passing by ref issue

[BobswYourUcle]

Hello,

I am new to passing by reference and I am not sure I am doing this the right way.

I have the below loop, i want to add a list of fileList to the list FileGroup within the loop. However when I add the list

groupList.addAll(fileList)

and then clear the list with

fileList.clear()

so I can continue the loop (creating more file groups) I loose the file list I just added to the group list, leaving me with nothing.

To fix this I created a method addFileList that just adds the list, as seen below.

My question is is this an acceptable way to do this? or is this just plain wrong?

...
 List<FileGroup> groupList = new ArrayList<FileGroup>();
 List<FileInfo> fileList = new ArrayList<FileInfo>();
 
 for(int i = 0; i < files.size(); i ++)
 { 
	if(validateAdditionalfiles(files.get(i-1), files.get(i))) //if valid add to the list
	{
		fileList.add(files.get(i));
	 }
	 else
	 {	//when invalid add the current file list to a group and keep going
//		groupList.add(new FileGroup(fileList)); //I did it like this first
		addfileList(groupList, fileList); //Now i use this menthos
		fileList.clear(); //then i clear the list so I can continue
		 
		if(validateFirstFlight(minsUntilDeparture))
		{
			fileList.add(files.get(i));
		}	
	}
}
	
private static void addFileList(List<FileGroup> groupList, List<FlightInfo> fileList) 
{
	 List<fileList> tmpList = new ArrayList<fileList>();

	 tmpList.addAll(fileList);
	 groupList.add(new FileGroup(tmpList));
}

Thanks

[Junky]

Java does not pass by reference. Java does pass by value. No ifs, buts or maybes. When you pass an object reference as a parameter then you pass that reference by value. It is important that you get that clear in your head.

As to your problem, you can have a List of Lists. Or you can create a Foo class that holds a list of objects and then create a List of Foo objects. The advantage of the second option is that you can add helper methods to the class to add, remove search etc the List.

[BobswYourUcle]

Thanks,

Could you please give me a small example of how I would do this with a list of list? I am new to lists, and well java..

Thanks again.

[Junky]

List<List<FileInfo>> list = ....

[JeffGrigg]

I think it would help if you got your code to the point where it compiles first.

For example, the method name on this line:

				addfileList(groupList, fileList); // Now i use this menthos

needs to match this line exactly:

	private static void addFileList(List<FileGroup> groupList, List<FlightInfo> fileList) {

Then you probably need to decide if the 'fileList' variable (and parameter) is a List of FileInfo, or a List of FlightInfo.

This line probably contains error(s):

		List<fileList> tmpList = new ArrayList<fileList>();

Now once you get it to compile, and maybe to run, I suspect that it might make more sense for the FileGroup class to contain a List of things than to have a List that directly contains another List.

[stchman]

I used to do some programming in C and C++ and from my understanding passing by reference means passing by memory location.

Since Jave has a JRE with a garbage collector, passing by reference is not needed. You simply pass the variable directly.

[Tolls]

Instead of faffing about with this:

		addfileList(groupList, fileList); //Now i use this menthos
		fileList.clear(); //then i clear the list so I can continue

just create a new list each time, since that's what you want:

fileList = new ArrayList<FileInfo>();

[JosAH]

Since Jave has a JRE with a garbage collector, passing by reference is not needed. You simply pass the variable directly.

What has the garbage collector to do with this?

kind regards,

Jos