Java Programming for Beginners: Become a Java Programmer

What is a variable? Variable is used to store a value that you can use later.Each variable in Java has a specific type, which determines the size and layout of the variable's memory; the range of values that can be stored within that memory; and the set of operations that can be applied to the variable.

Data Types in JAVA

int => A 32-bit (4-byte) integer value e.g. 78, 90,100

short => A 16-bit (2-byte) integer value e.g. 9,10,100

long => A 64-bit (8-byte) integer value e.g. 12312313131,242342342424244

byte => An 8-bit (1-byte) integer value e.g. 1,10,11

float => A 32-bit (4-byte) floating-point value e.g. 1.98f, 56.89f

double => A 64-bit (8-byte) floating-point value e.g. 78.90, 67.56

char => A 16-bit character using the Unicode encoding scheme e.g. 'A','C','X'

boolean => A true or false value e.g. true or false.

You must declare all variables before they can be used. Following are valid examples of variable declaration and initialization in Java:

int a, b, c;// Declares three ints, a, b, and c.
int a = 10, b = 10;// Example of initialization 
byte B = 22;// initializes a byte type variable B. 
double pi = 3.14159; // declares and assigns a value of PI. 
char a = 'a';// the char variable a iis initialized with value 'a'

Java Scanner class: There are various ways to read input from the keyboard, the java.util.Scanner class is one of them.

Commonly used methods of Scanner class

• public String next() => it returns the next token from the scanner.
• public String nextLine()=> it moves the scanner position to the next line and returns the value as a string.
• public byte nextByte() =>it scans the next token as a byte.
• public short nextShort() =>it scans the next token as a short value.
• public int nextInt() =>it scans the next token as an int value.
• public long nextLong() => it scans the next token as a long value.
• public float nextFloat() =>it scans the next token as a float value.
• public double nextDouble() =>it scans the next token as a double value.

If-else Statement

The if-else statement in Java language is used to execute the code if condition is true or false. The syntax of if-else statement is given below:

if(expression){
//code to be executed if condition is true
}else{
//code to be executed if condition is false
}

There may be a situation when you need to execute a block of code several number of times. In general, statements are executed sequentially: The first statement in a function is executed first, followed by the second, and so on.

Programming languages provide various control structures that allow for more complicated execution paths.

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

Java programming language provides the following types of loop to handle looping requirements.

while loop

Repeats a statement or group of statements while a given condition is true. It tests the condition before executing the loop body.

for loop

Execute a sequence of statements multiple times and abbreviates the code that manages the loop variable.

do...while loop

Like a while statement, except that it tests the condition at the end of the loop body

Classes in Java:

A class is a blue print from which individual objects are created.

public class Dog
{
    String breed;
    int age;
    String color;
    void barking(){}
    void hungry(){}
    void sleeping(){} 
}

Objects in Java:

If we consider the real-world we can find many objects around us, Cars, Dogs, Humans, etc. All these objects have a state and behavior.

If we consider a dog, then its state is - name, breed, color, and the behavior is - barking, wagging, running

The idea behind inheritance in java is that you can create new classes that are built upon existing classes. When you inherit from an existing class, you can reuse methods and fields of parent class, and you can add new methods and fields also.

Inheritance represents the IS-A relationship, also known as parent-child relationship.

• For Method Overriding (so runtime polymorphism can be achieved).
• For Code Reusability

Syntax of Java Inheritance

class Subclass-name extends Superclass-name
{
//methods and fields
}

Abstract class in Java

A class that is declared with abstract keyword, is known as abstract class in java. It can have abstract and non-abstract methods (method with body).

Abstraction in Java

Abstraction is a process of hiding the implementation details and showing only functionality to the user.

Another way, it shows only important things to the user and hides the internal details for example sending sms, you just type the text and send the message. You don't know the internal processing about the message delivery.

Abstraction lets you focus on what the object does instead of how it does it.

Ways to achieve Abstaction

There are two ways to achieve abstraction in java

1. Abstract class (0 to 100%)
2. Interface (100%)

Interface in Java

An interface in java is a blueprint of a class. It has static constants and abstract methods only.

The interface in java is a mechanism to achieve fully abstraction. There can be only abstract methods in the java interface not method body. It is used to achieve fully abstraction and multiple inheritance in Java.

Java Interface also represents IS-A relationship.

It cannot be instantiated just like abstract class.

Why use Java interface?

There are mainly three reasons to use interface. They are given below.

• It is used to achieve fully abstraction.
• By interface, we can support the functionality of multiple inheritance.
• It can be used to achieve loose coupling.

Java Array

Array is a collection of similar type of elements that have contiguous memory location.

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

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

Advantage of Java Array

• Code Optimization: It makes the code optimized, we can retrieve or sort the data easily.
• Random access: We can get any data located at any index position.

Disadvantage of Java Array

• Size Limit: We can store only fixed size of elements in the array. It doesn't grow its size at runtime. To solve this problem, collection framework is used in java

What is exception ??

Exception is an abnormal condition.

In java, exception is an event that disrupts the normal flow of the program. It is an object which is thrown at runtime.

What is exception handling

The exception handling in java is one of the powerful mechanism to handle the runtime errors so that normal flow of the application can be maintained.

Advantage of Exception Handling

The core advantage of exception handling is to maintain the normal flow of the application. Exception normally disrupts the normal flow of the application that is why we use exception handling.

Let's take a scenario:

statement 1;
statement 2;
statement 3;
statement 4;
statement 5;//exception occurs
statement 6;
statement 7;
statement 8;
statement 9;
statement 10;

Suppose there is 10 statements in your program and there occurs an exception at statement 5, rest of the code will not be executed i.e. statement 6 to 10 will not run. If we perform exception handling, rest of the statement will be executed. That is why we use exception handling in java.