Handy Lambdas

Hi guys!

Do you still not use Lambdas in Java? Today I will show you how event-based projects can benefit from them.

Nice small listeners

Lambdas could be considered as a handy replacement for anonymous classes. Vaadin is an event-based framework. So, before Java 8 our PIM project had tons of code like this:

button.addClickListener(new Button.ClickListener() {
 
 private static final long serialVersionUID = -8692988480268369082L;

 public void buttonClick(ClickEvent event) {
     layout.addComponent(new Label("Thank you for clicking"));
 }
});

Note, how much clearer the code is when it uses a Lambda expression instead of the anonymous class:

button.addClickListener(event -> layout.addComponent(new Label("Thank you for clicking"))); 

The following unnecessary code was removed:

• new keyword together with the listener interface name
• serialVersionUID field
• the single interface method declaration

Even if a listener method has several lines, it also looks good:

button.addClickListener(event -> {
    Label label = new Label("Thank you for clicking");
    layout.addComponent(label);
});

Another benefit of Lambdas is direct access to parent class fields without adding parent class name before this keyword like:

    this.field

Instead of

    MyClass.this.field

Eclipse trick

Eclipse has a useful shortcut to automatically convert similar anonymous classes into Lambda expressions. For this purpose do the following:
1. Delete the serialVersionUID line. Note, the listener interface name is underlined as a warning.
2. Place the keyboard cursor on the listener interface name and press Ctrl+1
3. In the appeared popup menu select Convert to lambda expression

Lambdas in Stream API

Another useful application of Lambda expressions is Stream API. It is a kind of replacement for iterators. Streams are especially useful if you need to do many sequential manipulations on Collection items or process big Collections effectively.

Below is a simplified example from the PIM project code that represents a list of Entity objects as a String. The code does the following steps:

1. Retains only entities from the Master catalog
2. Sorts entities by ID
3. Forms String representation for each Entity object
4. Concatenates all Entity String representations into one String like 1@1,3@1,7@1

String result = entityList.stream() 
    .filter(entity -> MASTER.equals(entity.getCatalogId()))
    .sorted(Comparator.comparing(Entity::getId))
    .map(entity -> entity.getId() + AT + entity.getCatalogId())
    .collect( Collectors.joining( COMMA ) );

Think how much more code it would require to do the same with usual for operator.

I hope I persuaded you to learn a bit more about Lambda expressions and start using them in your applications.

bye!