Java学习笔记：(15)使可变性最小化

不可变类是实例不能被修改的类。每个实例中包含的所有信息都必须在创建该实例的时候就提供，并在对象的整个声明周期内固定不变。例如String、BigInteger和BigDecimal类。不可变类更易于设计、实现和使用。

设计不可变类的原则：

1. 不要提供任何修改对象状态的方法
2. 保证类不会被扩展
3. 使所有域都是final的
4. 使所有的域都是私有的
5. 确保对于任何可变组件的互斥访问

// Immutable class - pages 76-78 public final class Complex { private final double re; private final double im; private Complex(double re, double im) { this.re = re; this.im = im; } public static Complex valueOf(double re, double im) { return new Complex(re, im); } public static Complex valueOfPolar(double r, double theta) { return new Complex(r * Math.cos(theta), r * Math.sin(theta)); } public static final Complex ZERO = new Complex(0, 0); public static final Complex ONE = new Complex(1, 0); public static final Complex I = new Complex(0, 1); // Accessors with no corresponding mutators public double realPart() { return re; } public double imaginaryPart() { return im; } public Complex add(Complex c) { return new Complex(re + c.re, im + c.im); } public Complex subtract(Complex c) { return new Complex(re - c.re, im - c.im); } public Complex multiply(Complex c) { return new Complex(re * c.re - im * c.im, re * c.im + im * c.re); } public Complex divide(Complex c) { double tmp = c.re * c.re + c.im * c.im; return new Complex((re * c.re + im * c.im) / tmp, (im * c.re - re * c.im) / tmp); } @Override public boolean equals(Object o) { if (o == this) return true; if (!(o instanceof Complex)) return false; Complex c = (Complex) o; // See page 43 to find out why we use compare instead of == return Double.compare(re, c.re) == 0 && Double.compare(im, c.im) == 0; } @Override public int hashCode() { int result = 17 + hashDouble(re); result = 31 * result + hashDouble(im); return result; } private int hashDouble(double val) { long longBits = Double.doubleToLongBits(re); return (int) (longBits ^ (longBits >>> 32)); } @Override public String toString() { return "(" + re + " + " + im + "i)"; } }

不可变对象本质上是线程安全的，它们不要求同步。所以，不可变对象可以被自由的共享。

不可变对象的缺点是：对于每一个不同的值都需要一个单独的对象。

使类成为不可子类化的另一种方法是将类的构造函数私有化，然后提供静态工厂来产生对象。

// Immutable class with static factories instead of constructors public class Complex { private final double re; private final double im; private Complex(double re, double im) { this.re = re; this.im = im; } public static Complex valueOf(double re, double im) { return new Complex(re, im); } // Remainder unchanged }