Organizational units of an OOP program

Tuesday, Jul 6, 2021
Design Java

We will use Java as a medium to see how an object-oriented program is typically organized.


These are the valid words and punctuation marks that make up a language. For example, in english, hello is a word but lkjdlf is not. Similarly in Java, public is a valid token but \ by itself is not. In this sense, tokens are the most fundamental building blocks of a program.


Expressions are the core of any programming language and are combinations of various tokens like operators, variables and values. For example, in Java, 2 + 3 is a valid expression but + 2 3 is not. Expressions are evaluated by the language to get a value the type of which is also the type of expression. We can think of expressions as phrases.


If expressions can be thought of as phrases then statements are complete sentences. A statement contains expressions and perform basic actions in a program. There are various kinds of statements:


Methods are named sequence of statements that can be run as often as needed by calling the method.


A class is a collection of related methods. Everything resides inside a class in Java. In the example below, we use methods from two different classes - Scanner and System and create a new class - Convert3


A package is a collection of related classes. In the example below, we make use of a class from a package called java.util. As the name suggests, this package might contain utility classes.

An example program

The following example program illustrates all the units discussed above.

// import statement
// import class(es) from other packages
import java.util.Scanner;

// create a new class
public class Convert3 {
  // create a new method, main, inside class Convert3
  public static void main(String[] args) {
     * Converts temperature in degree Celcius to temperture in Kelvin.
     * */
    // declaration statement
    // declare new variables of type double
    double celcius, kelvin;
    // declare a new variable of type Scanner
    Scanner in = new Scanner(;
    // print statement
    System.out.print("How many degreec C? ");
    // assignment statement
    // read one double value using a method of Scanner class
    // update the stored value of celcius to new value
    celcius = in.nextDouble();
    // another assignment statement
    // compute expression on the right hand side
    // update the stored value of kelvin to new value
    kelvin = celcius + 273.15;
    // print statement
    System.out.printf("%.2f C = %.2f K\n",
                      celcius, kelvin);