Constructor

We have been creating objects using the new operator and the class name with ().

Sphere3 sphere = new Sphere3();

The () make this look like a method, and it almost is: Sphere3() is an example of a constructor.

constructor

A special method we use with new to create instances. It has the same name as the class of which it is a member.

Constructor Parameters

A constructor can take parameters and be overloaded.

We often use parameters to initialize fields.

The class below has two constructors, one of which initializes the radius field to the argument passed to the constructor.

public class Sphere4 {
  public static double PI = 3.14159;

  public double radius;

  public Sphere4() {          //NO-ARG CONSTRUCTOR
  }

  public Sphere4(double r) {  //OVERLOADED CONSTRUCTOR
    radius = r;
  }

  public double getVolume() {
    double vol = 4.0 / 3.0 * PI  * radius * radius * radius;
    return vol;
  }

  public static double calculateCircumference(double r) {
    return PI * 2 * r;
  }
}

A constructor with no parameters defined is sometimes called a no-arg constructor for the class.

Practice Exercise

A constructor cannot have a return type. This class appears to define a one-argument constructor, but really just has a method with the same name as the class. This is legal, but don't do it.

public class Item2 {
  public double cost;
  
  public void Item2(double c) {
    cost = c;
  }
}

Practice Exercise

When writing the word constructor, we may use the abbreviation ctor.

Drill

Objects/com.example.objs.drills.Dog * Add a constructor with parameters to initialize breed and weight. * Add a constructor with parameters to initialize name, breed, and weight. * Create three dog instances and call the displayDogInfo method on each Dog object.

Do not add a no-arg constructor.

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