JAVA OOP CHEAT SHEET
Object Oriented Programming in Java Inheritance
Java is an Object Oriented Programming language
that produces software for multiple platforms. An
object-based application in Java is concerned with
declaring classes, creating objects from them and
interacting between these objects.
//Declaring and Initializing an object
Test t = new Test();
Java Object
Modifiers in Java
Class A {
//your parent class code
}
Class B extends A {
//your child class code
}
Single Inheritance
Class A {
//your parent class code
}
Class B extends A {
//your code
}
Class C extends B {
//your code
}
Multi Level Inheritance
Class A {
//your parent class code
}
Class B {
//your parent class code
}
Class C extends A,B {
//your child class code
}
Multiple Inheritance
class Test {
// class body
member variables
methods
}
Java Class
Constructors
class Test{
/* Added by the Java Compiler at the Run Time
public Test(){
}
*/
public static void main(String args[]) {
Test testObj = new Test();
}
}
Default Constructor
public class Test {
int appId;
String appName;
//parameterized constructor with two parameters
Test(int id, String name){
this.appId = id;
this.appName = name;
}
void info(){
System.out.println("Id: "+appId+" Name: "+appName);
}
public static void main(String args[]){
Test obj1 = new Test(11001,"Facebook");
Test obj2 = new Test(23003,"Instagram");
obj1.info();
obj2.info();
}
}
Parameterized Constructor
Scope Private Default Protected Public
Same class Yes Yes Yes Yes
Same package subclass No Yes Yes Yes
Same package non-subclass No Yes Yes Yes
Different package subclass No No Yes Yes
Different package non
-
subclass
No No No Yes
Access Modifiers
Type Scope
Static Makes the attribute dependent on a class
Final Once defined, doesn’t allow any changes
Abstract Makes the classes and methods abstract
Synchronized Used to synchronize the threads
Non - Access Modifiers
J ava C e r ti f i c a t io n
T r a i n i n g
Class A {
//your parent class code
}
Class B extends A {
//your child class code
}
Class C extends A {
//your child class code
}
Hierarchical Inheritance
A A
/ \ |
B C (OR) B
/ \ / \
D E C D
Hybrid Inheritance
Polymorphism
class Calculator {
static int add(int a, int b){
return a+b;
}
static double add( double a, double b){
return a+b;
}
public static void main(String args[]){
System.out.println(Calculator.add(123,17));
System.out.println(Calculator.add(18.3,1.9));
}
}
Compile Time Polymorphism
public class Mobile{
void sms(){System.out.println("Mobile class");}
}
//Extending the Mobile class
public class OnePlus extends Mobile{
//Overriding sms() of Mobile class
void sms(){
System.out.println(" OnePlus class");
}
public static void main(String[] args) {
OnePlus smsObj= new OnePlus();
smsObj.sms();
}
}
Run Time Polymorphism
Abstraction
public abstract class MyAbstractClass
{
public abstract void abstractMethod();
public void display(){
System.out.println("Concrete method");
}
}
Abstract Class
//Creating an Interface
public interface Bike { public void start(); }
//Creating classes to implement Bike interface
class Honda implements Bike{
public void start() {
System.out.println("Honda Bike");
} }
class Apache implements Bike{
public void start() {
System.out.println("Apache Bike");
} }
class Rider{
public static void main(String args[]){
Bike b1=new Honda();
b1.start();
Bike b2=new Apache();
b2.start();
} }
Interface
public class Artist {
private String name;
//getter method
public String getName() { return name; }
//setter method
public void setName(String name) { this.name = name; }
}
public class Show{
public static void main(String[] args){
//creating instance of the encapsulated class
Artist s=new Artist();
//setting value in the name member
s.setName(“BTS");
//getting value of the name member
System.out.println(s.getName());
}
}
Encapsulation
