Reduce memory foot print by replace anonymous class with singleton. But need to refactor the design more

Question

So I have this method that get executed repeatedly

public static boolean isReady(String dirPath, int numPdfInPrintJob){
    File dir = new File(dirPath);
    String[] fileList = dir.list(new FilenameFilter(){
        public boolean accept(File file, String filename) {
            return (filename.toLowerCase().endsWith(".pdf"));
        }
    });
    if(fileList.length >= numPdfInPrintJob) return true;
    else return false;  
}

This method using anonymous class that will create a new instance of FilenameFilter every time invoked and I invoke this method a lot. So I want to make this anonymous class into a singleton. So my initial thought is to create a new singleton class that look like this

public class PdfFileNameFilter implements FilenameFilter{
    private PdfFileNameFilter(){} //non-instantible

    //guarantee to only have one instance at all time
    public static final PdfFileNameFilter INSTANCE = new PdfFileNameFilter();

    public boolean accept(File dir, String name) {
        return (name.toLowerCase().endsWith(".pdf"));
    }
}

Can I refactor this a bit more. I need to do ZipFileNameFilter as well, and maybe many different file extension filter. Dont want to create a class for each filter. I need to refactor this design a bit more. Maybe interface come into place somewhere here.

Answer 1

If all you wanted to do was reduce memory usage you could have done

private static final FilenameFilter PDF_FILES = new FilenameFilter(){
    public boolean accept(File file, String filename) {
        return (filename.toLowerCase().endsWith(".pdf"));
    }
}

If you want to create a singleton, the simplest way is

public enum PdfFileNameFilter implements FilenameFilter {
    INSTANCE;

    public boolean accept(File dir, String name) {
        return (name.toLowerCase().endsWith(".pdf"));
    }
}

Answer 2

It seems simpler to me to just use your existing anonymous class and make one instance that all your method invocations use.

private static final FilenameFilter PDF_FILTER = new FilenameFilter() {
    public boolean accept(File file, String filename) {
        return (filename.toLowerCase().endsWith(".pdf"));
    }
}

public static boolean isReady(String dirPath, int numPdfInPrintJob){
    File dir = new File(dirPath);
    String[] fileList = dir.list(pdfFilter);
    if(fileList.length >= numPdfInPrintJob) return true;
    else return false;  
}

This is a case where subclassing and making a singleton seems to be a tad overkill: you simply want only one instance to use right here, whereas a singleton is used when there is only one instance you will ever want to use.