Struggling with java

[Nick Smithson]

I'm a fairly new java student. Near the end of last semester we kinda jumped into UML, constructors and making classes in separate files and all of that pretty quick and I was already struggling with it. My professor doesn't have time to help students after class because he has to meet with 1st and 2nd semester freshmen to ensure they're on track for graduation with their classes and stuff, which eats up all of his office hours and he's the only java instructor. It's a community college, so it is what it is. Over the break I was going to try to go through my book and try to get more practice, but ended up in a pretty bad car accident and now I feel like I'm starting out behind the 8 ball. I seem to have forgotten some of the stuff we did last semester, and have been going back through my book (Tony Gaddis - Starting Out With Java 6e) trying to remember some of this, and am now snowed in with an assignment to work on. I've got some of it done, but am stuck. This is what I'm supposed to make:

Instructions:

Implement the following class:
-----------------------------------
Inventory
-----------------------------------
- quantity : int
- width : double
- cost : double
- name : String
-----------------------------------
+ Inventory()
+ Inventory (q : int, w : double, c : double, n : String)
+ getTotalCost() : double
+ display() : void
-----------------------------------
Only create the fields listed above.
Create sets and gets for each field.
All constructors should perform their traditional role. Only use set methods within the constructor.
getTotalCost returns the product of the quantity multiplied by the cost.
display should display all the fields of the object as well as the result of getTotalCost. Only use get methods within display.
Use all methods at least once in either the Inventory or InventoryDemo
Create a InventoryDemo class which uses the Inventory class. (Each class is in its own file.)
Create main.
Create an inventory item i1 using the no argument constructor.
Use display to display the results.
Create an inventory item i2 using values of: quantity =20, width=2, cost=3.25 and name = Regular
Use display to display the results.
Create a loop that decrements the quantity of i2 by 1, starting at 10 and ending at 1 (includes 10 & 1).
Perform the following steps within the loop for each iteration:
If the totalcost is greater than 20, display the special message “Reduce inventory”.
If the totalcost is 20 or less, do not display a special message.
Use display to display the result of every iteration.
Create an inventory item i3 which is an array containing 3 items.
Use the no argument constructor to load information into each item of i3.
Place display inside of a loop to display the results of array i3.

This is what I have so far:

Inventory.java

public class Inventory {

	private int quantity;
	private double width;
	private double cost;
	private String name;
	
	//~~~~~~(Constructors)~~~~~~ pg. 348 - 352
	
	//no arg constructor 
	public Inventory {
		setQuantity(0);
        setWidth(0.0);
        setCost(0.0);
        setName("");
	}
	
	//overloaded arg constructor
	public Inventory(int q, double w, double c, String n) {
		setQuantity(q);
		setWidth(w);
		setCost(c);
		setName(n);
	}
	
	
	//~~~~~~(Methods)~~~~~~ ch. 5
	
	//Total cost method
	public double getTotalCost(){
		int tot = getQuantity * getCost;
		return tot;
	}

	//display method
	public void display(){
		String show = "Quantity: " + getQuantity() + \n +  
					   "Width: " + getWidth() + \n + 
					   "Cost: " + getCost() + \n +
					   "Name: " + getName();
		return show;
	}
	
	
	//~~~~~~(Sets and Gets)~~~~~~ pg. 340
	
	//quantity set and get
    public void setQuantity(int q1) {
		quantity = q1;
    }
	
    public int getQuantity() {
		return quantity;
    }
	
	
	//width set and get
    public void setWidth(double w1) {
		width = w1;
    }

	public double getWidth() {
		return width;
    }
	
	
	//cost set and get
    public void setCost(double c1) {
		cost = c1;
    }

	public double getCost() {
		return cost;
    }
	
	
	//name set and get
    public void setName(String n1) {
		name = n1;
    }

    public String getName() {
		return name;
    }
	
	
}

InventoryDemo.java

public class InventoryDemo {
	
    public static void main(String[] args) {
		
		//inventory items 1 and 2
		Inventory i1 = new Inventory();
		i1.display();
		Inventory i2 = new Inventory(20, 2.0, 3.25, "Regular");
		i2.display();
		
		for(int i = 10; i >= 1; --i){
			i2.setQuantity(i);
                }
		
	}
	
}

I'm having a hard time remembering how to do with with two files. I thought that when I used command prompt (it's pointing at the correct location for the files) I was supposed to do javac InventoryDemo.java and it would compile both InventoryDemo and Inventory, but instead all I got was a list of 46 errors and I don't understand what the problem is. I wanted to go ahead and try to compile this much before continuing on.

The other problem I'm having, is that I'm not sure how to tackle the last part of the assignment:

If the totalcost is greater than 20, display the special message “Reduce inventory”.
If the totalcost is 20 or less, do not display a special message.
Use display to display the result of every iteration.
Create an inventory item i3 which is an array containing 3 items.
Use the no argument constructor to load information into each item of i3.
Place display inside of a loop to display the results of array i3.

