构造器与多态

1.构造器的调用顺序

例：

class Real{    public Real(String index) {        // TODO Auto-generated constructor stub        System.out.println("Real()"+index);    }}class Meal{    Real r= new Real("Meal");    Meal() {        // TODO Auto-generated constructor stub        System.out.println("Meal()");    }}class Bread{    Bread() {        // TODO Auto-generated constructor stub        System.out.println("Bread()");    }}class Cheese{    Cheese() {        System.out.println("Cheese()");    }}class Lettuce{    Lettuce() {        // TODO Auto-generated constructor stub        System.err.println("Lettuce()");    }}class PortableLunch{    Real r= new Real("PortableLunch");    PortableLunch(){        System.out.println("PortableLunch()");    }}public class Sandwich extends PortableLunch{    Bread b= new Bread();    Cheese c= new Cheese();    Lettuce l= new Lettuce();    Sandwich(){        System.out.println("Sandwich()");    }    public static void main(String[] args) {        // TODO Auto-generated method stub        new Sandwich();    }}

2、继承与清理

例：

class Characteristic {    private String s;    Characteristic(String s) {        this.s = s;        System.out.println("Creating Characteristic " + s);    }    protected void dispose() {        System.out.println("disposing Characteristic " + s);    }}class Description {    private String s;    Description(String s) {        this.s = s;        System.out.println("Creating Description " + s);    }    protected void dispose() {        System.out.println("disposing Description " + s);    }}class LivingCreature {    private Characteristic p = new Characteristic("is alive");    private Description t = new Description("Basic Living Creature");    LivingCreature() {        System.out.println("LivingCreature()");    }    protected void dispose() {        System.out.println("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() {        System.out.println("Animal()");    }    protected void dispose() {        System.out.println("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() {        System.out.println("Amphibian()");    }    protected void dispose() {        System.out.println("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() {        System.out.println("Frog()");    }    protected void dispose() {        System.out.println("Frog dispose");        t.dispose();        p.dispose();        super.dispose();    }    public static void main(String[] args) {        Frog frog = new Frog();        System.out.println("Bye!");        frog.dispose();    }} /* * Output

3.构造器内部的多态方法的行为
例：

class Glyph {    void draw() {        System.out.println("Glyph.draw()");    }    Glyph() {        System.out.println("Glyph() before draw()");        draw();        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() {        System.out.println("RoundGlyph.draw(), radius = " + radius);    }}public class PolyConstructors {    public static void main(String[] args) {        new RoundGlyph(5);    }}/**输出结果Glyph() before draw()RoundGlyph.draw(), radius = 0Glyph() after draw()RoundGlyph.RoundGlyph(), radius = 5*/

1.当创建RoundGlyph 对象时，会先为创建该对象分配足够的内存，存储空间清理，
基本类型被初始化为0，变量radius 被初始化为0，
2.再调用基类的构造器，基类构造器
中的draw()方法被子类覆盖，所以draw()调用的是子类RoundGlyph 中的方法，但是子类还没有完成初始化，所以容易出问题

4.协变返回类型
子类覆盖版本返回的类型可以是父类返回类型的子类
例：

class Grain{    public String toString(){ return "Grain"; }}class Wheat extends Grain{    public String toString(){ return "Wheat"; }}class Mill{    Grain process(){ return new Grain(); }}class WheatMill extends Mill{    Wheat process() { return new Wheat(); }}public class CovariantReturn{    public static void main(String[] args) {        Mill m = new Mill();        Grain g = m.process();        System.out.println(g);        m = new WheatMill();        g = m.process();        System.out.println(g);    }}/*OutputGrainWheat*/

5.用继承进行设计

纯继承：（is-a）,是一种纯替代，父类和子类有着完全形同的接口
扩展：子类中有额外的方法，子类中的扩展方法基类不能访问，当向上转型时会丢失新方法

向下转型不安全，当转到错误类型容易报错

例：

class Useful {    public void f() {        System.out.println("Useful f()");    }    public void g() {        System.out.println("Useful g()");    }}class MoreUseful extends Useful {    public void f() {        System.out.println("MoreUseful f()");    }    public void g() {        System.out.println("MoreUseful g()");    }    public void u() {        System.out.println("MoreUseful u()");    }    public void v() {    }    public void w() {    }}public class RTTI {    public static void main(String[] args) {        Useful[] x = { new Useful(), new MoreUseful() };        x[0].f();        x[1].g();        // x[1].u();        ((MoreUseful) x[1]).u();        ((MoreUseful) x[0]).u();// Exception thrown.    }}