The difference between Scala’s traits and Java’s interface, as well as the Scala trait’s own type and dependency injection

1. The difference between Scala’s traits and java interfaces

There are some differences in concepts and usage between traits in Scala and interfaces in Java:

  1. Default implementation: In Java, an interface can only define the signature of a method but not a default implementation. In Scala’s characteristics, in addition to defining method signatures, you can also define the specific implementation of the method. In this way, in a class that mixes in a trait, you can directly use the default implementation of the methods defined in the trait.

  2. Multiple inheritance: In Java, a class can only inherit singly, but it can implement multiple interfaces. In Scala, a class can be mixed with multiple traits, achieving the effect of multiple inheritance. This makes Scala’s traits more flexible and can resolve conflicts caused by multiple inheritance.

  3. Definition of fields: Both traits and interfaces can define fields, but in Scala traits, fields can contain specific initial values. In Java interfaces, fields can only be constants (i.e. static final fields).

  4. Constructors: Neither traits nor interfaces can directly define constructors. In Java, interfaces cannot have constructors, and in Scala, traits cannot have explicit constructors. However, traits can define abstract methods with parameters, which is equivalent to defining a constructor that requires passing parameters.

  5. Linearization of traits: Traits in Scala have linearization properties, which means that method calls in the trait will be parsed in linearized order. This feature makes traits more flexible and controllable.

In general, traits in Scala are more powerful and flexible than interfaces in Java. Traits can contain default implementations of methods, support multiple inheritance, and can define fields and abstract methods with parameters. The linearization properties of traits also make method parsing more controllable. These features make traits in Scala more flexible and convenient when implementing code reuse and component design.

2. Scala trait definition and trait mixing and trait superposition rules

2.1 Scala trait definition and trait mixing

In the Scala language, traits (characteristics) are used to replace the concept of interfaces. That is to say, when multiple classes have the same traits (characteristics), this trait (characteristics) can be isolated and declared using the keyword trait.
Traits in Scala can have abstract properties and methods, or concrete properties and methods. A class can be mixed with multiple traits. This feels similar to abstract classes in Java.
Scala introduces trait features, which can replace Java interfaces and supplement the single inheritance mechanism.

  • Basic syntax:
trait trait name {
trait subject
}
  • example:
trait PersonTrait {<!-- -->

// declare properties
var name:String = _

//Declaration method
def eat():Unit={<!-- -->
}

// abstract properties
var age:Int

//Abstract method, the characteristic of speaking language, because the language is uncertain, it is defined as abstract method
def speakLanguage():Unit
}
  • Classes inherit certain characteristics:
    Basic syntax for class inheritance traits:
    A class has a certain trait (characteristic), which means that this class satisfies all the elements of this trait (characteristic), so when using it, the extends keyword is also used. If there are multiple traits or a parent class exists, then you need Use the with keyword to connect.
    1) Basic syntax:
    No parent class: class classname extends trait 1 with trait 2 with trait 3 …
    There is a parent class: class class name extends parent class with trait 1 with trait 2 with trait 3…
    2) Description:
    (1) The relationship between classes and traits: use inheritance.
    (2) When a class inherits traits, the first connective is extends, followed by with.
    (3) If a class inherits both traits and parent class, the parent class should be written after extends.
    3) Case practice:
    (1) Traits can have both abstract methods and concrete methods
    (2) A class can be mixed with multiple traits
    (3) All Java interfaces can be used as Scala traits
    (4) Dynamic mixing: flexibly expands the functions of the class
    • (4.1) Dynamic mixing: Mixing in a trait when creating an object without mixing the class into the trait.
    • (4.2) If there are unimplemented methods in the mixed trait, they need to be implemented.

4. Trait superposition rules

Since a class can be mixed into multiple traits, and traits can have specific attributes and methods, if the mixed traits have the same method (the method name, parameter list, and return value are the same), inheritance conflicts will inevitably occur. question.

  • Conflicts are divided into the following two types:

  • The first one is that two traits (TraitA, TraitB) mixed into a class (Sub) have the same specific methods, and there is no relationship between the two traits. To solve this kind of conflict problem, directly re-do it in the class (Sub). Write conflict methods.

  • Second, a class (Sub) mixed into two traits (TraitA, TraitB) has the same specific method, and the two traits inherit from the same trait (TraitC), and the so-called “diamond problem” is solved. To solve the conflict problem, Scala adopts the strategy of trait superposition.

    The so-called trait superposition is to superimpose conflicting methods in multiple mixed traits.

  • example:

trait Ball {<!-- -->
def describe(): String = {<!-- -->
"ball"
}
}

trait Color extends Ball {<!-- -->
override def describe(): String = {<!-- -->
"blue-" + super.describe()
}
}

trait Category extends Ball {<!-- -->
override def describe(): String = {<!-- -->
"foot-" + super.describe()
}
}

class MyBall extends Category with Color {<!-- -->
override def describe(): String = {<!-- -->
"my ball is a " + super.describe()
}
}
object TestTrait {<!-- -->
def main(args: Array[String]): Unit = {<!-- -->
println(new MyBall().describe())
}
}
  • operation result:

4.1 Trait overlay execution sequence

Thinking: Does super.describe() in the above case call the method in the parent trait?
When a class is mixed with multiple traits, Scala will sort all traits and their parent traits in a certain order, and the super.describe() call in this case actually calls the next trait after sorting. describe() method in . The sorting rules are as follows:

in conclusion:
(1) Super in the case does not represent its parent trait object, but represents the next trait in the above superposition sequence, that is, super in MyClass refers to Color, super in Color refers to Category, and super in Category refers to On behalf of Ball.