This is the list of errors if it would help:

C:\Users\######\Documents\Java Programs>javac InventoryDemo.java
.\Inventory.java:39: error: <identifier> expected
public Inventory {
^
.\Inventory.java:40: error: invalid method declaration; return type required
setQuantity(0);
^
.\Inventory.java:40: error: illegal start of type
setQuantity(0);
^
.\Inventory.java:41: error: invalid method declaration; return type required
setWidth(0.0);
^
.\Inventory.java:41: error: illegal start of type
setWidth(0.0);
^
.\Inventory.java:42: error: invalid method declaration; return type required
setCost(0.0);
^
.\Inventory.java:42: error: illegal start of type
setCost(0.0);
^
.\Inventory.java:43: error: invalid method declaration; return type required
setName("");
^
.\Inventory.java:43: error: illegal start of type
setName("");
^
.\Inventory.java:47: error: class, interface, or enum expected
public Inventory(int q, double w, double c, String n) {
^
.\Inventory.java:49: error: class, interface, or enum expected
setWidth(w);
^
.\Inventory.java:50: error: class, interface, or enum expected
setCost(c);
^
.\Inventory.java:51: error: class, interface, or enum expected
setName(n);
^
.\Inventory.java:52: error: class, interface, or enum expected
}
^
.\Inventory.java:58: error: class, interface, or enum expected
public double getTotalCost(){
^
.\Inventory.java:60: error: class, interface, or enum expected
return tot;
^
.\Inventory.java:61: error: class, interface, or enum expected
}
^
.\Inventory.java:64: error: class, interface, or enum expected
public void display(){
^
.\Inventory.java:65: error: illegal character: '\'
String show = "Quantity: " + getQuantity() + \n +
^
.\Inventory.java:66: error: illegal character: '\'
"Width: " + getWidth() + \n +
^
.\Inventory.java:67: error: illegal character: '\'
"Cost: " + getCost() + \n +
^
.\Inventory.java:69: error: class, interface, or enum expected
return show;
^
.\Inventory.java:70: error: class, interface, or enum expected
}
^
.\Inventory.java:76: error: class, interface, or enum expected
public void setQuantity(int q1) {
^
.\Inventory.java:78: error: class, interface, or enum expected
}
^
.\Inventory.java:80: error: class, interface, or enum expected
public int getQuantity() {
^
.\Inventory.java:82: error: class, interface, or enum expected
}
^
.\Inventory.java:86: error: class, interface, or enum expected
public void setWidth(double w1) {
^
.\Inventory.java:88: error: class, interface, or enum expected
}
^
.\Inventory.java:90: error: class, interface, or enum expected
public double getWidth() {
^
.\Inventory.java:92: error: class, interface, or enum expected
}
^
.\Inventory.java:96: error: class, interface, or enum expected
public void setCost(double c1) {
^
.\Inventory.java:98: error: class, interface, or enum expected
}
^
.\Inventory.java:100: error: class, interface, or enum expected
public double getCost() {
^
.\Inventory.java:102: error: class, interface, or enum expected
}
^
.\Inventory.java:106: error: class, interface, or enum expected
public void setName(String n1) {
^
.\Inventory.java:108: error: class, interface, or enum expected
}
^
.\Inventory.java:110: error: class, interface, or enum expected
public String getName() {
^
.\Inventory.java:112: error: class, interface, or enum expected
}
^
InventoryDemo.java:7: error: no suitable constructor found for Inventory(no arguments)
Inventory i1 = new Inventory();
^
constructor Inventory.Inventory(<any>) is not applicable
(actual and formal argument lists differ in length)
constructor Inventory.Inventory(<any>) is not applicable
(actual and formal argument lists differ in length)
constructor Inventory.Inventory(<any>) is not applicable
(actual and formal argument lists differ in length)
constructor Inventory.Inventory(<any>) is not applicable
(actual and formal argument lists differ in length)
InventoryDemo.java:8: error: cannot find symbol
i1.display();
^
symbol: method display()
location: variable i1 of type Inventory
InventoryDemo.java:9: error: no suitable constructor found for Inventory(int,double,double,String)
Inventory i2 = new Inventory(20, 2.0, 3.25, "Regular");
^
constructor Inventory.Inventory(<any>) is not applicable
(actual and formal argument lists differ in length)
constructor Inventory.Inventory(<any>) is not applicable
(actual and formal argument lists differ in length)
constructor Inventory.Inventory(<any>) is not applicable
(actual and formal argument lists differ in length)
constructor Inventory.Inventory(<any>) is not applicable
(actual and formal argument lists differ in length)
InventoryDemo.java:10: error: cannot find symbol
i2.display();
^
symbol: method display()
location: variable i2 of type Inventory
InventoryDemo.java:13: error: cannot find symbol
i2.setQuantity(i);
^
symbol: method setQuantity(int)
location: variable i2 of type Inventory
InventoryDemo.java:14: error: cannot find symbol
if (i2.getTotalCost() <= 20.0)
^
symbol: method getTotalCost()
location: variable i2 of type Inventory
InventoryDemo.java:16: error: cannot find symbol
System.out.println(i2.display());
^
symbol: method display()
location: variable i2 of type Inventory
46 errors

C:\Users\######\Documents\Java Programs>

Line numbers for the errors may be incorrect because I removed the comment documentation header I had to put at the top because it includes my name and personal info.

