Dinesh on Java

Constructors may include parameters of various types. When the constructor is invoked using the new operator, the types must match those that are specified in the constructor definition.

Java provides a default constructor which takes no arguments and performs no special actions or initializations, when no explicit constructors are provided.

The only action taken by the implicit default constructor is to call the superclass constructor using the super() call. Constructor arguments provide you with a way to provide parameters for the initialization of an object.

Below is an example of a Cuboid class containing 2 constructors. (one default and one parameterized constructor).

public class Cuboid {

 int length;
 int breadth;
 int height;
 public int getVolume() {
  return (length * breadth * height);
 }
 Cuboid () {
  length = 10;
  breadth = 10;
  height = 10;
 }
 Cuboid (int l, int b, int h) {
  length = l;
  breadth = b;
  height = h;
 }
 public static void main(String[] args) {
  Cuboid cubeObj1, cubeObj2;
  cubeObj1 = new Cuboid ();
  cubeObj2 = new Cuboid (10, 20, 30);
  System.out.println("Volume of Cuboid 1 is : " + cubeObj1.getVolume());
  System.out.println("Volume of Cuboid 1 is : " + cubeObj2.getVolume());
 }
}

Note: If a class defines an explicit constructor, it no longer has a default constructor to set the state of the objects. If such a class requires a default constructor, its implementation must be provided. Any attempt to call the default constructor will be a compile time error if an explicit default constructor is not provided in such a case.

Java Overloaded Constructors:

Like methods, constructors can also be overloaded. Since the constructors in a class all have the same name as the class, />their signatures are differentiated by their parameter lists. The above example shows that the Cube1 constructor is overloaded one being the default constructor and the other being a parameterized constructor.

It is possible to use this() construct, to implement local chaining of constructors in a class. The this() call in a constructorinvokes the an other constructor with the corresponding parameter list within the same class. Calling the default constructor to create a Cube object results in the second and third parameterized constructors being called as well. Java requires that any this() call must occur as the first statement in a constructor.

Below is an example of a cube class containing 3 constructors which demostrates the this() method in Constructors context

public class Cuboid2 {

 int length;
 int breadth;
 int height;
 public int getVolume() {
  return (length * breadth * height);
 }
 Cuboid2 () {
  this(10, 10);
  System.out.println("Finished with Default Constructor");
 }
 Cuboid2 (int l, int b) {
  this(l, b, 10);
  System.out.println("Finished with Parameterized Constructor having 2 params");
 }
 Cuboid2 (int l, int b, int h) {
  length = l;
  breadth = b;
  height = h;
  System.out.println("Finished with Parameterized Constructor having 3 params");
 }
 public static void main(String[] args) {
  Cuboid2 cubeObj1, cubeObj2;
  cubeObj1 = new Cuboid2 ();
  cubeObj2 = new Cuboid2 (10, 20, 30);
  System.out.println("Volume of Cuboid2 1 is : " + cubeObj1.getVolume());
  System.out.println("Volume of Cuboid2 2 is : " + cubeObj2.getVolume());
 }
}

Constructor Chaining:

Every constructor calls its superclass constructor. An implied super() is therefore included in each constructor which does not include either the this() function or an explicit super() call as its first statement. The super() statement invokes a constructor of the super class.
The implicit super() can be replaced by an explicit super(). The super statement must be the first statement of the constructor.
The explicit super allows parameter values to be passed to the constructor of its superclass and must have matching parameter types A super() call in the constructor of a subclass will result in the call of the relevant constructor from the superclass, based on the signature of the call. This is called constructor chaining.

Below is an example of a class demonstrating constructor chaining using super() method.

class Cube {

 int length;
 int breadth;
 int height;
 public int getVolume() {
  return (length * breadth * height);
 }
 Cube() {
  this(10, 10);
  System.out.println("Finished with Default Constructor of Cube");
 }
 Cube(int l, int b) {
  this(l, b, 10);
  System.out.println("Finished with Parameterized Constructor having
         2 params of Cube");
 }
 Cube(int l, int b, int h) {
  length = l;
  breadth = b;
  height = h;
  System.out.println("Finished with Parameterized Constructor having
         3 params of Cube");
 }
}

public class SpecialCube extends Cube {

 int weight;
 SpecialCube() {
  super();
  weight = 10;
 }
 SpecialCube(int l, int b) {
  this(l, b, 10);
  System.out.println("Finished with Parameterized Constructor having
         2 params of SpecialCube");
 }
 SpecialCube(int l, int b, int h) {
  super(l, b, h);
  weight = 20;
  System.out.println("Finished with Parameterized Constructor having
         3 params of SpecialCube");
 }
 public static void main(String[] args) {
  SpecialCube specialObj1 = new SpecialCube();
  SpecialCube specialObj2 = new SpecialCube(10, 20);
  System.out.println("Volume of SpecialCube1 is : "
    + specialObj1.getVolume());
  System.out.println("Weight of SpecialCube1 is : "
    + specialObj1.weight);
  System.out.println("Volume of SpecialCube2 is : "
    + specialObj2.getVolume());
  System.out.println("Weight of SpecialCube2 is : "
    + specialObj2.weight);
 }
}

The super() construct as with this() construct: if used, must occur as the first statement in a constructor, and it can only be used in a constructor declaration. This implies that this() and super() calls cannot both occur in the same constructor. Just as the this() construct leads to chaining of constructors in the same class, the super() construct leads to chaining of subclass constructors to superclass constructors.
if a constructor has neither a this() nor a super() construct as its first statement, then a super() call to the default constructor in the superclass is inserted.

Note: If a class only defines non-default constructors, then its subclasses will not include an implicit super() call. This will be flagged as a compile-time error. The subclasses must then explicitly call a superclass constructor, using the super() construct with the right arguments to match the appropriate constructor of the superclass.

Below is an example of a class demonstrating constructor chaining using explicit super() call.

class Cube {

 int length;
 int breadth;
 int height;
 public int getVolume() {
  return (length * breadth * height);
 }
 Cube(int l, int b, int h) {
  length = l;
  breadth = b;
  height = h;
  System.out.println("Finished with Parameterized Constructor having
        3 params of Cube");
 }
}

public class SpecialCube1 extends Cube {

 int weight;
 SpecialCube1() {
  super(10, 20, 30); //Will Give a Compilation Error without this line
  weight = 10;
 }
 public static void main(String[] args) {
  SpecialCube1 specialObj1 = new SpecialCube1();
  System.out.println("Volume of SpecialCube1 is : "+ specialObj1.getVolume());
 }
}