Filling Arraylist using recursion in Java

Question

I am trying to fill a list myClassTypes with recursive calls to getMyClassTypes(). But it gets initialized every time there is recursive call to the method. And, I am losing few values because of this. I understand that the problem is with declaring locally: List myClassTypes = new ArrayList<>(); But, I don't want to declare the list at the class level. Here is the code I am working with:

private List<MyClassType> getMyClassTypes(final String path) throws SVNException {

    final Collection<SVNDirEntry> svnDirectoryEntries = svnRepository.getDir(path, -1, null, (Collection<SVNDirEntry>) null);
    final Iterator<SVNDirEntry> svnDirectoryEntryIterator = svnDirectoryEntries.iterator();
    final List<MyClassType> myClassTypes = new ArrayList<>();

    while (svnDirectoryEntryIterator.hasNext()) {
        final SVNDirEntry entry = svnDirectoryEntryIterator.next();
        final String fileName = entry.getName();

        if (!path.isEmpty() && path.matches(SUB_DIRECTORY_NAME_PATTERN) && fileName.endsWith(".xml")) {                
            final MyClassType myClassType = new MyClassType(path, fileName);
            myClassTypes.add(myClassType);
        }

        /*
         * Check if the entry is a directory recursively.
         */
        if (someCondition) {
            getMyClassTypes((path.equals("")) ? entry.getName() : path + "/" + entry.getName());
        }
    }
    return myClassTypes;
}

How to manage the 'myClassTypes' to contain all the values even after 'n times' recursive calls?

you mean you will call getMyClassTypes n times?

bremen_matt
– bremen_matt

2018-04-26 11:10:35 +00:00
Commented Apr 26, 2018 at 11:10 — bremen_matt
– bremen_matt, Commented Apr 26, 2018 at 11:10

Eran · Accepted Answer · 2018-04-26 11:18:21Z

Pass a List<MyClassType> instance as argument to the method instead of creating it inside the method.

This way to create just one ArrayList instance, and you can even change the return type to void.

private void getMyClassTypes(final String path, List<MyClassType> myClassTypes) throws SVNException 
{
    final Collection<SVNDirEntry> svnDirectoryEntries = svnRepository.getDir(path, -1, null, (Collection<SVNDirEntry>) null);
    final Iterator<SVNDirEntry> svnDirectoryEntryIterator = svnDirectoryEntries.iterator();

    while (svnDirectoryEntryIterator.hasNext()) {
        final SVNDirEntry entry = svnDirectoryEntryIterator.next();
        final String fileName = entry.getName();

        if (!path.isEmpty() && path.matches(SUB_DIRECTORY_NAME_PATTERN) && fileName.endsWith(".xml")) {                
            final MyClassType myClassType = new MyClassType(path, fileName);
            myClassTypes.add(myClassType);
        }

        /*
         * Check if the entry is a directory recursively.
         */
        if (someCondition) {
            getMyClassTypes((path.equals("")) ? entry.getName() : path + "/" + entry.getName(), myClassTypes);
        }
    }
}

Thiyagu · Accepted Answer · 2018-04-26 11:12:07Z

1

Add the return value of the recursive call to the current myClassTypes and return it.

if (someCondition) {
    myClassTypes.addAll(getMyClassTypes((path.equals("")) ? entry.getName() : path + "/" + entry.getName()));
}

answered Apr 26, 2018 at 11:12

Thiyagu

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4 Comments

Eran Over a year ago

This has the downside of creating multiple ArrayList instances when you only need one of them.

Thiyagu Over a year ago

@Eran True. If the recursion depth is too long and it proves to affect performance, then we can pass it as a parameter as you suggest.

Thiyagu Over a year ago

@Eran I felt this is a bit more cleaner (not mutate the input parameter)

code_buddy Over a year ago

@Eran: Thanks for the idea. Worked fine. That's a pretty solution. (Sorry that I am not allowed to add +1 to answer as my level is too low on stackoverflow)

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