Shulou(Shulou.com)06/03 Report--

Big data Scala series of characteristics, the definition of characteristics, except for the use of the keyword trait, is no different from the class definition.

Attributes are used to share interfaces or attributes between classes. Both classes and objects can inherit attributes, which cannot be instantiated, so there are no parameters.

Once attributes are defined, you can use extends or with to mix attributes into the class.

1 characteristics used as an interface

Definition of traits:

Trait Logger {

/ / this is an abstract method. Methods that are not implemented in attributes are abstract by default and do not need to be modified by the abstract keyword

Def log (msg:String)

}

The realization of traits by subclasses:

Class ConsoleLogger extends Logger {

/ / override abstract methods without override

Def log (msg:String) {println (msg)}

}

2 with the characteristics of concrete realization

Trait ConsoleLogger {

/ / Note that the interface is different from that in Java

Def log (msg:String) {println (msg)}

}

The use of traits

Class SavingAccount extends Account with ConsoleLogger {

Def withdraw (amount:Double) {

If (amount > balance) log ("Insufficent funds")

Else balance-= amount

}

}

3 objects with characteristics

Scala has its own Logged characteristics, but it has not been implemented.

Trait Logged {

Def log (msg:String) {}

}

If this attribute is used in the class definition

/ / in this class, the log information will not be recorded

Class SavingAccount extends Account with Logged {

Def withdraw (amount:Double) {

If (amount > balance) log ("Insufficent funds")

Else balance-= amount

}

}

Standard ConsoleLogger extends from Logger

Class ConsoleLogger extends Logger {

/ / override abstract methods without override

Def log (msg:String) {println (msg)}

}

You can add this attribute when you create an object:

Val acct1=new SavingAccount with ConsoleLogger

In this way, the same kind of object can be created, but different characteristics can be added.

Val acct2=new SavingAccount with FileLogger

4 multiple superimposed qualities

You can add multiple attributes that call each other to a class or object, and the order in which the attributes are executed depends on the order in which the attributes are added

Trait Logged {

Def log (msg:String)

}

Trait ConsoleLogger extends Logged {

/ / override abstract methods without override

Def log (msg: String) = {println (msg)}

}

/ / time stamp log

Trait TimestampLogger extends ConsoleLogger {

Override def log (msg: String) {

Super.log (s "${java.time.Instant.now ()} $msg")

}

}

/ / truncate overly lengthy log information

Trait ShortLogger extends ConsoleLogger {

Val maxLength = 15

Override def log (msg: String) {

Super.log (

If (msg.length balance) log ("Insufficent funds")

Else balance = balance-amount

}

}

Object test {

Def main (args: Array [String]): Unit = {

Val acct1 = new SavingAccount with ConsoleLogger with TimestampLogger with ShortLogger

Val acct2 = new SavingAccount with ConsoleLogger with ShortLogger with TimestampLogger

Acct1.withdraw (100.0)

Acct2.withdraw (100.0)

}

}

/ / res:

/ / the log method of ShortLogger is executed first, then its super.log calls the log method of TimestampLogger, and finally calls the method of ConsoleLogger to print out the information.

2018-06-15T16:50:28.448Z Insufficent.

/ / first, the log method of TimestampLogger is executed, then its super.log calls the log method of ShortLogger, and finally calls the method of ConsoleLogger to print out the information.

2018-06-15T1.

5 unified programming with special features

Import scala.collection.mutable.ArrayBuffer

Trait Pet {

Val name: String

}

Class Cat (val name: String) extends Pet

Class Dog (val name: String) extends Pet

Val dog = new Dog ("Harry")

Val cat = new Cat ("Sally")

Val animals = ArrayBuffer.empty [Pet]

Animals.append (dog)

Animals.append (cat)

Animals.foreach (pet = > println (pet.name)) / / Prints Harry Sally

The characteristics that Mixins uses for class composition:

Abstract class A {

Val message: String

}

Class B extends A {

Val message = "I'm an instance of class B"

}

/ / trait C here is mixin

Trait C extends A {

Def loudMessage = message.toUpperCase ()

}

Class D extends B with C

Val d = new D

Println (d.message) / / I'm an instance of class B

Println (d.loudMessage) / / I'M AN INSTANCE OF CLASS B

6 characteristics used as a rich interface

/ / pay attention to the combination of abstract methods and concrete methods

Trait Logger {def log (msg: String)

Def info (msg: String) {log ("INFO:" + msg)}

Def warn (msg: String) {log ("WARN:" + msg)}

Def severe (msg: String) {log ("SEVERE:" + msg)}

}

Class Account {

Protected var balance:Double = 0

}

Class SavingsAccount extends Account with Logger {

Def withdraw (amount: Double) {

If (amount > balance) severe ("Insufficient funds") else "you can do this"}

Override def log (msg: String) {println (msg)}

}

Object test {

Def main (args: Array [String]): Unit = {

Val acc = new SavingsAccount

Acc.withdraw (100)

}

}

/ / result

SEVERE: Insufficient funds

7 concrete fields and abstract fields in the characteristics

A field in a trait that has an initial value is concrete, otherwise it is abstract.

Trait ShortLogger extends Logged {

Val maxLength = 15 / / specific field

}

So how does the subclass that inherits this attribute get this field? Scala puts the field directly into a subclass that inherits the specialization, rather than being inherited. For example:

Class SavingsAccount extends Account with ConsoleLogger with ShortLogger {

Var interest = 0.0

Def withdraw (amount: Double) {

If (amount > balance) log ("Insufficient funds")

Else...

}

}

Abstract fields in attributes must be overridden in concrete subclasses:

Trait ShortLogger extends Logged {

Val maxLength: Int// abstract field

Override def log (msg: String) {

Super.log (if (msg.length)

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