Help with ArrayList?

[orry kaplan]

Hi, i am trying to add these pieces of data into an ArrayList however i just want to show what i have done so far because i think i am going the right way about it but then i could be wrong :/

This is the question anyway i am answering:

Add to the array list the following data using the add method:

Name
Age
Fred
21
Jo
43
Zoe
37

import java.util.ArrayList;


public class CS1702_Lab5 {
	public static void main(String args[])
	{
		Data x = new Data("Fred",41);
		x.Print();
		
		ArrayList<Object> arr = new ArrayList<Object>();
		arr.add(21);
		arr.add("Fred");
		arr.add(43);
		arr.add("Jo");
		arr.add(37);
		arr.add("Zoe");

[orry kaplan]

orry kaplan wrote:Hi, i am trying to add these pieces of data into an ArrayList however i just want to show what i have done so far because i think i am going the right way about it but then i could be wrong :/

This is the question anyway i am answering:

Add to the array list the following data using the add method:

Name    Age
Fred       21
Jo          43
Zoe       37

import java.util.ArrayList;


public class CS1702_Lab5 {
	public static void main(String args[])
	{
		Data x = new Data("Fred",41);
		x.Print();
		
		ArrayList<Object> arr = new ArrayList<Object>();
		arr.add(21);
		arr.add("Fred");
		arr.add(43);
		arr.add("Jo");
		arr.add(37);
		arr.add("Zoe");

[Carey Brown]

You seem to have a Data class that holds a name and an age. Might be better if this was renamed to something more meaningful, such as 'Person'.
On line 10 you have

ArrayList<Object>

This should probably be more like

ArrayList<Person>

because 'Object' doesn't give you robust type checking.
Then you'll notice that arr.add(21) etc. all break. That is because you'll need to be adding Persons.

[orry kaplan]

Carey Brown wrote:You seem to have a Data class that holds a name and an age. Might be better if this was renamed to something more meaningful, such as 'Person'.
On line 10 you have

ArrayList<Object>

This should probably be more like

ArrayList<Person>

because 'Object' doesn't give you robust type checking.
Then you'll notice that arr.add(21) etc. all break. That is because you'll need to be adding Persons.

Thanks for the help, but would you say what i have done so far for this question is correct?

[Carey Brown]

orry kaplan wrote:Thanks for the help, but would you say what i have done so far for this question is correct?

It may compile, I can't tell if it would run because you provided incomplete code, but if I were your instructor I'd say no. If you've already gotten to the chapter on ArrayList then you should have already understood how to use constructors and do simplistic class definitions. You have a 'Data' class, where did it come from? What does the source code look like?

[Carey Brown]

On line 7 you use 'Data' correctly. You have to use the same approach with your 'arr.add(...)' calls.

[orry kaplan]

Carey Brown wrote:

orry kaplan wrote:Thanks for the help, but would you say what i have done so far for this question is correct?

It may compile, I can't tell if it would run because you provided incomplete code, but if I were your instructor I'd say no. If you've already gotten to the chapter on ArrayList then you should have already understood how to use constructors and do simplistic class definitions. You have a 'Data' class, where did it come from? What does the source code look like?

I have a sub class called Data that looks like so:

public class Data 
{
	private String name;
	private int age;
	Data(String n,int a)
	{
		name = n;
		age = a;
	}
	public String GetName()
	{
		return(name);
	}
	public void SetName(String n)
	{
		name = n;
	}
	public int GetAge()
	{
		return(age);
	}
	public void SetAge(int a)
	{
		age = a;
	}
	public void Print()
	{
		System.out.print(("("+GetName()));
		System.out.print(",");
		System.out.print(GetAge());
		System.out.print(") ");
	}
}

And my code i have so far but not sure if going the right way about it is like so:

import java.util.ArrayList;


public class CS1702_Lab5 {
	public static void main(String args[])
	{
		Data x = new Data("Fred",21);
		x.Print();
		Data p = new Data("Jo",43);
		x.Print();
		Data o = new Data("Zoe",37);
		x.Print();
		
		ArrayList<Data> arr = new ArrayList<Data>();
		arr.add("Fred",21);
		arr.add("Jo",43);
		arr.add("Zoe",37);
		
		for(int i=0;i<arr.size();++i)
		{
		System.out.println(arr.get(i));
		}
		System.out.println(arr);

>

[Carey Brown]

Well, the compiler should complain with this
arr.add("Fred",21);
You are half way there. Make a new Data object with "Fred" and 21 and then 'add()' that new object.

[Carey Brown]

This line will work but will will only print the hash of the object, not the object's data in a meaningful format.

