Java Tutorial : Class Structure

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Understanding Java Class Structure: A Beginner's Guide

In object-oriented programming with Java, everything revolves around classes and objects. A class is essentially a blueprint or template from which individual objects are created. If you are starting your Java programming journey, understanding the fundamental structure of a class is your first key milestone.

1. Core Components of a Java Class

Every Java class is defined using the class keyword followed by the class name. Within the curly braces { }, a class primarily consists of two elements:

• Variables (Attributes / Properties / State): These hold data representing the characteristics of an object.
• Methods (Functions / Behaviors / Actions): These define the operations or tasks that an object can perform.

2. Example: Creating a Person Class

Let's look at a practical demonstration. Suppose we want to model a Person:

public class Person {
    // Variables (Attributes)
    String name;
    int age;

    // Method (Behavior)
    public void displayMessage() {
        System.out.println("I am " + name + ", I am " + age + " years old.");
    }
}

3. The Role of the main() Method

A Java application can contain multiple classes, but at least one class must contain the main() method. When your program executes, the Java Virtual Machine (JVM) searches for this entry point to begin execution.

The standard syntax is:

public static void main(String[] args)

4. Instantiating Objects and Using the Dot Operator

To bring a class to life, you create an instance (object) using the new operator. Once created, you access its variables and invoke its methods using the dot (.) operator.

public class PersonDemo {
    public static void main(String[] args) {
        // Creating an object of class Person
        Person peter = new Person();

        // Accessing variables using dot operator
        peter.name = "Peter";
        peter.age = 53;

        // Calling method using dot operator
        peter.displayMessage();
    }
}

5. Expected Program Output

I am Peter, I am 53 years old.

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Java Tutorial : Class Structure

Java Tutorial : Class Structure

Java Tutorial : Class Structure

Java Tutorial : Class Structure

Java Tutorial : Class Structure

Person.java

/**
 * Person class is the blueprint from which individual Person objects can be
 * created.
 */
public class Person
{
    String name;
    int age;

    public void displayMsg()
    {
        System.out.println("I am " + name);
        System.out.println("I am " + age + " yrs. old");
    }
}

PersonDemo.java

public class PersonDemo
{
    public static void main(String[] args)
    {
        Person peter = new Person();
        /*
         * Setting Attributes/Properties/Characteristics.
         */
        peter.name = "Peter";
        peter.age = 53;

        peter.displayMsg();
    }
}

Output

I am Peter
I am 53 yrs. old

To Download PersonDemoStructure Project Click the below link:
https://sites.google.com/site/javaee4321/java/PersonDemoStructure.zip?attredirects=0&d=1

See also:

• All JavaEE Videos Playlist
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• Servlets Tutorial
• All Design Patterns Links
• JDBC Tutorial
• Java Collection Framework Tutorial
• JAVA Tutorial