is the primary benefit of Encapsulation?

Answer 28

The main benefit of encapsulation is the ability to modify our implemented code without breaking the code of others who use our code. With this Encapsulation gives maintainability, flexibility and extensibility to our code.

Question 29

What is an Interface?

Answer 29

An interface is a collection of abstract methods. A class implements an interface, thereby inheriting the abstract methods of the interface.

Question 30

Give some features of Interface?

Answer 30

• Interface cannot be instantiated
• An interface does not contain any constructors.
• All of the methods in an interface are abstract.

Question 31

What are the two ways in which Thread can be created?

Answer 31

Thread can be created by: implementing Runnable interface, extending the Thread class.

Question 32

Explain garbage collection in Java?

Answer 32

It uses garbage collection to free the memory. By cleaning those objects that is no longer reference by any of the program.

Question 33

Define immutable object?

Answer 33

An immutable object can’t be changed once it is created.

Question 34

What is Comparable Interface?

Answer 34

It is used to sort collections and arrays of objects using the collections.sort() and java.utils. The objects of the class implementing the Comparable interface can be ordered.

Question 35

What is function overloading?

Answer 35

If a class has multiple functions by same name but different parameters, it is known as Method Overloading.

Question 36

What is function overriding?

Answer 36

If a subclass provides a specific implementation of a method that is already provided by its parent class, it is known as Method Overriding.

Question 37

Difference between Overloading and Overriding?

Answer 37

Method overloading increases the readability of the program. Method overriding provides the specific implementation of the method that is already provided by its super class parameter must be different in case of overloading, parameter must be same in case of overriding.

Question 38

What is final class?

Answer 38

Final classes are created so the methods implemented by that class cannot be overridden. It can’t be inherited.

Question 39

What is NullPointerException?

Answer 39

A NullPointerException is thrown when calling the instance method of a null object, accessing or modifying the field of a null object etc.

Question 40

What are the ways in which a thread can enter the waiting state?

Answer 40

A thread can enter the waiting state by invoking its sleep() method, by blocking on IO, by unsuccessfully attempting to acquire an object’s lock, or by invoking an object’s wait() method. It can also enter the waiting state by invoking its (deprecated) suspend() method.

Question 41

How does multi-threading take place on a computer with a single CPU?

Answer 41

The operating system’s task scheduler allocates execution time to multiple tasks. By quickly switching between executing tasks, it creates the impression that tasks execute sequentially.

Question 42

What is the difference between yielding and sleeping?

Answer 42

When a task invokes its yield() method, it returns to the ready state. When a task invokes its sleep() method, it returns to the waiting state.

Question 43

Why Vector class is used?

Answer 43

The Vector class provides the capability to implement a growable array of objects. Vector proves to be very useful if you don’t know the size of the array in advance, or you just need one that can change sizes over the lifetime of a program.

Question 44

What are Wrapper classes?

Answer 44

These are classes that allow primitive types to be accessed as objects. Example: Integer, Character, Double, Boolean etc.

Question 45

What is the difference between static and non-static variables?

Answer 45

A static variable is associated with the class as a whole rather than with specific instances of a class. Non-static variables take on unique values with each object instance.

Question 46

What is Serialization and deserialization?

Answer 46

Serialization is the process of writing the state of an object to a byte stream. Deserialization is the process of restoring these objects.

Question 47

Explain the use of subclass in a Java program?

Answer 47

Sub class inherits all the public and protected methods and the implementation. It also inherits all the default modifier methods and their implementation.

Question 48

What’s the difference between constructors and other methods?

Answer 48

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.

Question 49

Is there any limitation of using Inheritance?

Answer 49

Yes, since inheritance inherits everything from the super class and interface, it may make the subclass too clustering and sometimes error-prone when dynamic overriding or dynamic overloading in some situation.

Question 50

When is the ArrayStoreException thrown?

Answer 50

When copying elements between different arrays, if the source or destination arguments are not arrays or their types are not compatible, an ArrayStoreException will be thrown.

Question 51

What’s the difference between the methods sleep() and wait()?

Answer 51

The code sleep(2000); puts thread aside for exactly two seconds. The code wait(2000), causes a wait of up to two 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.

Question 52

When ArithmeticException is thrown?

Answer 52

The ArithmeticException is thrown when integer is divided by zero or taking the remainder of a number by zero. It is never thrown in floating-point operations.

Question 53

What is a transient variable?

Answer 53

A transient variable is a variable that may not be serialized during Serialization and which is initialized by its default value during de-serialization.

Question 54

What is synchronization?

Answer 54

Synchronization is the capability to control the access of multiple threads to shared resources. synchronized keyword in java provides locking which ensures mutual exclusive access of shared resource and prevent data race.

Question 55

Does garbage collection guarantee that a program will not run out of memory?

Answer 55

Garbage collection does not guarantee that a program will not run out of memory. It is possible for programs to use up memory resources faster than they are garbage collected. It is also possible for programs to create objects that are not subject to garbage collection.

Question 56

What is the difference between an Interface and an Abstract class?

Answer 56

An abstract class can have instance methods that implement a default behavior. An Interface can only declare constants and instance methods, but cannot implement default behavior and all methods are implicitly abstract. An interface has all public members and no implementation.

Question 57

What is the difference between error and an exception?

Answer 57

An error is an irrecoverable condition occurring at runtime. Such as OutOfMemory error. Exceptions are conditions that occur because of bad input etc. e.g. FileNotFoundException will be thrown if the specified file does not exist.

Question 58

What are the advantages of ArrayList over arrays?

Answer 58

ArrayList can grow dynamically and provides more powerful insertion and search mechanisms than arrays.

Question 59

Why deletion in LinkedList is fast than ArrayList?

Answer 59

Deletion in linked list is fast because it involves only updating the next pointer in the node before the deleted node and updating the previous pointer in the node after the deleted node.

Question 60

How do you decide when to use ArrayList and LinkedList?

Answer 60

If you need to frequently add and remove elements from the middle of the list and only access the list elements sequentially, then LinkedList should be used. If you need to support random access, without inserting or removing elements from any place other than the end, then ArrayList should be used.

Question 61

Describe life cycle of thread?

Answer 61

A thread is an execution in a program. The life cycle of a thread include −

• Newborn state
• Runnable state
• Running state
• Blocked state
• Dead state

Question 62

Why Generics are used in Java?

Answer 62

Generics provide compile-time type safety that allows programmers to catch invalid types at compile time. Java Generic methods and generic classes enable programmers to specify, with a single method declaration, a set of related methods or, with a single class declaration, a set of related types.

Question 63

What is daemon thread?

Answer 63

Daemon thread is a low priority thread, which runs intermittently in the back ground doing the garbage collection operation for the java runtime system.