Java

Question 1

What is an If Statement?

Answer 1

An if statement is a boolean expression followed by one more statements. The block of code inside the if statement will run if the boolean expression is true.

Question 2

What is a Switch statement?

Answer 2

A switch statement allows a variable to be tested for equality against a list of values. Each value is called a case, and the variable being switched on is checked for each case.

Question 3

What are Loop statements in Java?

Answer 3

A loop statement allows us to execute a statement or group of statements multiple times.

Question 4

What is a While loop?

Answer 4

A while loop statement in Java programming language repeatedly executes a target statement as long as a given condition is true.

Question 5

What is a For loop?

Answer 5

A for loop is a repetition control structure that allows you to efficiently write a loop that needs to be executed a specific number of times.

Question 6

What is a do while loop?

Answer 6

A do…while loop is similar to a while loop, except that a do…while loop is guaranteed to execute at least one time.

Question 7

What is an enhanced For loop?

Answer 7

This is mainly used to traverse collection of elements including arrays.

Question 8

What is the usage of “This” keyword?

Answer 8

In java, this is a reference variable that refers to the current object.
Here is given the 6 usage of java this keyword.

    this can be used to refer current class instance variable.
    this can be used to invoke current class method (implicitly)
    this() can be used to invoke current class constructor.
    this can be passed as an argument in the method call.
    this can be passed as argument in the constructor call.
    this can be used to return the current class instance from the method

Question 9

What is the use of “new” keyword in Java?

Answer 9

The new keyword is a Java operator that creates an object. The new operator instantiates a class by dynamically allocating(i.e, allocation at run time) memory for a new object and returning a reference to that memory.

Question 10

What is the use of the “super” keyword in Java?

Answer 10

The super keyword in Java is a reference variable which is used to refer immediate parent class object.

Question 11

What is the use of “static” keyword in Java?

Answer 11

The static keyword belongs to the class than an instance of the class. The static variable can be used to refer to the common property of all objects (which is not unique for each object), for example, the company name of employees, college name of students, etc.

Question 12

What is the use of “final” keyword in Java?

Answer 12

In the Java programming language, the final keyword is used in several contexts to define an entity that can only be assigned once. Once a final variable has been assigned, it always contains the same value

Question 13

What is a String?

Answer 13

A string is an object that represents a sequence of characters. For example text.

Question 14

What are some string handling methods?

Answer 14

char charAt(int index)  - Returns the character at the specified index. 
    int compareTo(Object o)  - Compares this String to another Object. 
    String concat(String str)  - Concatenates the specified string to the end of this string. 
    boolean equals(Object anObject) - Compares this string to the specified object. 
    boolean equalsIgnoreCase(String anotherString) - Compares this String to another String, ignoring case considerations.

Question 15

What is a package in Java?

Answer 15

A java package is a group of similar types of classes, interfaces and sub-packages.

Package in java can be categorized in two form, built-in package and user-defined package.

There are many built-in packages such as java, lang, awt, javax, swing, net, io, util, sql etc.

Question 16

What are advantages of packages?

Answer 16

Java package is used to categorize the classes and interfaces so that they can be easily maintained.
Java package provides access protection.
Java package removes naming collision.

Question 17

What are the 3 ways to access package outside of package?

Answer 17

import package.*;
import package.classname;
fully qualified name.
If you use package.* then all the classes and interfaces of this package will be accessible but not subpackages. 
If you import package.classname then only declared class of this package will be accessible.

Question 18

What is an Interface?

Answer 18

An interface is a reference type in Java. It is similar to class. It is a collection of abstract methods. A class implements an interface, thereby inheriting the abstract methods of the interface.
Along with abstract methods, an interface may also contain constants, default methods, static methods, and nested types. Method bodies exist only for default methods and static methods.
Writing an interface is similar to writing a class. But a class describes the attributes and behaviors of an object. And an interface contains behaviors that a class implements.
    Unless the class that implements the interface is abstract, all the methods of the interface need to be defined in the class.

