Clever Geek Handbook

Reading Java 8 Code

This piece of Java code is hard to understand. How is this DirExplorer created? DirExplorer class link https://github.com/ftomassetti/analyze-java-code-examples/blob/master/src/main/java/me/tomassetti/support/DirExplorer.java Cheers, Code below:

 new DirExplorer((level, path, file) -> path.endsWith(".java"), (level, path, file) -> {
        System.out.println(path);
        System.out.println(Strings.repeat("=", path.length()));
        try {
            new VoidVisitorAdapter<Object>() {
                @Override
                public void visit(ClassOrInterfaceDeclaration n, Object arg) {
                    super.visit(n, arg);
                    System.out.println(" * " + n.getName());
                }
            }.visit(JavaParser.parse(file), null);
            System.out.println(); // empty line
        } catch (ParseException | IOException e) {
            new RuntimeException(e);
        }
    }).explore(projectDir);
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2 answers


Let's refactor the old-style code for easier understanding:

Filter filter = new Filter() {
    @Override
    public boolean interested(int level, String path, File file) {
        return path.endsWith(".java");
    }
};

FileHandler fileHandler = new FileHandler() {
    @Override
    public void handle(int level, String path, File file) {
        // Your long implementation for FileHandler
    }
};
new DirExplorer(filter, fileHandler).explore(projectDir);

A variable filter

is an instance of an anonymous class implementation interface filter

, the interface filter

has only one method, so in Java 8 it is a functional interface , and the initialization code above can be abbreviated as lambda expression in Java 8 to:



Filter filter = (level, path, file) -> path.endsWith(".java");

FileHandler fileHandler = (level, path, file) -> {
    // Your implementation for FileHandler
};
new DirExplorer(filter, fileHandler).explore(projectDir);

And besides that, you can inline both variables, which results in the code looking like this:

new DirExplorer((level, path, file) -> path.endsWith(".java"), (level1, path1, file1) -> {
        // Your implementation for FileHandler
    }).explore(projectDir);
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When it's hard to read, I break it down into smaller, more readable chunks. Is it easier to understand?



    Filter filter  = (level, path, file) -> path.endsWith(".java");
    FileHandler fileHandler = (level, path, file) -> {

        System.out.println(path);
        System.out.println(Strings.repeat("=", path.length()));
        try {
            new VoidVisitorAdapter<Object>() {
                @Override
                public void visit(ClassOrInterfaceDeclaration n, Object arg) {
                    super.visit(n, arg);
                    System.out.println(" * " + n.getName());
                }
            }.visit(JavaParser.parse(file), null);
            System.out.println(); // empty line
        } catch (ParseException | IOException e) {
            new RuntimeException(e);
        }
    };

    new DirExplorer(filter, fileHandler).explore(projectDir);
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