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Visitor

Intent: Represent an operation to be performed on the elements of an object structure. Visitor lets you define a new operation without changing the classes of the elements on which it operates.

A common use case is a directory walker, with a related file visitor.

FileVisitor.java

public interface FileVisitor {
    void visitFile(File parent, File file);
}

It is important to add the parent, not only the file itself, to be able to reconstruct the directories hierarchy.

DirectoryWalker.java

public class DirectoryWalker {
    private final File startingDirectory;
 
    public DirectoryWalker(final String startingDirectoryName) {
        this.startingDirectory = new File(startingDirectoryName);
    }
 
    public void walk(FileFilter filter, FileVisitor visitor) {
        final LinkedList<File> pendingDirectories = new LinkedList<File>();
 
        visitor.visitFile(null, startingDirectory);
        pendingDirectories.add(startingDirectory);
 
        while (pendingDirectories.isEmpty() == false) {
            File parent = pendingDirectories.removeFirst();
            File[] fileList = parent.listFiles(filter);
            Arrays.sort(fileList);
            for (File file : fileList) {
                visitor.visitFile(parent, file);
                if (file.isDirectory()) pendingDirectories.add(file);
            }
        }
    }
}

The FileFilter allow to filter the file visited and Arrays.sort(fileList) allow to preserve the names ordering of the files.

visitor.visitFile(parent, file) is the call to the visitor.

Java, OOP, DesignPattern

java/design_patterns/visitor.txt · Last modified: 2010/08/12 by emilmont

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Visitor