Instance Variable in Java

Let’s Start with Something Simple

Vehicle.java file in folder path: K:\Instance Variable

import model.Bike;  import java.util.List;  import java.util.ArrayList;    public class Vehicle {  	public static void main( String[] args ){    		List<Bike> bikes = new ArrayList<>();  		Bike bike1 = new Bike();  		bikes.add(bike1);  		Bike bike2 = new Bike();  		bikes.add(bike2);  		Bike bike3 = new Bike();  		bikes.add(bike3);    		for(Bike v : bikes) {  			System.out.println(v.name);  		}    	}  }

Bike.java file in folder path: K:\Instance Variable\model

package model;    public class Bike {    	public String name = "Honda";  	public long color = 0x2E0456;  	public int gearOrNot = 1;    }

Execution with errors:

• javac Vehicle.java
  • Vehicle.java:1: error: cannot access Bike
  • import model.Bike;
  • ^
  • bad class file: .\model\Bike.class
  • class file contains wrong class: Bike
  • Please remove or make sure it appears in the correct subdirectory of the classpath.
• K:\Instance Variable>cd model
• K:\Instance Variable\model>javac Bike.java
• K:\Instance Variable\model>cd ..
• K:\Instance Variable>javac Vehicle.java
  • Vehicle.java:16: error: incompatible types: Bike cannot be converted to Vehicle                                                         for(Vehicle v : bikes) {                                                                                                                ^
  • Vehicle.java:17: error: cannot find symbol                                                                                                      System.out.println(v.name);                                                                                                                 ^
  • symbol:   variable name
  • location: variable v of type Vehicle
  • 2 errors
• K:\Instance Variable>javac Vehicle.java
• K:\Instance Variable>java Vehicle
• Honda
• Honda
• Honda

Notes

• We need to compile Bike.java first to prepare Bike.class
• Writing ‘package model;’ is a must as it shows the relative package path.
• Bike instance has the instance variable ‘name’ and not instance of Vehicle

Let’s increase the complexity

Vehicle.java file in folder path: K:\Instance Variable

import model.Bike;  import java.util.List;  import java.util.ArrayList;    public class Vehicle {  	public static void main( String[] args ){    		List<Bike> bikes = new ArrayList<>();  		Bike bike1 = new Bike();  		bikes.add(bike1);  		Bike bike2 = new Bike();  		bikes.add(bike2);  		Bike bike3 = new Bike();  		bikes.add(bike3);    		VehicleTypes vt = new VehicleTypes();    		int noOfWheels = vt.getWheels(bikes);  		System.out.println("Number of wheels: " +noOfWheels);  	}  }

 VehicleTypes.java file in folder path: K:\Instance Variable

//FOr demonstrating instance method  import java.util.List;  import model.Bike;    public class VehicleTypes {  	public int getWheels(List<Bike> bikes) {    		for(Bike v : bikes) {  			System.out.println(v.name);  		}    		return 2;  	}  }

Output

• K:\Instance Variable>javac VehicleTypes.java
• K:\Instance Variable>javac Vehicle.java
• K:\Instance Variable>java Vehicle
• Honda
• Honda
• Honda
• Number of wheels: 2

Notes

• VehicleTypes now has the method to deal with bikes object.
• We need to import bike in VehicleTypes.
• We create instance of VehicleTypes which helps us deal with bikes object. This reduces code in static void main method.
• 3 things are to be noted:
  • Bike.java has the data
  • VehicleTypes has the logic to process the data
  • Vehicle.java creates the data and passes it to the VehicleTypes instance for processing.

Do It Yourself

• Bike.java has data set to public which breaks the rules of encapsulation.
• Make the data of Bike.java private and use Getters and Setters as the only means to access the data.