【翻译十六】java-固定对象的定义方法

A Strategy for Defining Immutable Objects

The following rules define a simple strategy for creating immutable objects. Not all classes documented as "immutable" follow these rules. This does not necessarily mean the creators of these classes were sloppy — they may have good reason for believing that instances of their classes never change after construction. However, such strategies require sophisticated analysis and are not for beginners.

1. Don't provide "setter" methods — methods that modify fields or objects referred to by fields.
2. Make all fields final and private.
3. Don't allow subclasses to override methods. The simplest way to do this is to declare the class as final. A more sophisticated approach is to make the constructor private and construct instances in factory methods.
4. If the instance fields include references to mutable objects, don't allow those objects to be changed:
   • Don't provide methods that modify the mutable objects.
   • Don't share references to the mutable objects. Never store references to external, mutable objects passed to the constructor; if necessary, create copies, and store references to the copies. Similarly, create copies of your internal mutable objects when necessary to avoid returning the originals in your methods.

Applying this strategy to SynchronizedRGB results in the following steps:

1. There are two setter methods in this class. The first one, set, arbitrarily transforms the object, and has no place in an immutable version of the class. The second one, invert, can be adapted by having it create a new object instead of modifying the existing one.
2. All fields are already private; they are further qualified as final.
3. The class itself is declared final.
4. Only one field refers to an object, and that object is itself immutable. Therefore, no safeguards against changing the state of "contained" mutable objects are necessary.

After these changes, we have ImmutableRGB:

final public class ImmutableRGB {

    // Values must be between 0 and 255.
    final private int red;
    final private int green;
    final private int blue;
    final private String name;

    private void check(int red,
                       int green,
                       int blue) {
        if (red < 0 || red > 255
            || green < 0 || green > 255
            || blue < 0 || blue > 255) {
            throw new IllegalArgumentException();
        }
    }

    public ImmutableRGB(int red,
                        int green,
                        int blue,
                        String name) {
        check(red, green, blue);
        this.red = red;
        this.green = green;
        this.blue = blue;
        this.name = name;
    }

    public int getRGB() {
        return ((red << 16) | (green << 8) | blue);
    }

    public String getName() {
        return name;
    }

    public ImmutableRGB invert() {
        return new ImmutableRGB(255 - red,
                       255 - green,
                       255 - blue,
                       "Inverse of " + name);
    }
}

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译文：

固定对象的定义方法

　　下面的一些规则描述了固定对象定义的方法。并不是所有的"固定的"类都需要遵从这些规则。这并不意味着创建着是马虎的——他们有理由相信被构造的固定类永远不会被改变。然而，这种策略需要复杂的分析，不适合初学者。

　　1。不要提供“setter”方法-改变字段或对象的域名。

　　2。让所有的域都是固定的或者私有的。　

　　3。不要让子类覆盖父方法，做到这一点最简单的方法是用final.更复杂的方法是私有化构造函数，用工厂实例。

　　4。如果实例对象包括对非固定对象的引用，这些对象不允许改变。

　　　　不要提供改变可变对象的方法。

　　　　不要共享对可变对象的引用。决不保存通过构造函数传递的外部，可变对象的引用。如果必要，可以创建副本，副本的存储引用。同样，创建你内部使用的可变对象的副本，而不是直接使用原来的。

在SynchronizedRGB 中用上边的规则的使用步骤：

　　1。在这个类中有两个"setter"方法。第一个，设置，任意变换对象，并没有在一个固定类的地方。第二个，反之，可适应它创建一个新的对象而不是修改现有的。

　　2。所有对象都是私有的，更进异步成为固定的。

　　3。这个类本身被申明为固定的。

　　4。只有一个字段，指的是一个对象，该对象本身是不可变的。因此，没有保证反对改变可变对象包含的状态是必须的。

在这些改变之后，我们有 ImmutableRGB:

 final public class ImmutableRGB {

     // Values must be between 0 and 255.
     final private int red;
     final private int green;
     final private int blue;
     final private String name;

     private void check(int red,
                        int green,
                        int blue) {
         if (red < 0 || red > 255
             || green < 0 || green > 255
             || blue < 0 || blue > 255) {
             throw new IllegalArgumentException();
         }
     }

     public ImmutableRGB(int red,
                         int green,
                         int blue,
                         String name) {
         check(red, green, blue);
         this.red = red;
         this.green = green;
         this.blue = blue;
         this.name = name;
     }

     public int getRGB() {
         return ((red << 16) | (green << 8) | blue);
     }

     public String getName() {
         return name;
     }

     public ImmutableRGB invert() {
         return new ImmutableRGB(255 - red,
                        255 - green,
                        255 - blue,
                        "Inverse of " + name);
     }
 }