(2) If you want to call a specified method mixed into the trait, you can add a constraint: super[], for example:
super[Category].describe().

4.2 Summary of Scala’s trait superposition rules

Traits in Scala can be mixed in to classes to add new functionality. When a class mixes in multiple traits, certain superposition rules will be followed.
Here are the stacking rules for Scala traits:

  1. Linear superposition: Traits in Scala form an inheritance hierarchy, so linear superposition is used when superimposing traits, that is, all traits are linearized according to their inheritance relationship, and then all members are combined into a new whole.

  2. Method conflict: If there are methods with the same name between traits, a conflict will occur. At this point, the compiler checks whether these conflicting methods have the same method signature and return type. If it is, the method will only be introduced once; if not, you need to override the method in the class and provide a concrete implementation.

  3. Calling order: When calling mixed trait methods in a specific instance, the calling order of the methods is the same as the linearized order, that is, starting from the rightmost trait and calling it in sequence from the left until the leftmost trait is called.

  4. Initialization order: When the mixed-in trait has a constructor, the initialization order must also be in linear order. The rightmost trait’s constructor is executed first, then the constructors of each trait are executed sequentially to the left.

  5. super call: When calling a mixed trait method in a specific instance, you can use the super keyword to call the implementation of the parent trait with the same method signature. This calling method also needs to follow linearization order.
    In general, the superposition rules of traits in Scala are relatively complex, mainly including linear superposition, method conflict, calling order, initialization order and super call. These rules make classes mixed with traits more flexible, but also increase the complexity of code design and maintenance. Therefore, traits should be used with caution in actual development and attention should be paid to avoiding possible problems.

5. Self-type and dependency injection of Scala traits

Traits in Scala are a mechanism for grouping methods and fields together, similar to interfaces in Java. Traits can be mixed in by classes, thereby providing additional functionality to the class and achieving the effect of multiple inheritance.

Trait types (self types) are a way of limiting what types of classes a trait can be mixed into. By specifying a specific type as the self type when defining a trait, only classes that mix in that specific type can mix in the trait. This ensures that traits can only be used by specific types of classes, increasing the correctness and security of the code.

Dependency Injection (DI) is a design pattern used to decouple dependencies between components. In Scala, you can use trait types and dependency injection to achieve decoupling of components.

The effect of dependency injection can be achieved by using the dependent component as the self type of the trait, and then mixing it into the class where the component is needed. This can easily replace dependent implementations and improve the testability and maintainability of the code.

For example, suppose we have a class UserService that requires logging functionality:

trait Logger {<!-- -->
  def log(message: String): Unit
}

class UserService {<!-- -->
  this: Logger => // Use Logger as self type

  def register(username: String, password: String): Unit = {<!-- -->
    //Registration logic
    log(s"User '$username' registered.")
  }
}

Now we can define a class that implements Logger and mix it into UserService:

class ConsoleLogger extends Logger {<!-- -->
  def log(message: String): Unit = println(message)
}

val userService = new UserService with ConsoleLogger
userService.register("user123", "password")

Through dependency injection, we can easily replace the log implementation, such as using file logs or database logs, without modifying the code of UserService.

In summary, trait types and dependency injection in Scala are a powerful mechanism that can achieve code decoupling and flexible replacement of components. This design pattern improves code testability, maintainability, and scalability.

5.1 Trait self-type case 1

  • Trait self-type: The self-type can realize the function of dependency injection.
  • example:
//User class
class User(val name: String, val password: String){<!-- -->}

trait UserDao {<!-- -->
 //The User class is injected here
  _: User =>

  //Insert data into the database
  def insert(): Unit = {<!-- -->
    println(s"insert into db: ${<!-- -->this.name}")
  }
}

//Define registered user class
class RegisterUser(name: String, password: String) extends User(name, password) with UserDao

//test
object Test_TraitSelfType {<!-- -->
  def main(args: Array[String]): Unit = {<!-- -->
    val user = new RegisterUser("alice", "123456")
    user.insert()
  }
}

The _underscore here represents a wildcard character, which refers to the current UserDao trait.

5.2 Trait self-type case 2

/**
  * Dependency injection refers to the creation of dependent objects, which is completed by a third party, rather than the dependent object. We call this transfer of control relationship dependency injection or control inversion.
  * scala implements dependency injection through its own type restrictions
  */
trait Logger {<!-- --> def log(msg: String) }
 
trait Auth {<!-- -->
  //The own type is named auth, and it is limited to the fact that Logger must be carried when Auth is instantiated.
  auth: Logger =>
  def act(msg: String): Unit = {<!-- -->
    log(msg) //After limiting its own type, you can use methods in the carrying class
  }
}
 
object DI extends Auth with Logger {<!-- -->
  override def log(msg: String) = println(msg)
}
 
object Dependency_Injection {<!-- -->
  def main(args: Array[String]): Unit = {<!-- -->
    DI.act("I hope you will like it")
  }
}

5.3 The difference between traits and abstract classes

1. Prioritize the use of traits. It is convenient for a class to extend multiple traits, but it can only extend one abstract class.
2. If you need constructor parameters, use abstract classes. Because abstract classes can define constructors with parameters, but traits cannot (with parameterless constructors).