[FIXED] Java, a method declaration inside another method?

Issue

I'm studyin' some Java code written by my ex-colleague, and I've found something I cannot understand properly. Here we have a method, getGeneAvailableTaxonomies(), that seems to contain a method call with its own declaration, accept(). Is it true? Is it possibile?

Here's the code part: I cannot understand the meaning of the program from FilenameFilter() to the end.

public List<Integer> getGeneAvailableTaxonomies() {
    List<Integer> availableTaxon = new ArrayList<Integer>();

    File dataDirectory = new File(_currentApplicationPath, String.format("Data"));
    FilenameFilter filter = new FilenameFilter() {
        public boolean accept(File dir, String name) {
            return name.startsWith("gene_") && name.endsWith("_info.info"); //get all info files...
        }
    };
    String[] children = dataDirectory.list(filter);

    for(String child:children) {
        availableTaxon.add(Integer.parseInt(child.substring(child.indexOf("_")+1, child.lastIndexOf("_"))));
    }

    return availableTaxon;
}

Solution

The part of the code where you see the accept() method being defined is what's called an anonymous class.

FilenameFilter filter = new FilenameFilter() {
    public boolean accept(File dir, String name) {
        return name.startsWith("gene_") && name.endsWith("_info.info"); //get all info files...
    }
};

What's going on here is the creation of a new Class that is-a FilenameFilter. They're implementing/overriding the accept method. Think of it like this, but in one statement:

class MyFilenameFilter implements FilenameFilter {
    public boolean accept(File dir, String name) {
        return name.startsWith("gene_") && name.endsWith("_info.info"); 
    }
}

FilenameFilter filter = new MyFilenameFilter()

Answered By - csturtz
Answer Checked By - Marie Seifert (JavaFixing Admin)