Core Java

Question 1

Learning Objectives
To install the JDK using Synaptic
Package Manager.

Answer 1

To run java program, we need to install JDK and Java development kit

Question 2

Why Java?

Answer 2

Simple
Object oriented
Platform independent
Safe
High performance
Multi-threaded

Question 3

Types and applications of Java.

Answer 3

Types and Applications of Java
JSP or Java Server Pages
Java Applets
J2EE or Java Enterprise Edition
JavaBeans
Mobile Java

Question 4

Create a simple java program.

Answer 4

class HelloJava {
    public static void main(String[] args){
    System.out.println("My first Java program");
}
}

Description :-
Public - access modifier, visible to all
Static - no need to create object to invoke static method
Void - return type.
Main - starting point
System.out.println - print statement

Question 5

Naming Conventions in Java

Answer 5

The class name should be in CamelName
Example:- PayPal, iPhone, MasterCard.

The method name should be in mixed case
Also method name should be an verb
Example :- showString(), main(),
goToHelp().

Variable name should not begin with digits

We cannot use keywords for class, method and variable name
Example:- Cannot use keywords like
public, private, void, static and many
more.

Question 6

Install Eclipse on Ubuntu

Install Eclipse on Redhat

Answer 6

fetch software update
-> sudo apt-get update

install software
-> sudo apt-get install eclipse
Ubuntu - Similar OS
Debian
Kubuntu & Xubuntu

fetch software list
->sudo yum upate

install software
->sudo yum install eclipse

Redhat - Similar OS
Fedora
Centos
SUSE Linux

Question 7

—-Introduction to Eclipse—-

What is Eclipse?

Answer 7

Eclipse is a Integrated development environment. One can write, debug and run Ava program with ease

Question 8

Possible Errors

Answer 8

Missing semicolon
 Missing double quotes
Mis-match file name and class name
Typing print statement in lower case

Question 9

Programming Features Used in

Eclipse IDE

Answer 9

Auto completion
Syntax error highlighting
Error dialog box
Shortcut keys

Question 10

Data types in Java

Answer 10

There are in total 8 data types defined in Java programming known as the primitive type

Numerical data type
int, float

Question 11

Arithmetic Operations

Answer 11

Addition : +
Subtraction : -
Multiplication : *
Division : /
Modulo Operation(Remainder after division) : %

Question 12

String
Create strings
Add strings
Convert to lower case
Convert to upper case

Answer 12

String in Java, is a sequence of
characters

public class StringDemo {
    public static void main(String agrs[]){

        String greet = "HeLlO";
        String name = "JaVa";
        char SPACE = ' ';
        greet = greet.toLowerCase();
        name = name.toUpperCase();
        String msg = greet + SPACE + name;
        System.out.println(msg);
    }
}

Output :-
hello JAVA

Question 13

Type Conversion

Answer 13

Converting one data type to another

Question 14

Type Conversion

Answer 14

Converting one data type to another

Java facilitates type conversion in two forms:
Implicit (Automatic) Type Conversion
Explicit Type Conversion

Question 15

Implicit type conversion

Answer 15

Also called as widening conversion because the compiler converts the value of (smaller size) type to the boarder(large size)

byte ——— is convertible to ————-> short, int, long, float, or double
Short ——— is convertible to ————-> int, long, float, or double
char ——— is convertible to ————-> int, long, float, or double
int ——— is convertible to ————-> long, float, or double
long ——— is convertible to ————-> float or double
float ——— is convertible to ————-> double

Question 16

Explicit type conversion

Answer 16

It is also called as narrowing conversion(narrowing of broader data type to smaller data type). The type casting is done manually by the programmer and not by the compiler

Question 17

Integer toString(int i)
Float toString() Method

Answer 17

Java toString() method is a part of the Integer class of the java.lang package
used to convert integer value into String.

Syntax:-
Integer.toString(a)

Java toString() method is a part of the float class if the java.lang package
used to convert float value to string

Syntax
a.toString()

Question 18

Relational operators

Answer 18

Used to check the condition
The size pf boolean data type is 1 bit
It stores only two values(true/false)

Question 19

Logical operators

Answer 19

They are used to check multiple conditions

and &&
or ||
not !

Question 20

Conditional Statements

Answer 20

Control the flow of execution of the program

Types:
If : single conditional statement. Executes a block of statements

If-else : Single condition. Executes alternatives statements

if else if : Multiple conditions. Executes various sets of statements. Called as branching or decision making statement

nested if : An if statement inside another if statement

switch

Question 21

Rules for conditional statement, ternary operator

Answer 21

Add a semicolon for terminating the sentence
Do not add colon after the condition
Add block of code within curly braces

ternary operator : conditional operator providing similar results as if else statement. Short syntax and denoted by ?. Takes three operands at a time.

