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Java Constructor Parameters

In this chapter you will learn:

  1. How to pass in value with parameters for constructors
  2. Syntax for Java Constructor Parameters
  3. Example - Java Constructor Parameters
  4. How to declare object parameters for constructors

Description

The constructors can also have parameters. Usually the parameters are used to set the initial states of the object.

Syntax

Syntax for Java Constructor Parameters


class ClassName{//ww w . j  a v  a2 s  .  c  o  m

   ClassName(parameterType variable,parameterType2 variable2,...){ // constructor
    ...
   }
}

Example

In the the following demo code Rectangle class uses the parameters, w for width and h for height, from the constructors to initialize its width and height. 

 
class Rectangle {
  double width;/*from   ww  w .j a  va 2  s  .  c  o m*/
  double height;

  Rectangle(double w, double h) {
    width = w;
    height = h;
  }

  double area() {
    return width * height;
  }
}

public class Main {
  public static void main(String args[]) {
    Rectangle mybox1 = new Rectangle(10, 20);
    double area;
    area = mybox1.area();
    System.out.println("Area is " + area);
  }
}

The output from this program is shown here:

Example 2

Just like methods in a class the constructors can not only accept primitive type parameters it can also have the object parameters. Object parameters contains more information and can help us initialize the class.

The following Rectangle class has a constructor whose parameter is a Rectangle class. In this way we can initialize a rectangle by the data from another rectangle.

 
class Rectangle {
  double width;/*w w w.j a  v a  2s . co  m*/
  double height;

  Rectangle(Rectangle ob) { // pass object to constructor
    width = ob.width;
    height = ob.height;
  }

  Rectangle(double w, double h) {
    width = w;
    height = h;
  }

  // constructor used when no dimensions specified
  Rectangle() {
    width = -1; // use -1 to indicate
    height = -1; // an uninitialized
  }

  // constructor used when cube is created
  Rectangle(double len) {
    width = height = len;
  }

  double area() {
    return width * height;
  }
}

public class Main {
  public static void main(String args[]) {
    Rectangle mybox1 = new Rectangle(10, 20);
    Rectangle myclone = new Rectangle(mybox1);
    double area;
    // get volume of first box
    area = mybox1.area();
    System.out.println("Area of mybox1 is " + area);
    // get volume of clone
    area = myclone.area();
    System.out.println("Area of clone is " + area);

  }
}

The output:

Next chapter...

What you will learn in the next chapter:

  1. What is Java this keyword for
  2. How to use this keyword to reference class member variables
  3. How to reference class member variables
  4. Example - use this to reference instance variable
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