Java Tutorial/Class Definition/Constructor

Calling a Constructor From a Constructor

class Sphere {
  int radius = 0;
  double xCenter;
  double yCenter;
  double zCenter;
  Sphere() {
    radius = 1;
  }
  Sphere(double x, double y, double z) {
    this();
    xCenter = x;
    yCenter = y;
    zCenter = z;
  }
  Sphere(int theRadius, double x, double y, double z) {
    this(x, y, z);
    radius = theRadius;
  }
}

Class Initializer: during declaration

// ClassInitializer2.java
class ClassInitializer2 {
  static boolean bool = true;
  static byte by = 20;
  static char ch = "X";
  static double d = 8.95;
  static float f = 2.1f;
  static int i = 63;
  static long l = 2L;
  static short sh = 200;
  static String str = "test";
  public static void main(String[] args) {
    System.out.println("bool = " + bool);
    System.out.println("by = " + by);
    System.out.println("ch = " + ch);
    System.out.println("d = " + d);
    System.out.println("f = " + f);
    System.out.println("i = " + i);
    System.out.println("l = " + l);
    System.out.println("sh = " + sh);
    System.out.println("str = " + str);
  }
}

Duplicating Objects using a Constructor

class Sphere {
  int radius = 0;
  double xCenter;
  double yCenter;
  double zCenter;
  Sphere() {
    radius = 1;
  }
  Sphere(double x, double y, double z) {
    this();
    xCenter = x;
    yCenter = y;
    zCenter = z;
  }
  Sphere(int theRadius, double x, double y, double z) {
    this(x, y, z);
    radius = theRadius;
  }
  
  // Create a sphere from an existing object
  Sphere(final Sphere oldSphere) {
    radius = oldSphere.radius;
    xCenter = oldSphere.xCenter;
    yCenter = oldSphere.yCenter;
    zCenter = oldSphere.yCenter;
  }
  
}

Multiple Constructors

class Sphere {
  int radius = 0;
  Sphere() {
    radius = 1;
  }
  Sphere(int radius) {
    this.radius = radius;
  }
}

Order of constructor calls

class Meal {
  Meal() {
    System.out.println("Meal()");
  }
}
class Bread {
  Bread() {
    System.out.println("Bread()");
  }
}
class Cheese {
  Cheese() {
    System.out.println("Cheese()");
  }
}
class Lettuce {
  Lettuce() {
    System.out.println("Lettuce()");
  }
}
class Lunch extends Meal {
  Lunch() {
    System.out.println("Lunch()");
  }
}
class PortableLunch extends Lunch {
  PortableLunch() {
    System.out.println("PortableLunch()");
  }
}
class Sandwich extends PortableLunch {
  private Bread b = new Bread();
  private Cheese c = new Cheese();
  private Lettuce l = new Lettuce();
  public Sandwich() {
    System.out.println("Sandwich()");
  }
}
public class MainClass {
  public static void main(String[] args) {
    new Sandwich();
  }
}

Meal()
Lunch()
PortableLunch()
Bread()
Cheese()
Lettuce()
Sandwich()

The Default Constructor

public class MainClass {
  double radius;
  MainClass() {
  }
  // Class constructor
  MainClass(double theRadius) {
    radius = theRadius;
  }
}

Using Constructors

1. Every class must have at least one constructor.
2. If there is no constructors for your class, the compiler will supply a default constructor(no-arg constructor).
3. A constructor is used to construct an object.
4. A constructor looks like a method and is sometimes called a constructor method.
5. A constructor never returns a value
6. A constructor always has the same name as the class.
7. A constructor may have zero argument, in which case it is called a no-argument (or no-arg, for short) constructor.
8. Constructor arguments can be used to initialize the fields in the object.

The syntax for a constructor is as follows.

constructorName (listOfArguments) {
    [constructor body]
}