Practice›Java Fundamentals›Module 4. Classes & Objects›4.2 Constructors

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4.2 Constructors — the constructor

Lesson

Setting up a new object

A constructor is a special method that runs automatically when you create an object with new. Its job is to initialize the object — usually to fill in its fields. A constructor has the same name as the class and no return type.

Worked example

class Dog {
    String name;
    int age;

    Dog(String n, int a) {   // constructor: same name as the class
        name = n;
        age = a;
    }
}

Dog d = new Dog("Rex", 3);   // constructor runs, sets the fields

Now d.name is Rex and d.age is 3 the moment the object is built.

▶ Try it: the Dog class has a constructor. Run this, then change the values you pass to new Dog(...) and re-run.

class Dog {
    String name;
    int age;
    Dog(String n, int a) {
        name = n;
        age = a;
    }
}
public class Main {
    public static void main(String[] args) {
        Dog d = new Dog("Rex", 3);
        System.out.println(d.name + " " + d.age);
    }
}

When does a constructor run?

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What is a constructor's main job?

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The Dog constructor sets name and age. Type what this prints:

Dog d = new Dog("Rex", 3);
System.out.println(d.name);

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True or false?

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