Implementing Interfaces with `proxy` in Clojure

10.2.1 Implementing Interfaces with proxy

In this section, we delve into the powerful proxy macro in Clojure, which allows you to create anonymous classes that implement Java interfaces or extend classes. This capability is crucial for Java developers transitioning to Clojure, as it provides a seamless way to integrate Clojure code with existing Java libraries and frameworks.

Understanding the proxy Macro

The proxy macro in Clojure is a tool that allows you to create instances of anonymous classes that can implement one or more interfaces or extend a class. This is particularly useful when you need to interact with Java APIs that require you to implement specific interfaces or extend classes.

Syntax of proxy

The basic syntax of the proxy macro is as follows:

1(proxy [interface-or-class-name] [constructor-args]
2  (method-name [args] method-body)
3  ...)

• interface-or-class-name: A vector of interfaces or a single class that the proxy should implement or extend.
• constructor-args: A vector of arguments to pass to the superclass constructor.
• method-name: The name of the method you want to override.
• args: A vector of arguments for the method.
• method-body: The implementation of the method.

Creating Anonymous Classes with proxy

Let’s start with a simple example where we implement a Java interface using proxy. Suppose we have a Java interface Runnable that we want to implement in Clojure.

Java Example

In Java, you would typically implement Runnable like this:

1public class MyRunnable implements Runnable {
2    @Override
3    public void run() {
4        System.out.println("Running in Java");
5    }
6}

Clojure Example

In Clojure, using proxy, you can achieve the same functionality as follows:

1(def my-runnable
2  (proxy [java.lang.Runnable] []
3    (run []
4      (println "Running in Clojure"))))
5
6;; To use the proxy, you can pass it to a Java thread
7(.start (Thread. my-runnable))

Explanation:

• We use proxy to create an anonymous class that implements Runnable.
• The run method is overridden to print a message.
• We create a new Thread and pass the proxy instance to it.

Extending Classes with proxy

The proxy macro can also be used to extend Java classes. Let’s consider an example where we extend the java.util.TimerTask class.

Java Example

In Java, you might extend TimerTask like this:

1import java.util.TimerTask;
2
3public class MyTimerTask extends TimerTask {
4    @Override
5    public void run() {
6        System.out.println("Timer task executed");
7    }
8}

Clojure Example

In Clojure, using proxy, you can extend TimerTask as follows:

1(def my-timer-task
2  (proxy [java.util.TimerTask] []
3    (run []
4      (println "Timer task executed in Clojure"))))
5
6;; Schedule the task using a Timer
7(let [timer (java.util.Timer.)]
8  (.schedule timer my-timer-task 1000))

Explanation:

• We use proxy to extend TimerTask.
• The run method is overridden to print a message.
• We schedule the task using a Timer.

Overriding Methods with proxy

When using proxy, you can override multiple methods. Let’s see an example where we implement a custom MouseListener.

Java Example

In Java, you would implement MouseListener like this:

 1import java.awt.event.MouseListener;
 2import java.awt.event.MouseEvent;
 3
 4public class MyMouseListener implements MouseListener {
 5    @Override
 6    public void mouseClicked(MouseEvent e) {
 7        System.out.println("Mouse clicked");
 8    }
 9
10    @Override
11    public void mousePressed(MouseEvent e) {}
12
13    @Override
14    public void mouseReleased(MouseEvent e) {}
15
16    @Override
17    public void mouseEntered(MouseEvent e) {}
18
19    @Override
20    public void mouseExited(MouseEvent e) {}
21}

Clojure Example

In Clojure, using proxy, you can implement MouseListener as follows:

 1(import '[java.awt.event MouseListener MouseEvent])
 2
 3(def my-mouse-listener
 4  (proxy [MouseListener] []
 5    (mouseClicked [e]
 6      (println "Mouse clicked in Clojure"))
 7    (mousePressed [e])
 8    (mouseReleased [e])
 9    (mouseEntered [e])
10    (mouseExited [e])))
11
12;; Example usage with a Java component
13;; (.addMouseListener some-component my-mouse-listener)

Explanation:

• We use proxy to implement MouseListener.
• Only the mouseClicked method is given a body, while others are left empty.
• This proxy can be added to any Java component that supports mouse listeners.

Comparing proxy with Java Anonymous Classes

In Java, anonymous classes are often used to implement interfaces or extend classes on the fly. The proxy macro in Clojure serves a similar purpose but with a more concise syntax and the power of Clojure’s functional programming paradigm.

Key Differences

• Syntax: Clojure’s proxy is more concise and leverages the language’s macro capabilities.
• Functional Style: Clojure encourages a functional style, even when dealing with object-oriented constructs.
• Dynamic Typing: Clojure is dynamically typed, which can lead to more flexible code compared to Java’s static typing.

Practical Use Cases for proxy

The proxy macro is particularly useful in scenarios where you need to:

• Integrate with Java Libraries: Implement interfaces required by Java libraries.
• Extend Java Classes: Add custom behavior to existing Java classes.
• Event Handling: Implement event listeners for GUI applications.
• Testing: Create mock objects for testing purposes.

Try It Yourself

To deepen your understanding, try modifying the examples above:

• Exercise 1: Implement a java.lang.Comparable interface using proxy and compare two numbers.
• Exercise 2: Extend java.util.TimerTask to perform a different task, such as printing the current time.
• Exercise 3: Create a proxy for java.awt.event.ActionListener and handle button click events.

Visualizing proxy Usage

To better understand how proxy works, let’s visualize the flow of data and method calls using a diagram.

    classDiagram
	    class Proxy {
	        +method1()
	        +method2()
	    }
	    class Interface {
	        <<interface>>
	        +method1()
	        +method2()
	    }
	    Proxy --|> Interface

Diagram Explanation: This class diagram illustrates how a proxy instance implements an interface by providing concrete implementations for its methods.

Further Reading

For more information on using proxy and other Clojure features, consider exploring the following resources:

• Official Clojure Documentation
• ClojureDocs - Proxy
• Java Interoperability in Clojure

Exercises and Practice Problems

1. Implement a Custom Comparator: Use proxy to create a custom comparator for sorting strings by length.
2. GUI Event Handling: Implement a WindowListener using proxy to handle window events in a Java Swing application.
3. Mocking for Tests: Create a mock implementation of a Java interface using proxy for unit testing.

Key Takeaways

• The proxy macro in Clojure allows you to create anonymous classes that implement interfaces or extend classes, facilitating seamless Java interoperability.
• proxy is a powerful tool for integrating Clojure with Java libraries, handling events, and creating mock objects for testing.
• Understanding proxy is essential for Java developers transitioning to Clojure, as it bridges the gap between functional and object-oriented programming paradigms.

Now that we’ve explored how to implement interfaces with proxy in Clojure, let’s apply these concepts to enhance your Java-Clojure interoperability skills.

Quiz: Mastering proxy in Clojure