Object Oriented programming is a methodology based on objects instead of functions and procedures. OOP provides liberty to users to create objects as they want and create methods to handle the objects created. Like in Java, Groovy also supports OOPs principles.

1. Classes and objects

Object is a basic unit in OOP, and is an instance of the class. They have state and behaviour. Consider mobile as an object and let's see an example to understand state and behaviour.

State of the objects can be represented by variables and behaviour of the objects can be represented by methods. Object is a real-world or run-time entity.

Object : Mobile

State : Model, Brand, Color, Size

Behaviour : Make a call, send a message, Click a picture

Class is the blueprint of an object, which is also referred as user-defined data type with variables and functions. Class is a logical entity.

Class can contain:

• Fields or variables
• Methods
• Constructors
• Nested class and interfaces

How to create a Class

class keyword is required to create a class.

class class_name {  
    fields;  
    methods;  
}

Example

class mobile {
    public:    // access specifier which specifies that accessibility of class members 
    string name; // string variable (attribute)
    int cost; // int variable (attribute)
};

How to create a Object

mobile m1;

How to define methods in a class

public class greeting {
    static void hello(){
        println("Hello.. Happy learning!!");
    }

    public static void main(String[] args) {
    hello();
    }
}

Practice with more examples here

2. Inheritance

Inheritance is a process of acquiring parent properties and behaviour by a child automatically. Hence you can either reuse or extend parent properties or behaviour in the child class. The main advantage of inheritance is code reusability. Use the keyword extends to inherit from a class.

Type of Inheritance

• Single inheritance

  When a child inherits properties from a single parent

• Multiple inheritance

  When a child inherits its properties from two or more parents.

• Hierarchical inheritance

  When multiple children inherit properties from a parent

• Multilevel inheritance

  When a parent properties are inherited by a child which again get interited by another child. For example, B is inherited from A and C is inherited from B.

  A -> B -> C

• Hybrid inheritance

  Hybrid inheritance is a combination or two or more inheritance types.

3. Encapsulation

Encapsulation is a mechanism to protect private data 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 use public set and get methods to access private variables.

EmployeeDetails o = new EmployeeDetails();
o.setname("foo");
o.setempId(12345);
o.setsalary(20000);
println("Employee Name: " + o.getname());
println("Employee ID: " + o.getempId());
println("Employee Salary: " + o.getsalary());
 
class EmployeeDetails{
    private String name;
    private int empId;
    private int salary;

    //Getter and Setter methods
    public int getempId(){
        return empId;
    }

    public String getname(){
        return name;
    }

    public int getsalary(){
        return salary;
    }

    public void setempId(int newId){
        empId = newId;
    }

    public void setname(String newName){
        name = newName;
    }

    public void setsalary(int newSalary){
        salary = newSalary;
    }
}

Check result here

4. Polymorphism

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

Consider an example of installing an application in your mobile, where your base class is Mobile and method is installation() and its derived classes could be installApp1, installApp2, installApp3 etc which will have its own implementation but installation method can be common which can be inherited from its base class.

class News {
  public void alerts() {
    println("News Alerts");
  }
}

class Sports extends News {
  public void alerts() {
    println("subscribe Sports News");
  }
}

class Entertainment extends News {
  public void alerts() {
    println("Subscribe for Entertainment News");
  }
}

class Politics extends News {
  public void alerts() {
    println("Subscribe for politics alerts");
  }
}


News myNews = new News(); 
Sports mySports = new Sports();
Entertainment myEntertainment = new Entertainment();
Politics myPolitics = new Politics();
myNews.alerts();
mySports.alerts();
myEntertainment.alerts();
myPolitics.alerts();

Practice with more examples here