Syntax :-
Result = Expression ? operand 1 : operand 2

Question 22

Arrays

Answer 22

Marks: 15, 12, 13, 11, 12, 10
Names: Arun, Bala, David
Temperatures: 33.2, 31.7. 32.3
Rainfall: 25, 31, 29, 13, 27, 3

An array is a collection of similar data types

Question 23

Array Operations
Arrays.toString
copyOfRange() method

Answer 23

import java.utiil.Arrays;

Arrays.toString
Converts any datatype of of array to string
–> Arrays.toString(var_name)

copyOfRange() method
This method ensures that the continuity of of ranges is maintained. It receives a copy of array from starting index and the ending index of the array

(0,4) implies from index 0 to 3

int[] array = {1,2,3,4,5,6,7};
int[] array1 = Arrays.copyOfRange(array , 3,5);
for(int num : array1)
System.out.println(num);

–> 4,5

Question 24

Array sort()
Array fill()
Array copyOf()
Arrays.equals()

Answer 24

Array sort() method
Sort  method of array class. It accepts array and sorts them in ascending order. We can extend this feature by providing start_index and end_index

Arrays.sort(arr);

Array fill() method
fill() method of array class. We can assign a particular value to all array elements or in a specified range

Arrays.fill(arr, 5);

Array copyOf()
Similar to copyOfRange() method

Arrays.copyOf(arr, 4);

Arrays.equals() array
Compare two array by comparing each elements of the array.

Question 25

``` A class in real world A class in Java Structure of a Java Class ```

Answer 25

1. Whatever we can see in the world are all objects All the object can be categorized into special groups Each group is termed as class Example :- class of human being objects :- different people Properties :- Eyes, hands, legs Behaviors: seeing, eating, walking 2. Class is a blueprint, from which objects are created 3. Set of properties :- variables set of behaviors :- methods

Question 26

Members of a class What is object Reference Variables

Answer 26

Methods Fields/Variables ``` 2. Object is instance of class It stores its state in fields/variables It exposes its behavior through methods ``` 3. Variables that refer to objects is called as reference variables

Question 27

Instance Fields

Answer 27

Objects store their individual state in instance fields They are declared without the static keyword Non static fields are also known as instance variables or instance fields Accessing the fields using dot operator ``` Instance variables are variables that are declare inside a class but outside any method, class Student { String name; int age; } ```

Question 28

Java method

Answer 28

It is a collection of statements that performs a specific task

Question 29

Java constructor | Difference between constructor and method

Answer 29

``` Constructor is used to initialize instance variables. It is called creation of object. Constructor has the same name as the class name Example:- class Test{ Test(){ //constructor of the body } } ``` Constructor - no return type Method - return type Constructor - it is called using new operator Method is called using dot operator

Question 30

Parameterized constructor

Answer 30

A constructor that has parameter is called parametrized constructor. It can have more than one parameter When we call a parameterized constructor, values are passed to it, they are known as arguments

Question 31

this keyword

Answer 31

Within the constructor, this is the reference to the current object. It helps to avoid name conflicts between local variables and instance variables This is called explicit constructor invocation.

Question 32

Non static keyword | When is a non-static block executed?

Answer 32

Any code written between two curly brackets is a non-static block. ``` When is a non-static block executed? Non-static keyword is executed for each object that is created It executed before the constructor execution It can initialize instance members of the class Any other execution like calculation could also be given in the block ```

Question 33

What is Constructor Overloading?

Answer 33

Define multiple constructors for a class. | They must differ in numbers or types of parameters.

Question 34

What is method overloading

Answer 34

Define two or more methods with same name within a class. They must differ in number or types of parameters. These methods are called overloaded methods. The process is called method overloading.

Question 35

What is bufferReader? | How to make use of BufferedReader?

Answer 35

Class in Java which is used to read text from input stream Provides efficient way to read characters, array of characters and lines Import three classes 1. IO exception 2. InputStreamReader 3. BufferedReader ``` Syntax:- Syntax I InputStreamReader isr=new InputStreamReader(System.in); BufferedReader br= new BufferedReader(isr); ```

Question 36

Subclassing | Private member in subclass

Answer 36

``` It is a way to create a new class from existing class. The new class created is subclass, child class or derived class The already existing class is called super class or base class or parent class Subclass cannot directly access the private members of the superclass ```

Question 37

Superclass

Answer 37

It can have protected or private methods These methods can access private methods The subclass can also access private fields through these methods

Question 38

Method overriding Annotations

Answer 38

A method in the subclassis said to override the method in the parent class if: name return type argument list matches exactly ``` Annotations start with at sign character(@) provide data about a program have no direct effect on the operation of the code ``` ``` Override Annotation If a method is annotated with @Override, compiler generates error if: the method does not override a method declared in its superclass the method signature is different in its superclass ```

Question 39

Super keyword

Answer 39

A subclass can use a superclass data or method using the super keyword ``` The super keyword: refers to the instance variable of the parent class is used to invoke parent class constructor is used to invoke parent class method ```

Question 40

What is final keyword?

Answer 40

final is a keyword or reserved word in Java It can be applied to variables, methods or classes final variable is a variable whose value cannot be changed It will be a constant ``` final variable as static static final variables cannot be initialized in a constructor. They must be assigned a value with their declaration They must be declared in a static block Static variables are shared among all the objects of a class Creating a new object would change the static variable This is not allowed if the static variable is final ``` ``` final and private method private methods are not inherited by the child class So we can add a method getDetails() in the child class ```

Question 41

What is Polymorphism?

Answer 41

Polymorphism is an ability of an object to take on many forms Advantages of Polymorphism Reduction of complexity Code re-usability Types of Polymorphism Compile-time or Method Overloading or Static Binding Run-time polymorphism or Method Overriding or Dynamic Binding