[Henry Wong]

Two hints.

One. When you have a large list of errors -- only work on the first few. Syntax errors can confuse the compiler, and sometimes, it generates a long list of false positives (along with missing some errors).

Two. Regarding the first error. How do you declare a constructor? Did you happen to forget something?

Henry

[Nick Smithson]

OH! It needs parenthesis, even though it's no-arg. I was confused with that error message because it said it was an identifier issue, so I wasn't sure if I didn't capitalize something or what. So that brought it down a lot. Also fixed the multi line thing.

C:\Users\######\Documents\Java Programs>javac InventoryDemo.java
InventoryDemo.java:16: error: 'void' type not allowed here
System.out.println(i2.display());
^
.\Inventory.java:59: error: cannot find symbol
int tot = getQuantity * getCost;
^
symbol: variable getQuantity
location: class Inventory
.\Inventory.java:59: error: cannot find symbol
int tot = getQuantity * getCost;
^
symbol: variable getCost
location: class Inventory
.\Inventory.java:69: error: incompatible types: unexpected return value
return show;
^
4 errors

C:\Users\######\Documents\Java Programs>

So what are those errors about? I feel like there needs to be a whole lesson or something just on learning to read java errors. Some of them are hard to understand what it's talking about.

[Henry Wong]

Nicole Anon wrote:

C:\Users\######\Documents\Java Programs>javac InventoryDemo.java
InventoryDemo.java:16: error: 'void' type not allowed here
System.out.println(i2.display());
^

So what are those errors about?

Well, instead of looking at it from an error perspective, can you tell us what is that line supposed to do?

Henry

[Nick Smithson]

Well, instead of looking at it from an error perspective, can you tell us what is that line supposed to do?

It's supposed to call the display method in inventory.java and print out for each iteration of the loop.

	public void display(){
		String show = "Quantity: " + getQuantity() +  
					   "\nWidth: " + getWidth() + 
					   "\nCost: " + getCost() +
					   "\nName: " + getName();
		return show;
	}

[Paul Clapham]

    return show;

But this line doesn't make any sense in a method which is declared to return void (i.e. to not return anything).

[Nick Smithson]

Thank you all for the tips and such. I've finally gotten rid of errors and at least have output lol. I'm just stuck on the last part:

Create an inventory item i3 which is an array containing 3 items.
Use the no argument constructor to load information into each item of i3.
Place display inside of a loop to display the results of array i3.

		Inventory[] i3 = new Inventory[3];
		

I've got that much sorted, but how would I use the constructor to load info into each item?

[Jason Bullers]

The array you've created can hold three items at indices 0 through 2 (inclusive):

i3[0]
i3[1]
i3[2]

All you have left to do is assign to each of those "buckets" an Inventory object created by calling the no argument constructor:

i3[0] = new Inventory();
i3[1] = new Inventory();
i3[2] = new Inventory();

You could make this cleaner by using a loop instead.

There's also a "shortcut" for initializing arrays where they are declared. This is handy when you know exactly what (and how many things) will be in the array:

Inventory[] i3 = {  new Inventory(),  new Inventory(),  new Inventory() };

[Winston Gutkowski]

Nikki Smith wrote:Thank you all for the tips and such.

Well here's one more: Don't write so much code that you get a pile of errors when you compile.

My usual rule of thumb is to compile:

every time I declare a class.

every time I add a field.

every time I add a method (or constructor).

every 10 lines I write inside a method.

It may sound like a pain, but believe me, it saves a lot of frustration of the type you just went through.

The compiler is NOT forgiving, so the more you can do to keep your code clean, the easier things will be.

HIH

Winston

[Campbell Ritchie]

Winston Gutkowski wrote:. . . The compiler is NOT forgiving, . . .

But the compiler tries to help you. It tries to give you errors which will guide you to the right solution and it tries to find things before they cause your program to crash at runtime. So learn about error messages and they will help you with your programs.

Note unwarranted anthropomorphising of the compiler

[fred rosenberger]

Winston Gutkowski wrote:My usual rule of thumb is to compile:

every time I declare a class.

every time I add a field.

every time I add a method (or constructor).

every 10 lines I write inside a method.

Winston

The only thing I'd say different is I compile every 2-3 lines inside a method. But I think you've been programming longer than me, so you're probably better/more confident.

And this was one of the hardest lesson to learn, yet one of the most beneficial ones to me.