A Little Java, A Few Patterns

UnionHasPt类还需要重写forTrans方法

@Override
public boolean forTrans(Point q, Shape s) {
    return s.accept(new UnionHasPt(p.minus(q)));
}

new Top(new Anchovy(), 
  new Top(new Tuna(), 
    new Top(new Anchovy(), 
      new Top(new Tuna(), 
        new Top(new Anchovy(), new Bot())))))
.accept(new LtdSubst(3, new Salmon(), new Anchovy()))

new Top(new Salmon(), 
  new Top(new Tuna(), 
    new Top(new Salmon(), 
      new Top(new Tuna(), 
        new Top(new Salmon(), new Bot())))))

疑似应该是`3`而不是书中的`2`。

All bits of advice. The fifth one is missing somehow. 1. When specifying a collection of data, use abstract classes for datatypes and extended classes for variants. 2. When writing a function over a datatype, place a method in each of the variants that make up the datatype. If a field of a variant belongs to the same datatype, the method may call the corresponding method of the field to compute the function. 3. When writing a function that returns values of a datatype, use **new** to create these values. 4. When writing several functions for the same self-referential datatype, use **visitor** protocols so that all methods for a function can be found in a single class. 5. Missing.6. When the additional consumed values change for a self-referential use of a visitor, don't forget to create a new visitor. 7. When designing visitor protocols for many different types, create a unifying protocol using Object. 8. When extending a class, use overriding to enrich its functionality. 9. If a datatype may have to be extended, be forward looking and use a constructor-like(overridable) method so that visitors can be extended, too. 10. When modifications to objects are needed, use a class to insulate the operations that modify objects. Otherwise, beware the consequences of your actions.

Does that mean that anybody can write yy.p = new Bot() and thus change a pieman like yy? 36 No, because yy's type is Peiman, p isn't available. Only addTop, remTop, substTop, and occTop are visible.

Isn't it good that we didn't include p in Peiman? 37 Yes, with this trick we can prevent others from changing p (or parts of p) in strange ways. Everything is clear now.

Pieman类中的成员变量Pie p并不是private，如何理解书中的“p isn't available”呢？