第七章: Polymorphism

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分类专栏： ThinkingInJava 3学习笔记文章标签： binding constructor reference class hierarchy methods

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博客主要围绕多态性展开，介绍了方法调用的绑定，包括前绑定和后绑定。还阐述了基类构造函数的调用步骤，强调构造函数应尽量少做事，避免调用方法，仅可调用基类的final方法。并通过示例代码展示了多态性在构造函数中的体现。

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第七章: Polymorphism

1、 Upcasting revisited--Taking an object reference and treating it as a reference to its base type.(通过绑定来实现多态的方法调用)

Method-call binding方法调用的绑定，分前绑定和后绑定--early binding(When binding is performed before the program is run)、Late binding also called dynamic binding or run-time binding(means that the binding occurs at run time, based on the type of object)
定义抽象类：
abstract class Instrument {
  private int i; // Storage allocated for each
  public abstract void play(Note n);
  public String what() {
    return "Instrument";
  }
  public abstract void adjust();
}

2、Constructors and polymorphism

2.1)The base-class constructor is called. This step is repeated recursively such that the root of the hierarchy is constructed first, followed by the next-derived class, etc., until the most-derived class is reached.

2.2)Member initializers are called in the order of declaration.

2.3)The body of the derived-class constructor is called

a good guideline for constructors is, “Do as little as possible to set the object into a good state, and if you can possibly avoid it, don’t call any methods.” The only safe methods to call inside a constructor are those that are final in the base class. (This also applies to private methods, which are automatically final.)理解如下示例：

import com.bruceeckel.simpletest.*;

abstract class Glyph {

abstract void draw();

Glyph() {

    System.out.println("Glyph() before draw()");

    draw(); //call a dynamically-bound method(多态性：所有的东西都是继承下来的)

    System.out.println("Glyph() after draw()");

}

}

class RoundGlyph extends Glyph {

private int radius = 1;

RoundGlyph(int r) {

    radius = r;

    System.out.println(

      "RoundGlyph.RoundGlyph(), radius = " + radius);

}

void draw() {     //基类构造函数在调用的时侯，radius还没有初始化

    System.out.println(

      "RoundGlyph.draw(), radius = " + radius);

}

}

public class PolyConstructors {

private static Test monitor = new Test();

public static void main(String[] args) {

    new RoundGlyph(5);

    monitor.expect(new String[] {

      "Glyph() before draw()",

      "RoundGlyph.draw(), radius = 0",

      "Glyph() after draw()",

      "RoundGlyph.RoundGlyph(), radius = 5"

    });

}

} ///:~