TIJ-4Edition-多态

1、多态

　　将方法调用同方法主体关联起来的操作叫做绑定（分派）。

　　前期绑定：程序执行前由编译器和链接程序实现的绑定。

　　后期绑定：在运行时根据对象的类型确定调用方法。

　　Java中除了static方法和final方法外（private方法属于final方法），都是后期绑定的（构造器隐式地为static的）。

　　多态依靠后期绑定来实现，是一项让程序员将改变的事物和未变的事物分开的重要技术。

　　静态方法和private方法都是前期绑定的，因此，无法实现多态。

2、构造器和多态

　　复杂对象构造器调用顺序：

　　　　1、基类构造器，不断往上递归

　　　　2、按声名顺序调用成员的初始化方法

　　　　3、调用导出类构造器的主体

　　下面为TIJ中的代码：

//: polymorphism/Sandwich.java
// Order of constructor calls.
package polymorphism;
import static net.mindview.util.Print.*;

class Meal {
  Meal() { print("Meal()"); }
}

class Bread {
  Bread() { print("Bread()"); }
}

class Cheese {
  Cheese() { print("Cheese()"); }
}

class Lettuce {
  Lettuce() { print("Lettuce()"); }
}

class Lunch extends Meal {
  Lunch() { print("Lunch()"); }
}

class PortableLunch extends Lunch {
  PortableLunch() { print("PortableLunch()");}
}

public class Sandwich extends PortableLunch {
  private Bread b = new Bread();
  private Cheese c = new Cheese();
  private Lettuce l = new Lettuce();
  public Sandwich() { print("Sandwich()"); }
  public static void main(String[] args) {
    new Sandwich();
  }
} /* Output:
Meal()
Lunch()
PortableLunch()
Bread()
Cheese()
Lettuce()
Sandwich()
*///:~

 

3、继承与清理

　　继承链上需要清理时，必须在导出类中显式地调用基类的清理函数。

　　为了避免因为依赖而发生错误，清理的顺序要和初始化的顺序相反。

　　初始化时是从基类开始的，因此清理要从导出类开始。

　　下面为TIJ中的代码：

//: polymorphism/Frog.java
// Cleanup and inheritance.
package polymorphism;
import static net.mindview.util.Print.*;

class Characteristic {
  private String s;
  Characteristic(String s) {
    this.s = s;
    print("Creating Characteristic " + s);
  }
  protected void dispose() {
    print("disposing Characteristic " + s);
  }
}

class Description {
  private String s;
  Description(String s) {
    this.s = s;
    print("Creating Description " + s);
  }
  protected void dispose() {
    print("disposing Description " + s);
  }
}

class LivingCreature {
  private Characteristic p =
    new Characteristic("is alive");
  private Description t =
    new Description("Basic Living Creature");
  LivingCreature() {
    print("LivingCreature()");
  }
  protected void dispose() {
    print("LivingCreature dispose");
    t.dispose();
    p.dispose();
  }
}

class Animal extends LivingCreature {
  private Characteristic p =
    new Characteristic("has heart");
  private Description t =
    new Description("Animal not Vegetable");
  Animal() { print("Animal()"); }
  protected void dispose() {
    print("Animal dispose");
    t.dispose();
    p.dispose();
    super.dispose();
  }
}

class Amphibian extends Animal {
  private Characteristic p =
    new Characteristic("can live in water");
  private Description t =
    new Description("Both water and land");
  Amphibian() {
    print("Amphibian()");
  }
  protected void dispose() {
    print("Amphibian dispose");
    t.dispose();
    p.dispose();
    super.dispose();
  }
}

public class Frog extends Amphibian {
  private Characteristic p = new Characteristic("Croaks");
  private Description t = new Description("Eats Bugs");
  public Frog() { print("Frog()"); }
  protected void dispose() {
    print("Frog dispose");
    t.dispose();
    p.dispose();
    super.dispose();
  }
  public static void main(String[] args) {
    Frog frog = new Frog();
    print("Bye!");
    frog.dispose();
  }
} /* Output:
Creating Characteristic is alive
Creating Description Basic Living Creature
LivingCreature()
Creating Characteristic has heart
Creating Description Animal not Vegetable
Animal()
Creating Characteristic can live in water
Creating Description Both water and land
Amphibian()
Creating Characteristic Croaks
Creating Description Eats Bugs
Frog()
Bye!
Frog dispose
disposing Description Eats Bugs
disposing Characteristic Croaks
Amphibian dispose
disposing Description Both water and land
disposing Characteristic can live in water
Animal dispose
disposing Description Animal not Vegetable
disposing Characteristic has heart
LivingCreature dispose
disposing Description Basic Living Creature
disposing Characteristic is alive
*///:~

 

4、构造器内部的多态行为

　　如果在构造器内部调用了正在构造对象的某个动态绑定的方法会发生什么？

　　被覆盖的方法在对象完全构造之前就会被调用。this逸出。

　　这可能会造成很难发现的错误。

　　编写构造器时，应该尽量用简单的方法使对象进入正常状态，尽可能避免在构造器内部调用其他方法，防止this逸出！

　　构造器内部可以安全调用的是基类的final和private方法。这些方法都属于前期绑定（静态分派），不会发生多态行为。

　　下面例子是TIJ中的：

//: polymorphism/PolyConstructors.java
// Constructors and polymorphism
// don't produce what you might expect.
import static net.mindview.util.Print.*;

class Glyph {
  void draw() { print("Glyph.draw()"); }
  Glyph() {
    print("Glyph() before draw()");
    draw();    //这里调用的实际上是覆盖后的方法，但是此时导出类还没有构造完毕
    print("Glyph() after draw()");
  }
}    

class RoundGlyph extends Glyph {
  private int radius = 1;
  RoundGlyph(int r) {
    radius = r;
    print("RoundGlyph.RoundGlyph(), radius = " + radius);
  }
  void draw() {
    print("RoundGlyph.draw(), radius = " + radius);
  }
}    

public class PolyConstructors {
  public static void main(String[] args) {
    new RoundGlyph(5);
  }
} /* Output:
Glyph() before draw()
RoundGlyph.draw(), radius = 0
Glyph() after draw()
RoundGlyph.RoundGlyph(), radius = 5
*///:~