Scala in Impatient 习题解答5 类A1

1. class Counter {
       private var value = 0
   
       def increment() { if (value < Int.MaxValue) value += 1 }
       def current = value
   }
   

2. class BankAccount {
       private var b = 0
   
       def balance = b 
   
       def deposit(num: Int) = {
           if (num > 0) {
               b = b + num
           }
           b
       }
   
       def withdraw(num: Int) = {
           if (num > 0) {
               b = b - num
           }
           b
       }
   }
   

3. class Time(val hours: Int, val minutes: Int) {
   
       def before(other: Time): Boolean = {
           hours + minutes < other.hours + other.minutes
       }
   }
   

   scala> val a = new Time(3,21)
   a: Time = Time@13a09b0
   
   scala> val b = new Time(23,39)
   b: Time = Time@1da6308
   
   scala> a.before(b)
   res23: Boolean = true
   
   scala> b.before(a)
   res24: Boolean = false
   

4. class Time(val hours: Int, val minutes: Int) {
       private val m = hours * 60 + minutes
   
       def before(other: Time): Boolean = {
           m < other.m
       }
   }
   

5. class Student {
       var name: String = ""
       var id: Long = 0
   }
   
   $ javap -private Student.class 
   Compiled from "5.scala"
   public class Student implements scala.ScalaObject {
     private java.lang.String name;
     private long id;
     public java.lang.String name();
     public void name_$eq(java.lang.String);
     public long id();
     public void id_$eq(long);
     public Student();
   }
   

使用@BeanProperty增加兼容JavaBean的get和set方法

```scala
import scala.reflect.BeanProperty

class Student {
    @BeanProperty
    var name: String = _
    @BeanProperty
    var id: Long = 0
}
```

1. class Person(var age: Int) {
       if (age < 0) age = 0
   }
   

2. name应该设定为val以防止被修改

    class Person(val name: String) {
        private val nameArray = name.split(" ")
   
        def firstName = nameArray(0)
        def lastName = nameArray(1)
    }
   

3. class Car(val manufacturer: String, val model: String,
       val year:Int = -1, var number:String = "") {
   }
   

4. 感受下Java实现的

    public class Car { 
        private final String manufacturer;
        private final String model;
        private final int year;
        private String number;
   
        public Car(String manufacturer, String model) {
            this(manufacturer, model, -1, "");
        }
   
        public Car(String manufacturer, String model, int year) {
            this(manufacturer, model, year, "");
        }
   
        public Car(String manufacturer, String model, int year, String number) {
            this.manufacturer = manufacturer;
            this.model = model;
            this.year = year;
            this.number = number;
        }
   
        public String getManufacturer() {
            return this.manufacturer;
        }
   
        public String getModel() {
            return this.model;
        }
   
        public int getYear() {
            return this.year;
        }
   
        public String getNumber() {
            return this.number;
        }
   
        public void setNumber() {
            this.number = number;
        }
    }
   

5. 显然还是用primary constructor更简洁

   class Employee {
     private var n: String = "John Q. Public"
     var salary: Double = 0.0
   
     def this(name: String, salary: Double) {
       this()
       n = name
       this.salary = salary
     }
   
     def name = n
   }