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Java Constructors

A constructor is a special method that runs when an object is created. It sets the starting values of the new object.

A constructor runs at creation

The Dog constructor stores Buddy in the name field. The print line reads the field after creation, and the stored name appears.

class Dog {
  String name;

  Dog() {
    name = "Buddy";
  }
}

public class Main {
  public static void main(String[] args) {
    Dog dog = new Dog();
    System.out.println(dog.name);
  }
}

A parameter constructor fills the fields

The values Ada and 36 travel with the creation call. The constructor stores each value in its field.

class Person {
  String name;
  int age;

  Person(String n, int a) {
    name = n;
    age = a;
  }
}

public class Main {
  public static void main(String[] args) {
    Person person = new Person("Ada", 36);
    System.out.println(person.name);
    System.out.println(person.age);
  }
}

Two objects, two sets of values

Each creation call passes its own values. Each object keeps the values from its own call.

class Person {
  String name;
  int age;

  Person(String n, int a) {
    name = n;
    age = a;
  }
}

public class Main {
  public static void main(String[] args) {
    Person first = new Person("Ada", 36);
    Person second = new Person("Ben", 24);
    System.out.println(first.name + ", " + first.age);
    System.out.println(second.name + ", " + second.age);
  }
}

Example

class Book {
  String title;
  int pages;

  Book(String t, int p) {
    title = t;
    pages = p;
  }
}

public class Main {
  public static void main(String[] args) {
    Book first = new Book("Dune", 412);
    Book second = new Book("Emma", 474);
    System.out.println(first.title + ", " + first.pages + " pages");
    System.out.println(second.title + ", " + second.pages + " pages");
  }
}

Key points

  • A constructor carries the same name as its class.
  • A constructor has no return type.
  • A constructor runs once for each new object.
  • Values passed at creation land in the constructor parameters.
  • A constructor can store parameter values in fields.

In this example, the Book constructor takes a title and a page count. Each creation call passes its own pair of values to the constructor. The prints show Dune with 412 pages and Emma with 474 pages.

Note

A class with no written constructor still builds objects, because Java supplies a simple one.

Try it Yourself

Edit the code, then press Run.

Running is simulated

Exercise

What name does a constructor carry?