The Truth About Java’s Pass-by-Value Mechanism

Primitive type data is passed by value and objects are passed by reference. But wait a minute, what happens when the references are passed in the method calls? In this article, we will figure out the Truth About Java’s Pass-by-Value Mechanism.

Is it Pass by reference or Pass by Value?

In Java, when object references are passed, they are also passed as value. Hence, in a nutshell, we can say that Java is always “pass by value“. It could be confusing for the newbies but take a good look and you will understand what it means. Let us check it with an example code –

Sample Code

public class JavaPassByValueOrReferenceCheck {

	public static void main(String[] args) {
		Car car = new Car("Volkswagen");

		System.out.println(car.getName()); // Output::Volkswagen

		tryToChangeReference(car);

		System.out.println(car.getName()); // Output::Volkswagen

		tryToModifyReference(car);

		System.out.println(car.getName()); // Output::Hyundai

	}

	public static void tryToChangeReference(Car dc) {
		Car anotherCar = new Car("Ford");
		System.out.println(anotherCar.getName()); // Output::Ford

		dc = anotherCar; // DC started pointing to another car

		System.out.println(dc.getName()); // Output::Ford

		// Any change we do to dc, it will impact the data of anotherCar Object
		dc.setName("Innova");
		System.out.println(dc.getName()); // Output::Innova
		System.out.println(anotherCar.getName()); // Output::Innova

	}

	public static void tryToModifyReference(Car dc) {
		// dc is still pointing to the Volkswagen car from main method
		dc.setName("Hyundai");
		// Cross Reason to understand this :::x:::
		// If it was pass by reference, the last statement in the main method would have
		// given the output::Innova isn't it?? :)
	}

}

class Car {

	private String name;

	public Car(String name) {
		this.name = name;
	}

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

}

Try to run the above code in debug mode and you will see how it works. Also, you may comment or un-comment on the main method and see the different scenarios

Explanation Video

Still confused? Watch this Debug video.

Further Readings

• https://community.oracle.com/tech/developers/discussion/2110675/pass-by-reference-or-pass-by-value
• https://stackoverflow.com/a/40523
• http://www.javadude.com/articles/passbyvalue.htm
• https://community.oracle.com/tech/developers/discussion/comment/9034101#Comment_9034101

If you still have doubts and want to discuss, feel free to share your viewpoints in the comment section below. 🙂