Question 19

What are differences between interface and class?

Answer 19

You cannot instantiate an interface.
An interface does not contain any constructors.
All of the methods in an interface are abstract.
An interface cannot contain instance fields. The only fields that can appear in an interface must be declared both static and final.
An interface is not extended by a class, it is implemented by a class.
An interface can extend multiple interfaces.

Question 20

What are exceptions?

Answer 20

Exceptions are events that occur during the execution of programs that disrupt the normal flow of instructions (e.g. divide by zero, array access out of bound, etc.).

In Java, an exception is an object that wraps an error event that occurred within a method and contains:

Information about the error including its type 
The state of the program when the error occurred 
Optionally, other custom information

Question 21

What are checked and unchecked exceptions?

Answer 21

Checked: are the exceptions that are checked at compile time. If some code within a method throws a checked exception, then the method must either handle the exception or it must specify the exception using throws keyword. (array index out of range)
Unchecked are the exceptions that are not checked at compiled time.

Question 22

What is a try, catch and what purpose does finally serve?

Answer 22

A try statement is used to catch exceptions that might be thrown as your program executes. You should use a try statement whenever you use a statement that might throw an exception
A try block is always followed by a catch block, which handles the exception that occurs in the try block.
A finally block contains all the crucial statements that must be executed whether an exception occurs or not.

Question 23

What is an Array?

Answer 23

Java array is an object which contains elements of a similar data type. It is a data structure where we store similar elements. We can store only a fixed set of elements in a Java array.

Array in java is index-based, the first element of the array is stored at the 0 index.

Question 24

What is a Queue?

Answer 24

The queue collection is used to hold the elements about to be processed and provides various operations like the insertion, removal etc. It is an ordered list of objects with its use limited to insert elements at the end of the list and deleting elements from the start of list i.e. it follows the FIFO or the First-In-First-Out principle.

LinkedList, ArrayBlockingQueue and PriorityQueue are the most frequently used implementations.

Question 25

What is a List Interface?

Answer 25

The List interface extends Collection and declares the behavior of a collection that stores a sequence of elements.

Elements can be inserted or accessed by their position in the list, using a zero-based index.
A list may contain duplicate elements.
In addition to the methods defined by Collection, List defines some of its own, which are summarized in the following table.
Several of the list methods will throw an UnsupportedOperationException if the collection cannot be modified, and a ClassCastException is generated when one object is incompatible with another.

Question 26

What is an ArrayList?

Answer 26

It provides us dynamic arrays in Java. Though, it may be slower than standard arrays but can be helpful in programs where lots of manipulation in the array is needed.

    ArrayList inherits AbstractList class and implements List interface.
    ArrayList is initialized by a size, however the size can increase if collection grows or shrunk if objects are removed from the collection.
    Java ArrayList allows us to randomly access the list.
    ArrayList can not be used for primitive types, like int, char, etc. We need a wrapper class for such cases.

Question 27

What is the Set Interface?

Answer 27

A Set is a Collection that cannot contain duplicate elements. It models the mathematical set abstraction.
The Set interface contains only methods inherited from Collection and adds the restriction that duplicate elements are prohibited.
Set also adds a stronger contract on the behavior of the equals and hashCode operations, allowing Set instances to be compared meaningfully even if their implementation types differ.

Question 28

What is the Map Interface?

Answer 28

The Map interface maps unique keys to values. A key is an object that you use to retrieve a value at a later date.

Given a key and a value, you can store the value in a Map object. After the value is stored, you can retrieve it by using its key.
Several methods throw a NoSuchElementException when no items exist in the invoking map.
A ClassCastException is thrown when an object is incompatible with the elements in a map.
A NullPointerException is thrown if an attempt is made to use a null object and null is not allowed in the map.
An UnsupportedOperationException is thrown when an attempt is made to change an unmodifiable map.