Java Cheatsheet

Java is a very popular programming language. Java can be used to develop anything and almost everything like web applications, web servers, application servers, mobile applications and so on.

Basics

Sample Java program

import java.util.Date;

public class HelloWorld {
    public static void main(String[] args) {
        Date now = new Date();
        System.out.println("Hello World!");
        System.out.println("now: " + now);
    }
}

• import – used to import java libraries
• class keyword – to declare a class in java.
• public keyword – access modifier represents visibility. Public is visible to all.
• static keyword – no need to create an object to invoke the static method. There is no need to create an object to invoke the main method and it’s invoked by JVM and saves memory.
• void – return type of the method and it doesn’t return any value.
• main – starting point of the program.
• String[] args – for command line arguments
• System.out.println() – used to print statement.

To Compile a Java program

Go to Command prompt and navigate to the folder where java files are stored.

javac example.java

To Run Java program

java example 

Comments

// – single line comment

Data types

Two groups of data types

1. Primitive data types

Examples

int a = 99999; 
short b = 999; 
long c = 99999999999L;
byte d = 99;
double e = 99.99d;
boolean f = true;
char g = 'A';

2. Non-Primitive data types

Non-primitive data types specifies the complex data values. For example, strings, arrays and classes can be referred as Non-primitive data types.

Variables

data-type variable-name = value;

Naming convention of variables

• Case sensitive.
• Must should begin with a lower case letter. For second word should be a capital like this: firstName, pinCode etc.
• Cannot contain white spaces like first name.
• special characters like $ and _ can be used to begin a variable name.

Types of Variables

• Local Variables
• Instance Variables
• Static Variables

Class Sum {
    int n1 = 10; // value of Instance  Variables are are instance specific.
    static int n2 = 20; //static variable is Common for all the instances of the class.
    void sum(){
        int n3 = 30; //local variable and they are Inside the body of a method.
        int total = n1+n2+n3;
    }
}

Data Conversions

• String to Numberint i = Intege­r.p­ars­eIn­t(­str); double d = Double.pa­rse­Dou­ble­(s­tr);
• Other types to StringString str = String.va­lue­Of(­va­l);

Operators

Arrays

Array is a collection of similar data which is stored in continuous memory addresses.

data-type[] array-name; // one dimensional array
data-type[][] array-name; // two dimensional array

Example

String[] mobiles = {"iPhone", "Samsung", "OnePlus"};
for (int i = 0; i < mobiles.length; i++) {
  System.out.println(mobiles[i]);
}
mobiles[1] = "Oppo"; // change an array element

Conditional Statements

1. If

if(conditional-expression)
{
    //code
}

2. If-else

if(conditional-expression)
{
    //code
} else {
    //code
}

3. If-else-if ladder

if(conditional-expression-1)
{
    //code
} else if(conditional-expression-2) {
    //code
} else if(conditional-expression-3) {
    //code
}
....
else {
    //code
}

4. Nested-If

if(conditional-expression-1) {    
     //code    
          if(conditional-expression-2) {  
             //code
             if(conditional-expression-3) {
                 //code
             }  
    }    
}

5. Switch

switch(conditional-expression){    
case value1:    
 //code    
 break;  //optional  
case value2:    
 //code    
 break;  //optional  
...    
    
default:     
 //code to be executed when all the above cases are not matched;    
} 

Loops

1. For

for(Initialization; Condition; Increment/decrement){  
//code  
} 

2. While

while(condition){  
//code 
}  

3. Do-While

do{  
//code 
}while(condition); 

Collections

String Methods

OOPS

1. Class

Class is the blueprint of an object and class keyword is required to create a class.

class class_name {  
    fields;  
    methods;  
}

2. Object

Object is a basic unit in OOP, and is an instance of the class.

class_name obj_name;

3. Abstraction

Data abstraction is a technique which provides only the required data to be visible or accessible to outside world.
abstract keyword is used for classes and methods.

abstract class Mobiles { // abstract class
  public abstract void features();   // abstract method
  public void method-name() {
    //code
  }
}

4. Encapsulation

Encapsulation is a mechanism to protect private hidden from other users. It wraps the data and methods as a single bundle. private is the keyword used to declare the variables or methods as private. You can make public set and get methods to access private variables.

5. Polymorphism

Polymorphism gives the meaning many forms, usually it occurs when multiple classes are present and have been inherited.

class Child-class extends Parent-Class {
//code
}

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