Server Monitoring Authors: Carmen Gonzalez, AppDynamics Blog, Yeshim Deniz, Liz McMillan, Pat Romanski

Blog Feed Post

Java: Passing by Reference With a Twist

Currently I’m teaching a Java class online, and Vitaly O., one of my students, ran into an interesting situation. He sent me the program below, which, to his surprise, printed 1.

public class Main {

    public static void main(String[] args) {
        Integer t = new Integer(0);

    public static void test(Integer t) {


The topic of passing by value vs by reference is one of the difficult topics to understand for Java beginners. The code above can confuse junior Java developers, because the author of the program above ran into a bouquet of Java features, which I’ll explain (slowly) in this blog.

All Java textbooks (mine included) will tell you that objects are being passed by reference and primitives by value. That’s fine and understandable – nobody wants to copy large objects in memory . But what about the variables that point at an object and are being passed to a method as arguments like in test(t) line above?
First, let’s take care of a simpler case. Let’s replace the Integer with the class Car that looks like this:

public class Car {
   int salesmanWhoHasKeys;

Imagine a tiny car dealership with two salesmen that has a room only for one car. When a customer pops in, one of the salesmen takes the car keys to test drive THE car. The class TestCar will look similar to the class Main, but is not exactly the same.

public class TestCar {

    public static void main(String[] args) {
        Car t = new Car();


    public static void test(Car t) {

The program TestCar prints 2. Why? What makes it different from the program Main? Just bear with me for a moment. Let’s completely understand what the phrase “objects are passed by reference” means. There is only one car, remember? And Java doesn’t create a copy of the one and only instance of the object Car just to pass it to the method test(). But it does create a copy of the original pointer t inside the method test for method argument, which (to add to the confusion) is also named t.

Get used to the fact that we have one object in memory with two different pointers to it (t and ttt). To make things easier to understand, modify the method test() to look as follow:

   public static void test(Car ttt) {

The program still prints 2. So what’s the difference between dealing with the instance of a Car vs Integer? The wrapper class Integer is immutable. This means that you can’t change its value once it was assigned, which is not the case with the Car.
To make things worse for comprehension, the Java feature called autoboxing kicks in and the original program quietly creates instances of new wrapper Integer objects when it sees something like t=2. This line gets converted into t=new Integer(2)! Got it? So the value 2 has been assigned to a different Integer object via a different variable t.

And just to make sure that you clearly understand the whole confusion of the program Main, please answer my final question, “How many instances of the class Integer were created during the lifespan of the program Main?”

Who said two? Mary? Wrong! The right answer is three! The first one had the value of zero, the next line caused the creation of the another instance if Integer with the value of 1, and the third instance was created by the method test() with the value of 2. Don’t believe me? Step through the Main program in a debugger, and you’ll see three different ids assigned to the variable t.

Don’t you love Java? I do.

Read the original blog entry...

More Stories By Yakov Fain

Yakov Fain is a Java Champion and a co-founder of the IT consultancy Farata Systems and the product company SuranceBay. He wrote a thousand blogs (http://yakovfain.com) and several books about software development. Yakov authored and co-authored such books as "Angular 2 Development with TypeScript", "Java 24-Hour Trainer", and "Enterprise Web Development". His Twitter tag is @yfain

IoT & Smart Cities Stories
New competitors, disruptive technologies, and growing expectations are pushing every business to both adopt and deliver new digital services. This ‘Digital Transformation’ demands rapid delivery and continuous iteration of new competitive services via multiple channels, which in turn demands new service delivery techniques – including DevOps. In this power panel at @DevOpsSummit 20th Cloud Expo, moderated by DevOps Conference Co-Chair Andi Mann, panelists examined how DevOps helps to meet the de...
According to Forrester Research, every business will become either a digital predator or digital prey by 2020. To avoid demise, organizations must rapidly create new sources of value in their end-to-end customer experiences. True digital predators also must break down information and process silos and extend digital transformation initiatives to empower employees with the digital resources needed to win, serve, and retain customers.
In his keynote at 18th Cloud Expo, Andrew Keys, Co-Founder of ConsenSys Enterprise, will provide an overview of the evolution of the Internet and the Database and the future of their combination – the Blockchain. Andrew Keys is Co-Founder of ConsenSys Enterprise. He comes to ConsenSys Enterprise with capital markets, technology and entrepreneurial experience. Previously, he worked for UBS investment bank in equities analysis. Later, he was responsible for the creation and distribution of life ...
Smart Cities are here to stay, but for their promise to be delivered, the data they produce must not be put in new siloes. In his session at @ThingsExpo, Mathias Herberts, Co-founder and CTO of Cityzen Data, discussed the best practices that will ensure a successful smart city journey.
"Space Monkey by Vivent Smart Home is a product that is a distributed cloud-based edge storage network. Vivent Smart Home, our parent company, is a smart home provider that places a lot of hard drives across homes in North America," explained JT Olds, Director of Engineering, and Brandon Crowfeather, Product Manager, at Vivint Smart Home, in this SYS-CON.tv interview at @ThingsExpo, held Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA.
LogRocket helps product teams develop better experiences for users by recording videos of user sessions with logs and network data. It identifies UX problems and reveals the root cause of every bug. LogRocket presents impactful errors on a website, and how to reproduce it. With LogRocket, users can replay problems.
@CloudEXPO and @ExpoDX, two of the most influential technology events in the world, have hosted hundreds of sponsors and exhibitors since our launch 10 years ago. @CloudEXPO and @ExpoDX New York and Silicon Valley provide a full year of face-to-face marketing opportunities for your company. Each sponsorship and exhibit package comes with pre and post-show marketing programs. By sponsoring and exhibiting in New York and Silicon Valley, you reach a full complement of decision makers and buyers in ...
There are many examples of disruption in consumer space – Uber disrupting the cab industry, Airbnb disrupting the hospitality industry and so on; but have you wondered who is disrupting support and operations? AISERA helps make businesses and customers successful by offering consumer-like user experience for support and operations. We have built the world’s first AI-driven IT / HR / Cloud / Customer Support and Operations solution.
Data Theorem is a leading provider of modern application security. Its core mission is to analyze and secure any modern application anytime, anywhere. The Data Theorem Analyzer Engine continuously scans APIs and mobile applications in search of security flaws and data privacy gaps. Data Theorem products help organizations build safer applications that maximize data security and brand protection. The company has detected more than 300 million application eavesdropping incidents and currently secu...
Rafay enables developers to automate the distribution, operations, cross-region scaling and lifecycle management of containerized microservices across public and private clouds, and service provider networks. Rafay's platform is built around foundational elements that together deliver an optimal abstraction layer across disparate infrastructure, making it easy for developers to scale and operate applications across any number of locations or regions. Consumed as a service, Rafay's platform elimi...