Object methods

First we define two instance variables width and height representing a Rectangle's corresponding two parameters width and height:

public class Rectangle {

  // Instance variables representing a rectangle's parameters
  private double width, height;
...
}

Next we allow for changing these two parameters:

public class Rectangle {

  // Instance variables representing a rectangle's parameters
  private double width, height;

...
  /**
   * @param width Changing the rectangle's width
   */
  public void setWidth(double w) {
      width = w;
  }

  /**
   * @param height Changing the rectangle's height
   */
  public void setHeight(double height) {
      this.height = height;
  }
...
}

Note the subtle implementation difference between setWidth(...) and setHeight(...):

Both ways are perfectly legal. The complete implementation including all remaining methods reads:

/**
 * Representing rectangular shapes.
 *
 */
public class Rectangle {

  // Instance variables representing a rectangle's parameters
  private double width, height;

  /**
   *
   * @param width The rectangle's width
   * @param height The rectangle's height
   */
  public Rectangle (double width, double height) {
     setWidth(width);
     setHeight(height);
  }
  /**
   * @return The rectangle's area.
   */
  public double getArea() {
    return width * height;
  }

  /**
   * @return The rectangle's perimeter.
   */
  public double getPerimeter() {
    return 2 * (width + height);
  }

  /**
   * @return The rectangle's width.
   */
  public double getWidth() {
     return width;
  }
  /**
   * @param width The rectangle's new width
   */
  public void setWidth(double w) {
      width = w;
  }

  /**
   * @return The rectangle's height.
   */
  public double getHeight() {
     return height;
  }

  /**
   * @param height The rectangle's new height
   */
  public void setHeight(double height) {
      this.height = height;
  }
}

We define an instance variable radius inside our class Circle:

public class Circle {

  private double radius;
...
}

Next we implement our method to change a circle's radius:

public void setRadius(double r) {
  radius = r;
}

Note that we have chosen a different value for the method's formal radius parameter to be “r” rather than “radius”. Many people prefer to use radius here making it easier for a programmer to recognize the expected name in the generated Javadoc™:

public void setRadius(double radius) {
  this.radius = radius;
}

This requires the usage of the this keyword to distinguish the formal parameter in setRadius(double radius) from the instance variable previously being defined within our class Circle. In other words: We have to resolve a name shadowing conflict.

The rest of the implementation is (quite) straightforward. A complete class reads:

package step1;

/**
 * A circle of given radius
 *
 */
public class Circle {

  private double radius;

  /**
   * A new circle
   * @param radius The desired radius.
   */
  public Circle(double radius) {
    setRadius(radius);
  }

  /**
   * @return The circle's area.
   */
  public double getArea() {
    return radius * radius * Math.PI;
  }

  /**
   * @return The circle's perimeter.
   */
  public double getPerimeter() {
    return 2 * Math.PI * radius;
  }

  /**
   * @return The circle's radius.
   */
  public double getRadius() {
    return radius;
  }

  /**
   * @param radius Setting the circle's radius to a new value.
   */
  public void setRadius(double radius) {
    this.radius = radius;
  }
}

public class Rectangle {

  private double x, y, width, height;
 ...
  /**
   * @param x The rectangle's x center coordinate value
   */
  public void setX(double x) {
    this.x = x;
  }
  /**
   * @param y The rectangle's y center coordinate value
   */
  public void setY(double y) {
    this.y = y;
  }

public void writeSvg() {
    final int scale = 20;
    System.out.println(
        "<rect width='" + scale * width +"' height='" + scale * height +
        "' x='" + scale * x + "'" + " y='" + scale * y + "'" +
        "' style='fill:rgb(0,255,0);stroke-width:3;stroke:rgb(0,0,0)'/>");
  }
}

public class Circle {

  double x, y, radius;
...

