Object

Why Objects?

• Object encapsulates data with functions that operate on the data.

Introduction

• An object consists of named members,
  • field - stores a value
  • method - a function that can be invoked on the object
• When a method is invoked on an object, the function can access the object using the self variable.
• It is not possible for an object to have a field and a method with the same name.
• Need to use the new operator with a class to get an object.
• There are three visibility levels.
  • private - only visible within the object and its functions.
  • public - visible to any module.
  • protected - visible only within the same module.

Init Function

• The init method in the class initializes the object.
• Arguments to new are passed as arguments to init.
• The return type of init must be a subtype of error?.
  • If init returns (): The new returns the newly constructed object.
  • If init returns an error : The new returns that error.
  • If init does not specify a return type:
    • the return type defaults to ().
    • Then new will never return an error.

Sample

import ballerina/io;

class Customer {
    private string firstName;
    string lastName;
    public int id;

    function init(string firstName = "A", string lastName = "B", int id = 0) {
        self.firstName = firstName;
        self.lastName = lastName;
        self.id = id;
    }

    function getFullName() returns string {
        return self.firstName + " " + self.lastName;
    }
}

public function main() {
    //Calls init function with all parameters
    Customer cust1 = new ("Anupama", "Pathirage", 10032);
    string fullName = cust1.getFullName(); //Access  a method
    io:println(fullName);

    //Default values are used 
    Customer cust2 = new;
    int id = cust2.id; // Access a public id;
    io:println(id);

    //This inintialization is useful when the type of the object 
    //cannot be determined based on nthe context(e.g., when the LHS is var or union)
    var cust3 = new Customer("Bill", "Gates", 10031);
    string lastName = cust3.lastName;
    io:println(lastName);
}

Output

Anupama Pathirage
0
Gates