Instance Variable Example in Java

Introduction

Instance Variable is a classic Java console program that demonstrates the concept with complete source code and sample output. Variables store state in a program. Java distinguishes local, instance and static (class) variables.

This tutorial walks through the program line by line, explains how the logic works, and highlights best practices you can apply in your own code.

Definition

Variables are identifiers associated with values. They are declared by writing the variable’s type and name, and are optionally initialized in the same statement by assigning a value. Instance variables are declared inside a class but outside a method. They can been seen by all methods in the class.

Instance Variable Example Program

public class InstanceVariableDemo{
	public String str1;
		private String str2;
		public InstanceVariableDemo (String empName){
		str1 = empName;
	}
	public void salary(String empSal){
		str2 = empSal;
	}
    public void disp(){
    System.out.println( str1 );
    System.out.println( str2 );
	}
	public static void main(String args[]){
		InstanceVariableDemo obj = new InstanceVariableDemo("This is an instance variable and is visible to all child classes");
		obj.salary("The variable str2 is visible only inside methodOne");
		obj.disp();
	}
}

Sample Output

This is an instance variable and is visible to all child classes
The variable str2 is visible only inside methodOne

When to use

Use this instance variable example when learning or revising core Java syntax.

How it works

1. Execution begins in the main method — the JVM calls this method when you run the class.

2. str1 = empName; updates a variable used in the calculation or output.

3. str2 = empSal; updates a variable used in the calculation or output.

4. A println / print call writes text to the console — part of the sample output below.

5. A println / print call writes text to the console — part of the sample output below.

6. InstanceVariableDemo obj = new InstanceVariableDemo("This is an instance variable and is visible to all child classes"); updates a variable used in the calculation or output.

7. Compare your console output with the sample output for Instance Variable to confirm the program behaves correctly.

Best Practices

• Use meaningful variable and class names that describe their purpose.
• Compile and run the program locally — modify values to see how output changes.
• Read compiler errors carefully; they usually point to the exact line to fix.

Common Mistakes

• Copying code without understanding each line — practice by changing one statement at a time.
• Mismatching the public class name and the .java filename.
• Forgetting semicolons at the end of statements.