System.out.println(arr.get(i));

You'll need to add

public String toString()

method to Data to return a meaningful String, something similar to your Print() method but returning a String instead of printing it out.

[orry kaplan]

Carey Brown wrote:Well, the compiler should complain with this
arr.add("Fred",21);
You are half way there. Make a new Data object with "Fred" and 21 and then 'add()' that new object.

import java.util.ArrayList;

Do you mean like so?



public class CS1702_Lab5 {
	public static void main(String args[])
	{
		Data x = new Data("Fred",21);
		x.Print();
		Data p = new Data("Jo",43);
		x.Print();
		Data o = new Data("Zoe",37);
		x.Print();
		
		ArrayList<Data> arr = new ArrayList<Data>();
		arr.add(x);
		arr.add(p);
		arr.add(o);
		
		for(int i=0;i<arr.size();++i)
		{
		System.out.println(arr.get(i));
		}
		System.out.println(arr);

>

[Carey Brown]

Excellent.

[orry kaplan]

Carey Brown wrote:Excellent.

Thanks for the help. However when i print i get these results:

(Fred,21) (Fred,21) (Fred,21) Data@1540e19d
Data@677327b6
Data@14ae5a5
[Data@1540e19d, Data@677327b6, Data@14ae5a5]

i am sure these are the wrong results i should be looking for

[Carey Brown]

This is the hash I was mentioning. Not very useful is it?
Data@14ae5a5

You'll need a toString() method added to Data to provide meaningful formatting.

[orry kaplan]

Carey Brown wrote:This is the hash I was mentioning. Not very useful is it?
Data@14ae5a5

You'll need a toString() method added to Data to provide meaningful formatting.

Would this also change the fact that (Fred, 21) has been duplicated 3 times?

[Carey Brown]

orry kaplan wrote:Would this also change the fact that (Fred, 21) has been duplicated 3 times?

The duplication is because lines 11, 13, and 15 are all printing out 'x'.

[orry kaplan]

Carey Brown wrote:

orry kaplan wrote:Would this also change the fact that (Fred, 21) has been duplicated 3 times?

The duplication is because lines 11, 13, and 15 are all printing out 'x'.

Thanks, i didn't even notice haha. But that part where you said to print toString, i am still a little confused about how to do this?

[Carey Brown]

Google "java tostring tutorial" you'll find this
http://www.dreamincode.net/forums/topic/209515-basics-of-tostring/

[orry kaplan]

Carey Brown wrote:Google "java tostring tutorial" you'll find this
http://www.dreamincode.net/forums/topic/209515-basics-of-tostring/

import java.util.ArrayList;
import java.util.Arrays;


public class CS1702_Lab5 {
	public static void main(String args[])
	{
		Data x = new Data("Fred",21);
		x.Print();
		Data p = new Data("Jo",43);
		p.Print();
		Data o = new Data("Zoe",37);
		o.Print();
		
		ArrayList<Data> arr = new ArrayList<Data>();
		arr.add(x);
		arr.add(p);
		arr.add(o);
		
		String str = Arrays.toString(Data);
		System.out.println(arr);

	}

}

i still dont know if im using the toString correctly but does this seem the correct way of using it. i read the page you sent and a few others btw

[Carey Brown]

Remove line 20. I'm not even sure what this line would do. Does it give a compile error?

Line 21 would work or you could keep your loop:

for( int i=0 ; i < arr.size() ; i++ )
    System.out.println( arr.get(i) );

[Carey Brown]

A nit-pic:
An ArrayList is usually referred to as a 'list', not an array. Therefore your variable 'arr' would be clearer as 'list'.

[Carey Brown]

Carey Brown wrote:Line 21 would work or you could keep your loop:

for( int i=0 ; i < arr.size() ; i++ )
    System.out.println( arr.get(i) );

You could also use the newer style of for() loops:

for( Data data : arr )
    System.out.println( data );

[orry kaplan]

Carey Brown wrote:

Carey Brown wrote:Line 21 would work or you could keep your loop:

for( int i=0 ; i < arr.size() ; i++ )
    System.out.println( arr.get(i) );

You could also use the newer style of for() loops:

for( Data data : arr )
    System.out.println( data );

Could you please explain what actually goes into the toString? would it be toString(Data)    ?

[Carey Brown]

The tricky part here is that System.out.println() can only print Strings, so anything you put in the parenthesis has to be converted to a String if it isn't already. For objects it would implicitly call the object's toString() method. If you wanted to be verbose (i.e. non-implicit) you could have written this:

for( Data data : arr )
    System.out.println( data.toString() );

Directly calling toString() like this works but most people let the compiler insert it implicitly.

