Constructors

Syntax

public ClassName(parameters) {\n  // initialization code\n}

Examples

public class Point {
    private int x, y;

    public Point(int x, int y) {
        this.x = x;
        this.y = y;
    }

    public void display() {
        System.out.println("(" + x + ", " + y + ")");
    }
}

Point p = new Point(3, 4);
p.display();  // (3, 4)

public class Rectangle {
    private double width, height;

    public Rectangle() {
        this.width = 1;
        this.height = 1;
    }

    public Rectangle(double side) {  // square shortcut
        this.width = side;
        this.height = side;
    }

    public Rectangle(double width, double height) {
        this.width = width;
        this.height = height;
    }
}

Rectangle r1 = new Rectangle();          // 1x1
Rectangle r2 = new Rectangle(5);          // 5x5 square
Rectangle r3 = new Rectangle(4, 6);       // 4x6

public class User {
    private String name;
    private String role;

    public User(String name) {
        this(name, "member");  // calls the constructor below
    }

    public User(String name, String role) {
        this.name = name;
        this.role = role;
    }

    public void display() {
        System.out.println(name + " (" + role + ")");
    }
}

User u1 = new User("Fola");           // Fola (member)
User u2 = new User("Zain", "admin");   // Zain (admin)

public class Simple {
    private int value;
    // No constructor written - Java provides a default no-arg one automatically
}

Simple s = new Simple();  // works fine, uses default constructor

public class WithConstructor {
    private int value;

    public WithConstructor(int value) {  // once you write ANY constructor...
        this.value = value;
    }
}

// WithConstructor w = new WithConstructor();  // Error! default constructor no longer exists

Best practices

• Use this(...) to chain constructors and avoid duplicating initialization logic across multiple overloads
• Remember that defining any constructor removes the automatically provided no-argument default constructor
• Validate arguments inside constructors (e.g. throwing an exception for invalid values) to prevent objects from ever existing in an invalid state
• Keep constructors focused on initialization - avoid putting complex business logic inside them

At a glance

Purpose

General-purpose application development

File extension

.java

Runs in

Java Virtual Machine

Usually used with

JDK and Java libraries

Specifications & further reading

• Java Language Specification

Related Java documentation