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Azure Container Apps is a fully managed serverless container service that enables you to build and deploy modern, cloud-native Java applications and microservices at scale. It offers a simplified developer experience while providing the flexibility and portability of containers.

Of course, Azure Container Apps has really solid support for our ecosystem, from a number of build options, managed Java components, native metrics, dynamic logger, and quite a bit more.

To learn more about Java features on Azure Container Apps, you can get started over on the documentation page.

And, you can also ask questions and leave feedback on the Azure Container Apps GitHub page.

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Modern software architecture is often broken. Slow delivery leads to missed opportunities, innovation is stalled due to architectural complexities, and engineering resources are exceedingly expensive.

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1. Introduction

Constructors are the gatekeepers of object-oriented design.

In this tutorial, we’ll see how they act as a single location from which to initialize the internal state of the object being created.

Let’s forge ahead and create a simple object that represents a bank account.

2. Setting Up a Bank Account

Imagine that we need to create a class that represents a bank account. It’ll contain a Name, Date of Creation and Balance.

Also, let’s override the toString method to print the details to the console:

class BankAccount {
    String name;
    LocalDateTime opened;
    double balance;
    
    @Override
    public String toString() {
        return String.format("%s, %s, %f", 
          this.name, this.opened.toString(), this.balance);
    }
}

Now, this class contains all of the necessary fields required to store information about a bank account, but it doesn’t contain a constructor yet.

This means that if we create a new object, the field values wouldn’t be initialized:

BankAccount account = new BankAccount();
account.toString();

Running the toString method above will result in an exception because the objects name and opened are still null:

java.lang.NullPointerException
    at com.baeldung.constructors.BankAccount.toString(BankAccount.java:12)
    at com.baeldung.constructors.ConstructorUnitTest
      .givenNoExplicitContructor_whenUsed_thenFails(ConstructorUnitTest.java:23)

3. A No-Argument Constructor

Let’s fix that with a constructor:

class BankAccount {
    public BankAccount() {
        this.name = "";
        this.opened = LocalDateTime.now();
        this.balance = 0.0d;
    }
}

Notice a few things about the constructor which we just wrote. First, it’s a method, but it has no return type. That’s because a constructor implicitly returns the type of the object that it creates. Calling new BankAccount() now will call the constructor above.

Secondly, it takes no arguments. This particular kind of constructor is called a no-argument constructor.

Why didn’t we need it for the first time, though? It’s because when we don’t explicitly write any constructor, the compiler adds a default, no-argument constructor.

This is why we were able to construct the object the first time, even though we didn’t write a constructor explicitly. The default, no argument constructor will simply set all members to their default values.

For objects, that’s null, which resulted in the exception that we saw earlier.

4. A Parameterized Constructor

Now, a real benefit of constructors is that they help us maintain encapsulation when injecting state into the object.

So, to do something really useful with this bank account, we need to be able to actually inject some initial values into the object.

To do that, let’s write a parameterized constructor, that is, a constructor that takes some arguments:

class BankAccount {
    public BankAccount() { ... }
    public BankAccount(String name, LocalDateTime opened, double balance) {
        this.name = name;
        this.opened = opened;
        this.balance = balance;
    }
}

Now we can do something useful with our BankAccount class:

    LocalDateTime opened = LocalDateTime.of(2018, Month.JUNE, 29, 06, 30, 00);
    BankAccount account = new BankAccount("Tom", opened, 1000.0f); 
    account.toString();

Notice, that our class now has 2 constructors. An explicit, no argument constructor and a parameterized constructor.

We can create as many constructors as we like, but we probably would like not to create too many. This would be a little confusing.

If we find too many constructors in our code, a few Creational Design Patterns might be helpful.

5. A Copy Constructor

Constructors need not be limited to initialization alone. They can also be used to create objects in other ways. Imagine that we need to be able to create a new account from an existing one.

The new account should have the same name as the old account, today’s date of creation and no funds. We can do that using a copy constructor:

public BankAccount(BankAccount other) {
    this.name = other.name;
    this.opened = LocalDateTime.now();
    this.balance = 0.0f;
}

Now we have the following behavior:

LocalDateTime opened = LocalDateTime.of(2018, Month.JUNE, 29, 06, 30, 00);
BankAccount account = new BankAccount("Tim", opened, 1000.0f);
BankAccount newAccount = new BankAccount(account);

assertThat(account.getName()).isEqualTo(newAccount.getName());
assertThat(account.getOpened()).isNotEqualTo(newAccount.getOpened());
assertThat(newAccount.getBalance()).isEqualTo(0.0f);

6. A Chained Constructor

Of course, we may be able to infer some of the constructor parameters or give some of them default values.

