Constructors in Java

What is a constructor

A constructor is a special method that runs automatically when you create an object with new. Its job is to set up the object's starting values. It has the same name as the class and no return type.

The default constructor

class Student {
    String name;
    Student() {                 // default constructor
        name = "Unknown";
    }
}
// use:
Student s = new Student();
System.out.println(s.name);

A no-argument constructor sets default values. If you write no constructor at all, Java provides an invisible empty one so new still works.

Parameterised constructor

class Student {
    String name;
    int marks;
    Student(String n, int m) {   // parameterised
        name = n;
        marks = m;
    }
}
// use:
Student s = new Student("Aman", 75);
System.out.println(s.name + " " + s.marks);

A parameterised constructor takes values so you can set up an object with custom data at creation. This is cleaner than creating an object and setting each field afterwards.

Constructor overloading

class Box {
    int w, h;
    Box() { w = 1; h = 1; }            // default size
    Box(int side) { w = side; h = side; } // square
    Box(int w, int h) { this.w = w; this.h = h; } // custom
}

Having several constructors with different parameters is overloading. Java picks the one matching your arguments — new Box(), new Box(5), or new Box(4, 6).

Calling this()

class Box {
    int w, h;
    Box() {
        this(1, 1);            // call the other constructor
    }
    Box(int w, int h) {
        this.w = w; this.h = h;
    }
}

this() calls another constructor in the same class, so one can reuse another's setup instead of repeating code. It must be the first line in the constructor.

Constructor vs method

Common mistakes

• Adding a return type. A constructor has none — adding one makes it a method.
• Wrong name. A constructor must exactly match the class name.
• Losing the default. If you add any constructor, the free default one is gone — add it back if needed.
• this() not first. A this() call must be the first statement in the constructor.

Summary

• A constructor runs automatically when an object is created.
• It has the class's name and no return type.
• Default takes no arguments; parameterised takes values.
• Overloading provides several constructors with different parameters.
• this() calls another constructor to reuse its setup.

Next, learn this and super, or reuse code with inheritance.