So, you have a reference named 'data' which refers to an object of type Data. This object contains 'name' and 'age'. When toString() is called it has access to the values stored in 'name' and 'age' for that particular object. It is toString()'s job to present those values in a meaningful way.

Does this help?

[orry kaplan]

Carey Brown wrote:The tricky part here is that System.out.println() can only print Strings, so anything you put in the parenthesis has to be converted to a String if it isn't already. For objects it would implicitly call the object's toString() method. If you wanted to be verbose (i.e. non-implicit) you could have written this:

for( Data data : arr )
    System.out.println( data.toString() );

Directly calling toString() like this works but most people let the compiler insert it implicitly.

So, you have a reference named 'data' which refers to an object of type Data. This object contains 'name' and 'age'. When toString() is called it has access to the values stored in 'name' and 'age' for that particular object. It is toString()'s job to present those values in a meaningful way.

Does this help?

i understand what you are saying. you have cleared it up pretty well so thank you. but that code snip you put above, is that the solution to this? because even though i get that it cannot be printed unless it is toString, is there a way to print this list in a readable format without it as it seems a little confusing to use all the time.

[Carey Brown]

orry kaplan wrote:That code snip you put above, is that the solution to this? because even though i get that it cannot be printed unless it is toString, is there a way to print this list in a readable format without it as it seems a little confusing to use all the time.

Using toString() is considered the proper way to do it. You could have been extremely verbose like this:

for( Data data : arr )
{
        System.out.print(("("+data.GetName()));
        System.out.print(",");
        System.out.print(data.GetAge());
        System.out.println(") ");
}

This is not the path usually chosen because of the risks of errors being introduced during maintenance and modification. Also, toString() is quite often used during the debugging stage of development.

Note that inserting parenthesis, commas, field names (e.g. "Name:") into the String are all up to the designer. Whatever you need for meaningful output.

[Carey Brown]

Carey Brown wrote:You could have been extremely verbose like this:

for( Data data : arr )
{
        System.out.print(("("+data.GetName()));
        System.out.print(",");
        System.out.print(data.GetAge());
        System.out.println(") ");
}

In your case, because you have a print() method, you could have done this:

for( Data data : arr )
    data.print();

However, you COULD NOT have done this without a toString():

System.println( arr );

[orry kaplan]

Carey Brown wrote:

orry kaplan wrote:That code snip you put above, is that the solution to this? because even though i get that it cannot be printed unless it is toString, is there a way to print this list in a readable format without it as it seems a little confusing to use all the time.

Using toString() is considered the proper way to do it. You could have been extremely verbose like this:

for( Data data : arr )
{
        System.out.print(("("+data.GetName()));
        System.out.print(",");
        System.out.print(data.GetAge());
        System.out.println(") ");
}

This is not the path usually chosen because of the risks of errors being introduced during maintenance and modification. Also, toString() is quite often used during the debugging stage of development.

Note that inserting parenthesis, commas, field names (e.g. "Name:") into the String are all up to the designer. Whatever you need for meaningful output.

/
So can the toString be used on this program effectively

[orry kaplan]

Carey Brown wrote:

Carey Brown wrote:You could have been extremely verbose like this:

for( Data data : arr )
{
        System.out.print(("("+data.GetName()));
        System.out.print(",");
        System.out.print(data.GetAge());
        System.out.println(") ");
}

In your case, because you have a print() method, you could have done this:

for( Data data : arr )
    data.print();

However, you COULD NOT have done this without a toString():

System.println( arr );

import java.util.ArrayList;
import java.util.Arrays;


public class CS1702_Lab5 {
	public static void main(String args[])
	{
		Data x = new Data("Fred",21);
		x.Print();
		Data p = new Data("Jo",43);
		p.Print();
		Data o = new Data("Zoe",37);
		o.Print();
		System.out.println("");
		
		ArrayList<Data> list = new ArrayList<Data>();
		list.add(x);
	    list.add(p);
		list.add(o);
		
		for(int i = 0; i < list.size(); i++) {
        System.out.println(list.get(i).GetName());

	}

}
}

i have finalised this thinking i have finished the problem. do you think this seems correct?

[Carey Brown]

Yep. That should work.

Do you need to print age as well?

[orry kaplan]

Carey Brown wrote:Yep. That should work.

Do you need to print age as well?

Yeah I do but I wasn't sure how to add it in too, unless I duplicate the print line with GetName and change the second one to GetAge?

[Carey Brown]

Use the String concatenation operator (+)

System.out.println(list.get(i).GetName()+" "+list.get(i).GetAge());