java – 不可变的类和子类

If you want to enforce immutability,you cannot have subclasses.

这几乎是正确的,但并非完全如此.重申一下：

If you want to enforce immutability,you must ensure that all sub-classes are immutable.

允许子类化的问题在于,通常任何可以创建类的人都可以子类化任何公共非final类.

但是所有子类都必须调用其超类的构造函数之一.包私有构造函数只能由同一包中的子类调用.

如果您使用seal packages以便控制包中的哪些类,则可以限制子类化.首先定义一个你想要子类的类：

public abstract class ImmutableBaseClass {
  ImmutableBaseClass(...) {
    ...
  }
}

由于所有子类都必须能够访问超级构造函数,因此可以确保您定义的包中的所有子类遵循不可变的规则.

public final class ImmutableConcreteClass extends ImmutableBaseClass {
  public ImmutableConcreteClass(...) {
    super(...);
  }
}

要将此应用于您的示例,

public abstract class Employee {
  private final Id id;
  private final Name name;

  // Package private constructor in sub-classable class.
  Employee(Id id,Name name,...) {
    // Defensively copy as necessary.
  }
}

public final class Accountant extends Employee {
  // Public constructos allowed in final sub-classes.
  public Accountant(Id id,...) {
    super(id,name,...);  // Call to super works from same package.
  }
}

public final class ITWorker extends Employee {
  // Ditto.
  public ITWorker(Id id,...);
  }
}