# Pass by value or pass by reference ?

Paul Tomoiu

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Examples,

Concepts

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Published 2024-01-02

# Explanation

Generally speaking, the parameters could be passed to the called method in 2 ways:

Pass-by-reference: When a method is called, the method arguments reference the same variable in memory as the caller.
Pass-by-value: When a method is called, the caller passes a copy of the argument variables to the method (we will have 2 values/copies in memory).

Info

In Java, the parameters are always passed by-value. However, for objects, the "values" are references, so the behaviour under the scene is as using Passing-by-reference.

# Example - passing by value

Fot the primitives and Wrapper Classes (Integer, String, etc) the behaviour under the hood is as for passing by value. We have below a short example.

Let's consider a simple class named "MyClass".

package com.example.demo;

public class MyClass {
   public Integer myInteger;

    public MyClass(Integer myInteger) {
        this.myInteger = myInteger;
    }
    void multiplyBy(Integer val) {
        this.myInteger = this.myInteger * val;
    }
}

and a main class which use this class:

package com.example.demo;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class Demo1Application {

  public static void main(String[] args) throws InterruptedException {
    SpringApplication.run(Demo1Application.class, args);

    Integer myInteger1 = 2;
    MyClass myClass = new MyClass(myInteger1);

    myClass.multiplyBy(100);

    System.out.println("-------------------------");
    System.out.println("myInteger1= " + myInteger1);
    System.out.println("myClass.myInteger= " + myClass.myInteger);
    System.out.println("-------------------------");
  }
}

When we run the main class we will see:

-------------------------
myInteger1= 2
myClass.myInteger= 200
-------------------------

# Example - passing a Class as parameter (by reference)

Let's consider a simple class named "MyClass".

For classes
package com.example.demo;

public class MyClass {
    Car car;

    public MyClass(Car car) {
        this.car = car;
    }
    void modifyColor(String color) {
        car.setColor(color);
    }
}

and a main class which use this class:

package com.example.demo;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class Demo1Application {

    public static void main(String[] args) throws InterruptedException {
        SpringApplication.run(Demo1Application.class, args);

		System.out.println("-------------------------");

		Car car1 = new Car("green");
		System.out.println("car1 = " + car1);
		System.out.println("car1.color = " + car1.color);

		MyClass myClass = new MyClass(car1);
        myClass.modifyColor("yellow");

		System.out.println("-------------------------");
		System.out.println("car1 = " + car1);
		System.out.println("car1.color = " + car1.color);
		System.out.println("-------------------------");

    }
}

When we run the main class we will see:

-------------------------
car1 = com.example.demo.Car@70925b45
car1.color = green
-------------------------
car1 = com.example.demo.Car@70925b45
car1.color = yellow
-------------------------

Info

So, under the hood, when the myClass object is created, it will contain a pointer to the car1 object. All the operations inside this object are done on the same instance used by the main class.
This could be demonstrated by printing the hashed value of the address memory of the object.

# Example - passing an Array as parameter (by reference)

Fot the Arrays the behaviour is the same as for the Classes.

Let's consider a simple class named "MyClass" as below:

package com.example.demo;

public class MyClass {
    int[] myArray;

    public MyClass(int[] arr) {
        this.myArray = arr;
    }
    void modifyArray(int valueIn) {
        this.myArray[0] = valueIn;
    }
}

and a main class which use this class:

package com.example.demo;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class Demo1Application {

  public static void main(String[] args) throws InterruptedException {
    SpringApplication.run(Demo1Application.class, args);

    System.out.println("-------------------------");

    int[] myArray1 = {10, -7, -8, 3, 2, 0, -1};

    System.out.println("myArray1 = " + myArray1);
    System.out.println("myArray1[0] = " + myArray1[0]);

    MyClass myClass = new MyClass(myArray1);
    myClass.modifyArray(200);

    System.out.println("-------------------------");
    System.out.println("myArray1 = " + myArray1);
    System.out.println("myArray1[0] = " + myArray1[0]);
    System.out.println("-------------------------");

  }
}

When we run the main class we will see:

-------------------------
myArray1 = [I@46cc127b
myArray1[0] = 10
-------------------------
myArray1 = [I@46cc127b
myArray1[0] = 200
-------------------------

# Example - passing a Collection as parameter (by reference)

Fot the Arrays the behaviour is the same as for the Classes.

Let's consider a simple class named "MyClass".

package com.example.demo;

import java.util.ArrayList;

public class MyClass {
    ArrayList<String> myList;

    public MyClass(ArrayList<String> myList) {
        this.myList = myList;
    }
    void addElement(String element) {
        this.myList.add(element);
    }
}

and a main class which use this class:

package com.example.demo;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

import java.util.ArrayList;

@SpringBootApplication
public class Demo1Application {

    public static void main(String[] args) throws InterruptedException {
        SpringApplication.run(Demo1Application.class, args);

		System.out.println("-------------------------");

		ArrayList<String> carList = new ArrayList<>();
		
		carList.add("Audi");
		carList.add("Renault");

		System.out.println("carList = " + carList);
		System.out.println("carList.size = " + carList.size());

		MyClass myClass = new MyClass(carList);
		myClass.addElement("Ford");

		System.out.println("-------------------------");
		System.out.println("carList = " + carList);
		System.out.println("carList.size = " + carList.size());
		System.out.println("-------------------------");
    }
}

When we run the main class we will see:

-------------------------
carList = [Audi, Renault]
carList.size = 2
-------------------------
carList = [Audi, Renault, Ford]
carList.size = 3
-------------------------