Defining Traits

A trait is defined using the trait keyword:

1   trait Animal {
2       int hunger = 100
3       def  eat() { println "eating"; hunger -= 1 }
4       abstract int getNumberOfLegs()
5   }

As this code demonstrates, traits can have properties, methods, and abstract methods. If a class implements a trait, it must implement its abstract methods.

Using Traits

To use a trait, you use the implements keyword . For example:

 1   class Rocket  {
 2       String name
 3       def  launch() { println(name + " Take off!") }
 4   }
 5   trait MoonLander {
 6       def land() { println("${getName()} Landing!") }
 7       abstract String getName()
 8   }
 9   class  Apollo  extends  Rocket implements  MoonLander {
10   }                                                                    

So now you can do the following:

1   def  apollo = new  Apollo(name: "Apollo 12")
2   apollo.launch()
3   apollo.land()

This would generate the following output:

1   Apollo 12 Take off!
2   Apollo 12 Landing!

Unlike super-classes , you can use multiple traits in one class. Here is such an example:

1   trait Shuttle {
2       boolean canFly() { true }
3       abstract int getCargoBaySize()
4   }
5   class MoonShuttle  extends  Rocket
6       implements  MoonLander, Shuttle {
7       int getCargoBaySize() { 100 }
8   }

This would allow you to do the following:

1   MoonShuttle m = new  MoonShuttle(name: 'Taxi')
2   println "${m.name} can fly? ${m.canFly()}"
3   println "cargo bay: ${m.getCargoBaySize()}"
4   m.launch()
5   m.land()

And you would get the following output:

1   Taxi can fly? true          
2   cargo bay: 100
3   Taxi Take off!
4   Taxi Landing!

Summary

In this chapter, you learned about the following Groovy features:

• What traits are, which is similar to Java 8 interfaces.

• How to define a trait with fields and methods.

• How you can use multiple traits in one class.