  /**
   * @param x The circle's x center coordinate value
   */
  public void setX(double x) {
    this.x = x;
  }
  /**
   * @param y The circle's y center coordinate value
   */
  public void setY(double y) {
    this.y = y;
  }

public void writeSvg() {
  final int scale = 20;

    System.out.println(
        "<circle r='" + scale * radius +
        "' cx='" + scale * x + "'" + " cy='" + scale * y +
        "' style='fill:rgb(0,0,255);stroke-width:3;stroke:rgb(0,0,0)'/>");

  }
}

/**
 * Raise the employee's salary by the given percentage.
 *
 * Example: If the current annual salary is 30000 € then
 * raising by 2,5% will result in 30750 €
 *
 * @param percentage Raise the current salary by this percentage.
 *
 */
public void raiseSalary(double percentage) {
  salary *= (1 + percentage / 100);
}

/**
 * Print the employee's current state to standard output like e.g.:
 *
 * <pre>Age:25
Salary:30000.00</pre>
*/
public void print() {
  System.out.println("Age:" + age);
  System.out.format("Salary:%6.2f\n", salary);
}

Moving the Employee class from its company package to model changes its package declaration:

package company;

public class Employee {
   int age;
   double salary;
   ...

package model;

public class Employee {
   int age;
   double salary;
   ...

The movement also adds an additional import statement to your Driver class being required since both classes no longer reside in a common package:

package company;

public class Driver {

   public static void
     main(String[] args) {
      ...

package company;

import model.Employee;

public class Driver {

   public static void
       main(String[] args) {
      ...

Unfortunately a Driver class still residing in our company package will no longer compile due to a newly introduced access restriction:

package company;

public class Driver {

   public static void main(String[] args) {

      final Employee jim = new Employee(25, 30_000.00);

      jim.salary=40_000.00; // Compiler error:
                            // "The field Employee.salary is not visible"

      jim.print();

   }
}

This error message is in accordance with Table 4.1 from the “Encapsulation and Access Control” section of [Kurniawan]: The attributes age and salary have no access modifiers at all. Hence “default” access prohibits classes belonging to other packages accessing the attributes in question. More clearly: Members from class Driver have no permission to access either age or salary.

There are two possible ways to overcome this problem:

The novice reader may question the advantage of the second approach: Alien classes have essentially the same means changing an employee's salary as with the first approach.

There is however one apparent benefit: Developers do have better control including bug tracking options on offer:

public void setSalary(double salary) {
  if (salary < 0) {
  System.err.println("This is odd: A salary may not become negative!" +
                     " Ask your union about minimum wages.");
  } else {
    this.salary = salary;
  }
}

Depending on your type of project (Java, Maven,...) both your .java source files and generated .class files will be organized below their respective entry level directories. The following typical layout depends on your Java development environment / IDE:

Packages give rise to further sub directories both for source code and compiler generated byte code. The following example is a minor variation of our employee example. The model package has been refactored into company.model:

 Project root
 ┣━━━┳ src ❶
 ┃   ┣━━━┳ company
 ┃  ...  ┣━━━ Driver.java ━━━━━━━━┓
 ┃       ┗━━━┳ model              ┃ compile
 ┃           ┗━━━ Employee.java ━━╋━━━┓
 ┣━━━┳ target                     ┃   ┃ ❷
 ┃   ┣━━━┳ company                ┃   ┃
 ┃  ...  ┣━━━ Driver.class ◀━━━━━━┛   ┃
 ┃       ┗━━━┳ model                  ┃
 ┃           ┗━━━ Employee.class ◀━━━━┛
...

We implement our private String class in a package de.hdm_stuttgart.mi.sd1.tooling:

package de.hdm_stuttgart.mi.sd1.tooling;

public class String { ❶

    final java.lang.String s; ❷

    public String(final java.lang.String s ❸) {
        this.s = s;
    }

    public java.lang.String toString() ❹ {
        return s;
    }

    public java.lang.String ❺ sayHello() {
        return "Hello '" + s + "', good morning!";
    }
}

There are basically two ways using de.hdm_stuttgart.mi.sd1.tooling.String and java.lang.String in the same scope: