Thread: New to Objects...

I'm starting on a very simple program dealing with classes and objects and I'm confused about what's going on here...

Class file

 public class Employee { private String fname; private String lname; private String id; private double salary; Employee() {   fname = "--empty--"; lname = "--empty--"; id = "--empty--"; salary = 0; }   Employee(String f, String l, String i, String s) {   fname = f; lname = l; id = i; salary = 0; }   public void setfname (String f) {   fname = f; }   public String getfname() {   return fname; }   public void setlname(String l) {   lname=l; }   public String getlname() {   return lname; }   public void setid(String i) {   id=i; }   public String getid() {   return id; }   public void setsalary (double s) {   salary = s; }   public double getsalary() {   return salary; } }

This compiles fine

Here is the run program

 import javax.swing.*;   public class TestEmployee { public static void main (String[] args) { String firstn; String lastn; String idnum; double salary;   firstn = JOptionPane.showInputDialog(null,"Enter First Name"); lastn = JOptionPane.showInputDialog(null,"Enter Last Name"); idnum = JOptionPane.showInputDialog(null,"Enter ID Number"); salary = Double.parseDouble(JOptionPane.showInputDialog(null,"Enter Salary"));   Employee firstGuy = new Employee( firstn, lastn, idnum, salary);   firstn=JOptionPane.showInputDialog(null,"Enter First Name"); lastn=JOptionPane.showInputDialog(null,"Enter Last Name"); idnum=JOptionPane.showInputDialog(null,"Enter ID Number"); salary = Double.parseDouble(JOptionPane.showInputDialog(null,"Enter Salary"));   Employee secondGuy = new Employee(firstn, lastn, idnum, salary);   JOptionPane.showMessageDialog(null,"Firstname: " + firstGuy.getfname() + "\nLastname: " + firstGuy.getlname() + "\nID number: " + firstGuy.getid() + "\nSalary: " + firstGuy.getsalary());   JOptionPane.showMessageDialog(null,"Firstname: " + secondGuy.getfname() + "\nLastname: " + secondGuy.getlname() + "\nID number: " + secondGuy.getid() + "\nSalary: " + secondGuy.getsalary()); }   }

I can't seem to compile this very simple program. It makes me feel silly. Any help would be great. Here are the error messages.

TestEmployee.java:17: cannot find symbol
symbol : constructor Employee(java.lang.String,java.lang.String,java.la ng.String,double)
location: class Employee
Employee firstGuy = new Employee( firstn, lastn, idnum, salary);
^
TestEmployee.java:24: cannot find symbol
symbol : constructor Employee(java.lang.String,java.lang.String,java.la ng.String,double)
location: class Employee
Employee secondGuy = new Employee(firstn, lastn, idnum, salary);
^
2 errors

Originally Posted by copeg

Have a look at the constructor(s) for Employee (takes 4 strings) vs how it is attempted to be used (3 strings and 1 double)

Ah! Cool.

Now it compiles but when I run it no values are being stored in the salary field. It just displays 0.0?

Once I get some of these basics down I should be able to truck along.

What are you doing with the other 3 variables in the constructor?

Originally Posted by Junky

What are you doing with the other 3 variables in the constructor?

They are just strings for input.

Here is the corrected code

class code

 public class Employee { private String fname; private String lname; private String id; private double salary; Employee() {   fname = "--empty--"; lname = "--empty--"; id = "--empty--"; salary = 0; }   Employee(String f, String l, String i, double s) {   fname = f; lname = l; id = i; salary = s; }   public void setfname (String f) {   fname = f; }   public String getfname() {   return fname; }   public void setlname(String l) {   lname = l; }   public String getlname() {   return lname; }   public void setid(String i) {   id = i; }   public String getid() {   return id; }   public void setsalary (double s) {   salary = s; }   public double getsalary() {   return salary; } }

main code with array

 import javax.swing.*;   public class TestEmployeeArray {   public static void main (String[] args) {   String firstn; String lastn; String idnum; double salary;   Employee[] empArray = new Employee[5];   for (int i = 0; i < empArray.length; i++) {   firstn = JOptionPane.showInputDialog(null,"Enter First Name"); lastn = JOptionPane.showInputDialog(null,"Enter Last Name"); idnum = JOptionPane.showInputDialog(null,"Enter ID Number"); salary = Double.parseDouble(JOptionPane.showInputDialog(null,"Enter Salary"));   empArray[i] = new Employee(firstn, lastn, idnum, salary); }   for (int i = 0; i < empArray.length; i++) {   JOptionPane.showMessageDialog(null,"First Name: " + empArray[i].getfname() + "\nLast Name: " + empArray[i].getlname() + "\nID Number: " + empArray[i].getid() + "\nSalary: " + empArray[i].getsalary()); } JOptionPane.showMessageDialog(null,"Good Bye"); } }

For my hardest assignment in this chapter I am supposed to create a "class Rectangle" to represent a rectangle. The class needs to have the following...

1. Two double data fields named width and height that specify the width and height of a rectangle. The default values are 1 for both values.
2. A no-arg constructor that creates a default rectangle.
3. A constructor that creates a rectangle with the specified width and height.
4. A method named getArea() that returns the area of this rectangle.
5. A method named getPerimeter() that returns the perimeter.

Write a test program that creates two rectangles--one with width 4 and height 40 and the other with width 3.5 and height 35.9. Display the width, height, area, and perimeter of each rectangle in this order.

My teacher has also said he wants to add a default color - which means also add a setColor method to change the color of one of the rectangles. I'm a bit confused by this one. I should also state that he loves to change his mind and decide he wants user input for the rectangles, so I usually design that way to be covered.

Anywho, this is my class file so far.

 public class Rectangle {   //The private variables private double x; private double y; private double height; private double width; private double area; private double perimeter; private String color;   //default constructor that sets values public Rectangle() {   double x = 0; double y = 0; double height = 1; double width = 1; double area = 0; double perimeter = 0; String color = " "; } //non-default constructor that sets values based on input public Rectangle(double x, double y, double width, double height, double area, double perimeter, String color) {   setX(x); setY(y); setWidth(width); setHeight(height); setColor(color); setArea(area); setPerimeter(perimeter); }   //method for setting the height of the rectangle public void setHeight(double newHeight) {   if (newHeight < 0) {   throw new IllegalArgumentException("Invalid Data Entered!"); } else { height = newHeight; } }   //method for returning the height of the rectangle public double getHeight() {   return height; }   //method for setting the width of the rectangle public void setWidth(double newWidth) {   if (newWidth < 0) {   throw new IllegalArgumentException("Invalid Data Entered!"); } else {   width = newWidth; } }   //method for returning the width of the rectangle public double getWidth() {   return width; }   //method for setting the X coordinate public void setX(double newX){   if (newX < 0) {   throw new IllegalArgumentException("Invalid Data Entered!"); } else {   x = newX; } }   //method for returning the X coordinate public double getX() {   return x; } //method for setting the X coordinate public void setY(double newY){   if (newY < 0) {   throw new IllegalArgumentException("Invalid Data Entered!"); } else {   y = newY; } }   //method for returning the Y coordinate public double getY(){   return y; }   //method for setting the Area of the rectangle public void setArea(double newArea){   if (newArea < 0) {   throw new IllegalArgumentException("Invalid Data Entered!"); } else {   area = newArea; } }   //method for setting the Area of the rectangle public double getArea() {   return area; }   //method for setting the Area of the rectangle public void setPerimeter(double newPerimeter) {   if (newPerimeter < 0) {   throw new IllegalArgumentException("Invalid Data Entered!"); } else {   perimeter = newPerimeter; } } public double getPerimeter() {   return perimeter; } public void setColor(String color) {   color = " "; }   //method for returning the Color public String getColor() {   return color; }   //boolean method for comparing rectangles and seeing if the values are equal public boolean equals(Rectangle o) {   return (x == o.x && y == o.y && width == o.width && height == o.height); }   }

Could you guys look over my code and point out my logic errors. I could also use as much advice as possible pertaining to both this class file and the strategy for designing the test program.

Thanks a lot for your help.

Well after about 3 hours, I need some help with something I haven't learned yet.

Here is my first attempt at this program. My main goal was just to have it compile and show me the fields.

 import javax.swing.*;   public class TestRectangle {   public static void main (String[] args) {   Rectangle rectangle1 = new Rectangle(); rectangle1.setHeight1(Rectangle.height1); rectangle1.setWidth1(Rectangle.width1); rectangle1.setArea(Rectangle.area); rectangle1.setPerimeter(Rectangle.perimeter); JOptionPane.showMessageDialog(null, Rectangle.toString());   } }   //Class file class Rectangle {   public static int height1 = 4; public static int width1 = 40; public static double height2 = 1; public static double width2 = 1; public static double area = 0; public static double perimeter = 0; String color = "Red";   public Rectangle() { }   //First Rectangle  public int getHeight1(){ return height1; }   public void setHeight1(int newHeight1) { height1 = newHeight1; }   public int getWidth1(){ return width1; }   public void setWidth1(int newWidth1) { width1 = newWidth1; }   //Second Rectangle public double getHeight2(){ return height2; }   public void setHeight2(double newHeight2) { height2 = newHeight2; }   public double getWidth2(){ return width2; }   public void setWidth2(double newWidth2) { width2 = newWidth2; }   //Area and Perimeter public double getArea(){ return area; }   public void setArea(double newArea) { area = newArea; }   public double getPerimeter(){ return perimeter; }   public void setPerimeter(double newPerimeter) { perimeter = newPerimeter; }   //Color for Triangle public String getColor() { return color; }   public void setColor(String newColor) { color = newColor; }   public String toString() { return "Height = " + height1 + "\n" + "Width = " + width1 + "\n" + "Area = " + area + "\n" + "Perimeter = " + perimeter + "\n" + "Color = " + color; } }

TestRectangle.java:12: non-static method toString() cannot be referenced from a static context
JOptionPane.showMessageDialog(null, Rectangle.toString());
^
1 error

----jGRASP wedge2: exit code for process is 1.

Can someone straighten me out. I am loosing hair fast!

Why have you changed all the variables to static. Do you understand what static means? Static variables belongs to the class and not the object. All objects you create will have the same value. For example if a Person's name was static then every Person in the world would be called Fred.

Originally Posted by Junky

Why have you changed all the variables to static. Do you understand what static means? Static variables belongs to the class and not the object. All objects you create will have the same value. For example if a Person's name was static then every Person in the world would be called Fred.

OK, I changed the variables to just public.

 import javax.swing.*;   public class TestRectangle {   public static void main (String[] args) {   Rectangle rectangle1 = new Rectangle(); rectangle1.setHeight1(Rectangle.height1); rectangle1.setWidth1(Rectangle.width1); rectangle1.setArea(Rectangle.area); rectangle1.setPerimeter(Rectangle.perimeter); JOptionPane.showMessageDialog(null, Rectangle.toString());   } }   //Class file class Rectangle {   public int height1 = 4; public int width1 = 40; public double height2 = 1; public double width2 = 1; public double area = 0; public double perimeter = 0; String color = "Red";   public Rectangle() { }   //First Rectangle  public int getHeight1(){ return height1; }   public void setHeight1(int newHeight1) { height1 = newHeight1; }   public int getWidth1(){ return width1; }   public void setWidth1(int newWidth1) { width1 = newWidth1; }   //Second Rectangle public double getHeight2(){ return height2; }   public void setHeight2(double newHeight2) { height2 = newHeight2; }   public double getWidth2(){ return width2; }   public void setWidth2(double newWidth2) { width2 = newWidth2; }   //Area and Perimeter public double getArea(){ return area; }   public void setArea(double newArea) { area = newArea; }   public double getPerimeter(){ return perimeter; }   public void setPerimeter(double newPerimeter) { perimeter = newPerimeter; }   //Color for Triangle public String getColor() { return color; }   public void setColor(String newColor) { color = newColor; }   public String toString() { return "Height = " + height1 + "\n" + "Width = " + width1 + "\n" + "Area = " + area + "\n" + "Perimeter = " + perimeter + "\n" + "Color = " + color; } }

TestRectangle.java:8: non-static variable height1 cannot be referenced from a static context
rectangle1.setHeight1(Rectangle.height1);
^
TestRectangle.java:9: non-static variable width1 cannot be referenced from a static context
rectangle1.setWidth1(Rectangle.width1);
^
TestRectangle.java:10: non-static variable area cannot be referenced from a static context
rectangle1.setArea(Rectangle.area);
^
TestRectangle.java:11: non-static variable perimeter cannot be referenced from a static context
rectangle1.setPerimeter(Rectangle.perimeter);
^
TestRectangle.java:12: non-static method toString() cannot be referenced from a static context
JOptionPane.showMessageDialog(null, Rectangle.toString());
^
5 errors

What next? I'm seriously lost?

How about reading your instructions.

"Write a test program that creates two rectangles--one with width 4 and height 40 and the other with width 3.5 and height 35.9."

Rectangle rec1 = new Rectangle(4.0, 40.0);

Let see if you can go from there.

Originally Posted by Junky

How about reading your instructions.

"Write a test program that creates two rectangles--one with width 4 and height 40 and the other with width 3.5 and height 35.9."

Rectangle rec1 = new Rectangle(4.0, 40.0);

Let see if you can go from there.

Like I said before. My teacher has only covered about 30 minutes worth of material for the last part of the last class. I have not grasped the material fully yet.

I applied you code suggestion here

 import javax.swing.*;   public class TestRectangle {   public static void main (String[] args) {   Rectangle rectangle1 = new Rectangle(4, 40, 3.5, 35.9); rectangle1.setHeight1(Rectangle.height1); rectangle1.setWidth1(Rectangle.width1); rectangle1.setArea(Rectangle.area); rectangle1.setPerimeter(Rectangle.perimeter); JOptionPane.showMessageDialog(null, Rectangle.toString());   } }   //Class file class Rectangle {   public int height1 = 4; public int width1 = 40; public double height2 = 1; public double width2 = 1; public double area = 0; public double perimeter = 0; String color = "Red";   public Rectangle() { }   //First Rectangle  public int getHeight1(){ return height1; }   public void setHeight1(int newHeight1) { height1 = newHeight1; }   public int getWidth1(){ return width1; }   public void setWidth1(int newWidth1) { width1 = newWidth1; }   //Second Rectangle public double getHeight2(){ return height2; }   public void setHeight2(double newHeight2) { height2 = newHeight2; }   public double getWidth2(){ return width2; }   public void setWidth2(double newWidth2) { width2 = newWidth2; }   //Area and Perimeter public double getArea(){ return area; }   public void setArea(double newArea) { area = newArea; }   public double getPerimeter(){ return perimeter; }   public void setPerimeter(double newPerimeter) { perimeter = newPerimeter; }   //Color for Triangle public String getColor() { return color; }   public void setColor(String newColor) { color = newColor; }   public String toString() { return "Height = " + height1 + "\n" + "Width = " + width1 + "\n" + "Area = " + area + "\n" + "Perimeter = " + perimeter + "\n" + "Color = " + color; } }

TestRectangle.java:7: cannot find symbol
symbol : constructor Rectangle(int,int)
location: class Rectangle
Rectangle rectangle1 = new Rectangle(4, 40);
^
TestRectangle.java:8: non-static variable height1 cannot be referenced from a static context
rectangle1.setHeight1(Rectangle.height1);
^
TestRectangle.java:9: non-static variable width1 cannot be referenced from a static context
rectangle1.setWidth1(Rectangle.width1);
^
TestRectangle.java:10: non-static variable area cannot be referenced from a static context
rectangle1.setArea(Rectangle.area);
^
TestRectangle.java:11: non-static variable perimeter cannot be referenced from a static context
rectangle1.setPerimeter(Rectangle.perimeter);
^
TestRectangle.java:12: non-static method toString() cannot be referenced from a static context
JOptionPane.showMessageDialog(null, Rectangle.toString());
^
6 errors

I just need to see the proper form one time and I'll have it.

NOOOooooo!

Your instance variables should either have no values or their default values. You use the constructor to set their values with the values passed as parameters. Then there is no need to make all those calls to the setXXX methods. The area and perimeter should be calculated from the height and width values that you create the Rectangle object with.

Originally Posted by Junky

NOOOooooo!

Your instance variables should either have no values or their default values. You use the constructor to set their values with the values passed as parameters. Then there is no need to make all those calls to the setXXX methods. The area and perimeter should be calculated from the height and width values that you create the Rectangle object with.

