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Constructor in Java - Types, Examples and Best Practices

Constructor in Java

In the realm of Java programming, constructors play a pivotal role in the creation and initialization of objects. A constructor is a special method within a class that is invoked automatically when an object of that class is instantiated. It provides a means to set up the initial state of the object and perform any necessary setup tasks.

Types of Constructors in Java

  • Default Constructor in Java: If no constructor is explicitly defined in a class, Java provides a default constructor automatically. This constructor takes no arguments and initializes the object with default values. It’s often used when you don’t need specific initialization logic.
  • Parameterized Constructors in Java: Parameterized constructors are created by developers to accept parameters during object creation. These parameters are used to initialize the object’s fields based on the provided values. This allows for customization and flexibility in object initialization.
  • Copy Constructor in Java: A copy constructor is used to create a new object by copying the attributes of an existing object. This can be particularly useful when you need to duplicate an object’s data while maintaining separation between instances.
  • Chained Constructors (Constructor Overloading) in Java: In Java, you can define multiple constructors with different parameter lists. This is known as constructor overloading. One constructor can call another using the this() keyword, allowing you to reuse code and provide varying initialization options.

Examples of Java Constructors

Let’s discuss the Java Constructor Examples:

Consider a simple class Person that represents individuals with a name and age. Here’s how constructors can be employed to create and initialize Person objects:

public class Person {
// Fields
    private String name;
    private int age;
    // Default Constructor
    public Person() {
        name = "Unknown";
        age = 0;
    }  
    // Parameterized Constructor
    public Person(String personName, int personAge) {
        name = personName;
        age = personAge;
    }
    // Methods to access and manipulate data (getters and setters)   
    // Main method to demonstrate object creation
    public static void main(String[] args) {
        // Using the default constructor
        Person defaultPerson = new Person();
        System.out.println("Default Person: " + defaultPerson.name + " (" + defaultPerson.age + " years old)");   
        // Using the parameterized constructor
        Person customPerson = new Person("Alice", 28);
        System.out.println("Custom Person: " + customPerson.name + " (" + customPerson.age + " years old)");
    }
}

In this example, the constructors define the initial state of Person objects by setting the name and age fields. The default constructor initializes the object with default values, while the parameterized constructor allows customization during object creation.

Read on:- Java Tutorial!

Java Constructor Overloading Example

Here is an example of Constructor Overloading in Java:

public class BankAccount {

    private String accountHolder;

    private double balance;

    public BankAccount(String holder) {

        accountHolder = holder;

        balance = 0;

    }

    public BankAccount(String holder, double initialBalance) {

        accountHolder = holder;

        balance = initialBalance;

    }

    // Methods for deposit and withdrawal

    public static void main(String[] args) {

        BankAccount account1 = new BankAccount("John Doe");

        BankAccount account2 = new BankAccount("Jane Smith", 1000.0);

    }

}

Example of a Copy Constructor in Java

Consider a class called Student that represents student information:

public class Student {

    private String name;

    private int age;

    public Student(String name, int age) {

        this.name = name;

        this.age = age;

    }
    // Copy Constructor

    public Student(Student otherStudent) {

        this.name = otherStudent.name;

        this.age = otherStudent.age;

    }

    // Getters and setters

    public static void main(String[] args) {

        Student originalStudent = new Student("Alice", 20);

        Student copiedStudent = new Student(originalStudent); // Using the copy constructor

        System.out.println("Original Student: " + originalStudent.getName() + " (" + originalStudent.getAge() + " years old)");

        System.out.println("Copied Student: " + copiedStudent.getName() + " (" + copiedStudent.getAge() + " years old)");

    }

}

In this example, the copy constructor for the Student class takes another Student object as an argument and creates a new instance with the same attribute values. This allows you to create a separate copy of the originalStudent while maintaining distinct instances.

Learn the basics of OOPs in Java through this blog!

Constructor in Java Best Practices

  • Initialize Fields: Constructors should initialize all essential fields to meaningful default or provided values, ensuring that objects start in a consistent state.
  • Avoid Repetition: When possible, use constructor chaining to avoid code duplication. A constructor can call another constructor using this().
  • Document Your Constructors: Provide clear documentation for your constructors, explaining the purpose of each constructor and the parameters they accept.

Benefits of Constructors in Java

  • Object Initialization: Constructors ensure that objects are properly initialized when they are created, preventing instances with undefined states.
  • Customization: Parameterized constructors allow for customized object creation by accepting input values based on the intended use.
  • Controlled Initialization: Constructors enable you to enforce rules during object creation, ensuring that objects adhere to specific guidelines.

Conclusion

Constructors in Java are fundamental components of object-oriented programming in Java. They empower developers to create and initialize objects according to specific requirements, providing a seamless way to set up initial states and carry out preparatory tasks. By grasping the principles of constructors, developers can construct well-structured and efficient Java applications.

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About the Author

Senior Consultant Analytics & Data Science

Presenting Sahil Mattoo, a Senior Consultant Analytics & Data Science at Eli Lilly and Company is an accomplished professional with 14 years of experience across data science, analytics, and technical leadership domains, demonstrates a remarkable ability to drive business insights. Sahil holds a Post Graduate Program in Business Analytics and Business Intelligence from Great Lakes Institute of Management.