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Mastering Constructors in Java: Types and Examples

When diving into Java, one of the foundational concepts you’ll come across is constructors. Constructors play a critical role in how objects are created and initialized. In this post, you'll gain a clear understanding of constructors in Java, their importance, different types, and usage with practical examples.

You'll also explore the role of constructors in initializing objects and handling object creation in a variety of ways. So, let's dive in!

What Are Constructors in Java?

In Java, a constructor is a block of code used to initialize an object when it's created. It gets invoked automatically at the time of object creation, setting up the object's initial state. If no constructor is explicitly defined in a class, Java will call the default constructor.

Constructors differ from regular methods in two important ways:

Same Name as Class: A constructor must have the same name as the class it belongs to.
No Return Type: Constructors do not return any values, not even void.
Automatically called: The constructor is automatically invoked when an object is created using the new keyword, relieving you from the need to call it explicitly.

Why Are Constructors Important in Java?

Constructors are essential because they provide the framework for initializing new objects in a consistent manner. They ensure that every object starts with valid, meaningful data, making it easier to manage the state of an object throughout its lifecycle.

Once you understand constructors, you'll appreciate that they are automatically invoked when an object is created using the new keyword.

Types of Constructors in Java

There are three main types of constructors in Java:

No-Argument Constructor
Parameterized Constructor
Default Constructor

Let’s break each one down in detail.

1. No-Argument Constructor

A no-argument constructor is a constructor that doesn’t take any parameters. It initializes objects with default values or with values defined within the constructor.

Example:

class Rectangle {
    double length;
    double breadth;

    // No-argument constructor
    Rectangle() {
        length = 15.5;
        breadth = 10.67;
    }

    double calculateArea() {
        return length * breadth;
    }
}

class Main {
    public static void main(String[] args) {
        Rectangle myRectangle = new Rectangle();  // No-argument constructor is invoked
        double area = myRectangle.calculateArea();
        System.out.println("The area of the Rectangle: " + area);
    }
}

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Output: The area of the Rectangle is 165.385.

Here, the no-argument constructor initializes length and breadth with default values when a Rectangle object is created.

2. Parameterized Constructor

A parameterized constructor allows you to pass arguments to initialize an object with specific values. This flexibility enables you to create multiple objects with different initial states.

Example:

class Rectangle {
    double length;
    double breadth;

    // Parameterized constructor
    Rectangle(double l, double b) {
        length = l;
        breadth = b;
    }

    double calculateArea() {
        return length * breadth;
    }
}

class Main {
    public static void main(String[] args) {
        Rectangle myRectangle = new Rectangle(20, 30);  // Parameterized constructor is invoked
        double area = myRectangle.calculateArea();
        System.out.println("The area of the Rectangle: " + area);
    }
}

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Output: The area of the Rectangle is 600.0.

Here, the parameterized constructor accepts length and breadth as arguments, allowing us to set custom values for each object.

3. Default Constructor

If no constructor is defined in a class, Java provides a default constructor. This constructor initializes the instance variables with default values (e.g., null for objects, 0 for numbers).

Example:

class Circle {
    double radius;

    double calculateArea() {
        return Math.PI * radius * radius;
    }
}

class Main {
    public static void main(String[] args) {
        Circle myCircle = new Circle();  // Default constructor is invoked
        System.out.println("Radius: " + myCircle.radius);  // Output will be 0.0, the default value
    }
}

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Since the Circle class does not explicitly define any constructor, Java provides a default one that initializes radius to 0.0.

Constructor Overloading in Java

Java allows constructor overloading, where a class can have multiple constructors with different argument lists. Each constructor performs a unique task based on the parameters passed.

Example:

class Student {
    String name;
    int age;

    // No-argument constructor
    Student() {
        name = "Unknown";
        age = 0;
    }

    // Parameterized constructor
    Student(String n, int a) {
        name = n;
        age = a;
    }

    void displayInfo() {
        System.out.println("Name: " + name + ", Age: " + age);
    }
}

class Main {
    public static void main(String[] args) {
        Student student1 = new Student();  // Calls no-argument constructor
        Student student2 = new Student("Alice", 20);  // Calls parameterized constructor

        student1.displayInfo();  // Output: Name: Unknown, Age: 0
        student2.displayInfo();  // Output: Name: Alice, Age: 20
    }
}

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In this case, the class Student has two constructors: one with no arguments and another with parameters (name and age). Java distinguishes between them based on the number and type of arguments passed when creating an object.

The this Keyword in Constructors

In Java, the this keyword is used to refer to the current instance of the class. It's useful when constructor parameters have the same names as instance variables, helping to avoid ambiguity.

Example:

class Employee {
    String name;
    double salary;

    // Parameterized constructor
    Employee(String name, double salary) {
        this.name = name;  // 'this' refers to the current object's instance variable
        this.salary = salary;
    }

    void display() {
        System.out.println("Employee Name: " + name);
        System.out.println("Salary: " + salary);
    }
}

class Main {
    public static void main(String[] args) {
        Employee emp = new Employee("John", 50000);  // Using parameterized constructor
        emp.display();
    }
}

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In this example, this.name refers to the instance variable, while name without this refers to the parameter passed to the constructor.

建構函式與方法：有什麼差別？

Constructor	Method
Must have the same name as the class	Can have any name
No return type (not even void)	Must have a return type
Invoked automatically when an object is created	Called explicitly by the programmer
Used to initialize objects	Used to perform actions or computations