OK, I don't understand. As I said I work better with "seeing the logic" rather than being told. Never built an object . No nothing of how constructors work. You're naming parts of an object that I have just started hearing about.

Even my teacher shows me examples of the proper syntax so we can start seeing through the code and get to the heart of the matter.

Rectangle rec1 = new Rectangle(4.0, 40.0); Rectangle rec2 = new Rectangle(3.5, 35.9);   class Rectangle { private double height; private double width;   public Rectangle(double w, double h) { width = w; height - h; }   // other constructors and methods }

It really isn't that difficult. Sorry to be tough but if you are struggling with this you are in for a world of hurt over the rest of the course.

Check me...

I have two codes that both work. One code based off of the example in my book, and the example you gave me.

Book example:

 import javax.swing.JOptionPane; public class TestRectangle { public static void main(String[] args) {   Rectangle rectangle = new Rectangle();   rectangle.setHeight(40); rectangle.setWidth(4); rectangle.setArea(0); rectangle.setColor("Yellow");   JOptionPane.showMessageDialog(null, rectangle.toString());   Rectangle rectangle2 = new Rectangle();   rectangle2.setHeight2(35.9); rectangle2.setWidth2(3.5); rectangle2.setArea(0); rectangle2.setColor("Blue");   JOptionPane.showMessageDialog(null, rectangle2.toString2()); } }   class Rectangle {   private int height; private int width; private double height2; private double width2; private double area; private double perimeter; private String color = "white";   public Rectangle() { }   //Int Rectangle public int getHeight() { return height; }   public void setHeight(int newHeight) { height = newHeight; }   public int getWidth() { return width; }   public void setWidth(int newWidth) { width = newWidth; }   //Double Rectangle public double getHeight2() { return height2; }   public void setHeight2(double newHeight2) { height2 = newHeight2; }   public double getWidth2() { return width2; }   public void setWidth2(double newWidth2) { width2 = newWidth2; }   public double getArea(double width, double height) { return area; }   public void setArea(double newArea) { area = newArea; }   public double getPerimeter() { return perimeter; }   public void setPerimeter(double newPerimeter) { perimeter = newPerimeter; }     public String getColor() { return color; }   public void setColor(String newColor) { color = newColor; }   public String toString() { return "Rectangle # 1\n" + "\n" + "Height = " + height + "\n" + "Width = " + width + "\n" + "Area = " + (width * height) + "\n" + "Perimeter = " + (2 * width + 2 * height) + "\n" + "Color = " + color; }   public String toString2() { return "Rectangle # 2\n" + "\n" + "Height " + height2 + "\n" + "Width " + width2 + "\n" + "Area " + (width2 * height2) + "\n" + "Perimeter " + (2 * width2 + 2 * height2) + "\n" + "Color " + color; } }

Junky example

 import javax.swing.JOptionPane; public class TestRectangle { public static void main(String[] args) {   Rectangle rec1 = new Rectangle(4, 40);   JOptionPane.showMessageDialog(null, rec1.toString());   Rectangle rec2 = new Rectangle(3.5, 35.9);   JOptionPane.showMessageDialog(null, rec2.toString2());   } }   class Rectangle {   private int height; private int width; private int area; private int perimeter; private String color; private double height2; private double width2; private double area2; private double perimeter2; private String color2;   public Rectangle(int w, int h) {   width = w; height = h; area = (h * w); perimeter = (2 * h + 2 * w); color = "Red";   }   public Rectangle(double w2, double h2) {   width2 = w2; height2 = h2; area2 = (h2 * w2); perimeter2 = (2 * h2 + 2 * w2); color2 = "Yellow";   }   public String toString() {   return "Rectangle # 1\n" + "\n" + "Height = " + height + "\n" + "Width = " + width + "\n" + "Area = " + area + "\n" + "Perimeter = " + perimeter + "\n" + "Color = " + color;   }   public String toString2() {   return "Rectangle # 2\n" + "\n" + "Height " + height2 + "\n" + "Width " + width2 + "\n" + "Area " + area2 + "\n" + "Perimeter " + perimeter2 + "\n" + "Color " + color2;   } }

Please critique both sets of codes if you have time. I would love to go into class today ahead of the game for a change.