Lambda¶

Runnable Example¶

Lambda expressions (Project Lambda - JSR 335) are a new and important feature of the upcoming Java SE 8 platform (JSR 337). They can be used to represent one method interface (also known as functional interface) in a clear and concise way using an expression in the form of: (argument list) -> body

Classic Code¶

Before Java 8, we create and start a thread by creating an anonymous class that implements the Runnable interface, as shown in the following code:

Runnable task1 = new Runnable(){
	@Override
	public void run(){
		System.out.println("Task #1 is running");
	}
};
Thread thread1 = new Thread(task1);
thread1.start();

Or pass the anonymous class into the Thread’s constructor:

Thread thread1 = new Thread(new Runnable() {
	@Override
	public void run(){
		System.out.println("Task #1 is running");
	}
});
thread1.start();

Runnable Lambda Code¶

With Lambda expressions come with Java 8, the above code can be re-written more concisely. For example:

// Lambda Runnable
Runnable task2 = () -> { System.out.println("Task #2 is running"); };
// start the thread
new Thread(task2).start();

Collections Comparator Example¶

Sorting the list collection in the classic way¶

As a way mentioned in the article Sorting List Collections Examples, we can write a comparator to sort the list. For example, the following code creates a comparator which compares two books by their titles:

Comparator<Book> titleComparator = new Comparator<Book>() {
	public int compare(Book book1, Book book2) {
		return book1.getTitle().compareTo(book2.getTitle());
	}
};

Sorting the list collection in the Lambda way¶

Since Java 8 with Lambda expressions support, we can write a comparator in a more concise way as follows:

Comparator<Book> descPriceComp = (Book b1, Book b2) -> (int) (b2.getPrice() - b1.getPrice());

This comparator compares two books by their prices which cause the list to be sorted in descending order of prices, using the Lambda expression:

(Book b1, Book b2) -> (int) (b2.getPrice() - b1.getPrice());

Listener Example¶

Classic Listener Code¶

Before Java 8, it’s very common that an anonymous class is used to handle click event of a JButton, as shown in the following code:

JButton button = new JButton("Click Me!");
button.addActionListener(new ActionListener() {
	public void actionPerformed(ActionEvent evt) {
		System.out.println("Handled by anonymous class listener");
	}
});

Here, an anonymous class that implements the ActionListener interface is created and passed into the addActionListener() method. And as normal, the event handler code is placed inside the actionPerformed() method.

Using Lambda Listener Code¶

Because the ActionListener interface defines only one method actionPerformed(), it is a functional interface which means there’s a place to use Lambda expressions to replace the boilerplate code. The above example can be re-written using Lambda expressions as follows:

button.addActionListener(e -> System.out.println("Handled by Lambda listener"));