For example, we could just create a new bank account with only the name.

So, let’s create a constructor with a name parameter and give the other parameters default values:

public BankAccount(String name, LocalDateTime opened, double balance) {
    this.name = name;
    this.opened = opened;
    this.balance = balance;
}
public BankAccount(String name) {
    this(name, LocalDateTime.now(), 0.0f);
}

With the keyword this, we’re calling the other constructor.

We have to remember that if we want to chain a superclass constructor we have to use super instead of this.

Also, remember that this or super expression should always be the first statement.

7. Value Types

An interesting use of constructors in Java is in the creation of Value Objects. A value object is an object that does not change its internal state after initialization.

That is, the object is immutable. Immutability in Java is a bit nuanced and care should be taken when crafting objects.

Let’s go ahead and create an immutable class:

class Transaction {
    final BankAccount bankAccount;
    final LocalDateTime date;
    final double amount;

    public Transaction(BankAccount account, LocalDateTime date, double amount) {
        this.bankAccount = account;
        this.date = date;
        this.amount = amount;
    }
}

Notice, that we now use the final keyword when defining the members of the class. This means that each of those members can only be initialized within the constructor of the class. They cannot be reassigned later on inside any other method. We can read those values, but not change them.

If we create multiple constructors for the Transaction class, each constructor will need to initialize every final variable. Not doing so will result in a compilation error.

8. Conclusion

We’ve taken a tour through the different ways in which constructors build objects. When used judiciously, constructs form the basic building blocks of Object-Oriented design in Java.

The code backing this article is available on GitHub. Once you're logged in as a Baeldung Pro Member, start learning and coding on the project.
Baeldung Pro – NPI EA (cat = Baeldung)
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Baeldung Pro comes with both absolutely No-Ads as well as finally with Dark Mode, for a clean learning experience:

>> Explore a clean Baeldung

Once the early-adopter seats are all used, the price will go up and stay at $33/year.

Partner – Microsoft – NPI EA (cat = Spring Boot)
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Azure Container Apps is a fully managed serverless container service that enables you to build and deploy modern, cloud-native Java applications and microservices at scale. It offers a simplified developer experience while providing the flexibility and portability of containers.

Of course, Azure Container Apps has really solid support for our ecosystem, from a number of build options, managed Java components, native metrics, dynamic logger, and quite a bit more.

To learn more about Java features on Azure Container Apps, visit the documentation page.

You can also ask questions and leave feedback on the Azure Container Apps GitHub page.

Partner – Orkes – NPI EA (cat = Spring)
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Modern software architecture is often broken. Slow delivery leads to missed opportunities, innovation is stalled due to architectural complexities, and engineering resources are exceedingly expensive.

Orkes is the leading workflow orchestration platform built to enable teams to transform the way they develop, connect, and deploy applications, microservices, AI agents, and more.

With Orkes Conductor managed through Orkes Cloud, developers can focus on building mission critical applications without worrying about infrastructure maintenance to meet goals and, simply put, taking new products live faster and reducing total cost of ownership.

Try a 14-Day Free Trial of Orkes Conductor today.

Partner – Orkes – NPI EA (tag = Microservices)
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Modern software architecture is often broken. Slow delivery leads to missed opportunities, innovation is stalled due to architectural complexities, and engineering resources are exceedingly expensive.

Orkes is the leading workflow orchestration platform built to enable teams to transform the way they develop, connect, and deploy applications, microservices, AI agents, and more.

With Orkes Conductor managed through Orkes Cloud, developers can focus on building mission critical applications without worrying about infrastructure maintenance to meet goals and, simply put, taking new products live faster and reducing total cost of ownership.

Try a 14-Day Free Trial of Orkes Conductor today.

eBook – HTTP Client – NPI EA (cat=HTTP Client-Side)
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The Apache HTTP Client is a very robust library, suitable for both simple and advanced use cases when testing HTTP endpoints. Check out our guide covering basic request and response handling, as well as security, cookies, timeouts, and more:

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eBook – Java Concurrency – NPI EA (cat=Java Concurrency)
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Handling concurrency in an application can be a tricky process with many potential pitfalls. A solid grasp of the fundamentals will go a long way to help minimize these issues.

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eBook – Java Streams – NPI EA (cat=Java Streams)
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Since its introduction in Java 8, the Stream API has become a staple of Java development. The basic operations like iterating, filtering, mapping sequences of elements are deceptively simple to use.

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To get a better understanding on how Streams work and how to combine them with other language features, check out our guide to Java Streams:

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eBook – Persistence – NPI EA (cat=Persistence)
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