Java Lambdas - A Concise Example

I saw this on AngelikaLanger.com and wanted to extract the following portions for my own reference/reminder:

From the java.io.File source code, we have the following:

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public File[] listFiles(FileFilter filter) {  
    String ss[] = list();  
    if (ss == null) {  
        return null;  
    }  
    ArrayList<File> files = new ArrayList<>();  
    for (String s : ss) {  
        File f = new File(s, this);  
        if ((filter == null) || filter.accept(f)) {  
            files.add(f);  
        }  
    }  
    return files.toArray(new File[files.size()]);  
}  

The above method takes FileFilter as a parameter. The FileFilter interface is:

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public interface FileFilter {  
    boolean accept(File pathname);  
}  

An example usage of this interface (and the listFiles() method) is:

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File myDir = new File("/user/admin/deploy");  
if (myDir.isDirectory()) {  
    File[] files = myDir.listFiles(  
        new FileFilter() {  
            public boolean accept(File f) {   
                return f.isFile();   
            }   
        }   
    );  
}  

The above example uses an anonymous class. We can re-work the above example to use a lambda expression instead of an anonymous class:

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File myDir = new File("/user/admin/deploy");  
if (myDir.isDirectory()) {  
    File[] files = myDir.listFiles(  
        (File f) -> { return f.isFile(); }  
    );  
}  

We can do this, in this specific case, because FileFilter is a functional interface (it only has one method to be implemented).

We can go further (in this specific example) and use a method reference instead of the lambda expression:

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File myDir = new File("/user/admin/deploy");  
if (myDir.isDirectory()) {  
    File[] files = myDir.listFiles( File::isFile );   
}  