Java中构造器的初始化



package object;class Cup{Cup(int marker){System.out.println("Cup("+marker+")");}void f(int marker){System.out.println("f("+marker+")");}}class Cups {  static Cup cup1;  static Cup cup2;  static {    cup1 = new Cup(1);    cup2 = new Cup(2);  }  Cups(){  System.out.println("Cups()");  } }public class E08_StaticTest {  public static void main(String[] args) {    System.out.println("Inside main()");    //new Cups();    Cups.cup1.f(99);     }  static Cups cups1 = new Cups();  static Cups cups2 = new Cups();} /* Output:           Cup(1)           Cup(2)           Cups()           Cups()           Inside main()           f(99)         *///:~

初始化的顺序为先静态对象，后非静态对象：在进入函数的入口地址（main）之前，首先初始化静态对象cups1和cups2两个静态对象，当初始化cups1时，进入Cups的构造器，里面又有两个静态对象cup1和cup2->Cup的构造器->输出Cup(1)、Cup(2)和Cups()；当初始化cups2时，进入Cups的构造器，静态对象cup1和cup2已经创建，所以只输出Cups()；然后进入main函数，顺序执行println函数和f()方法。

需要注意的是：此示例在main函数开始之前所有的类都被加载了，实际情况通常并非如此，此示例中所有的事物都通过static联系起来。

package object;class Mug{    Mug(int marker)    {     System.out.println("Mug(" + marker + ")");    }    void f(int marker)    {     System.out.println("f(" + marker + ")");    }}public class Test{    Mug mug1;    Mug mug2;    {        mug1 = new Mug(1);        mug2 = new Mug(2);        System.out.println("mug1和mug2 初始化了");    }    Test()    {     System.out.println("Mugs()");    }    Test(int i)    {     System.out.println("Mugs(int)");    }    public static void main(String[] args)    {    System.out.println("Inside main()");        new Test();        System.out.println("new Mugs() completed");        new Test(1);        System.out.println("new Mugs(1) completed");    }}/*Output    Inside main()    Mug(1)    Mug(2)    mug1和mug2 初始化了    Mugs()    new Mugs() completed    Mug(1)    Mug(2)    mug1和mug2 初始化了    Mugs(int)    new Mugs(1) completed    *///:~

 这个例子中没有用到static关键字，则每次对匿名内部类进行初始化时相应的操作都会执行，而不是将带有static的对象只执行一次。

//: polymorphism/E15_PolyConstructors2.java/****************** Exercise 15 ****************** Add a RectangularGlyph to PolyConstructors.java* and demonstrate the problem described in this* section.***********************************************/package operators;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);}}class RectangularGlyph extends Glyph {private int width = 4;private int height = 5;RectangularGlyph(int width, int height) {this.width = width;this.height = height;System.out.println("RectangularGlyph.RectangularGlyph(), width = " +width + ", height = " + height);}void draw() {System.out.println("RectangularGlyph.draw(), area = " + width *height);}}public class E03_Aliasing2 {public static void main(String[] args) {new RoundGlyph(5);new RectangularGlyph(2,2);}} /* Output:Glyph() before draw()RoundGlyph.draw(), radius = 0Glyph() after draw()RoundGlyph.RoundGlyph(), radius = 5Glyph() before draw()RectangularGlyph.draw(), area = 0Glyph() after draw()RectangularGlyph.RectangularGlyph(), width = 2, height = 2*///:~

 上面这个例子在子代对象未完全构造之前调用了子代中的方法draw()；结果就出现了隐藏错误。new RoundGlyph(5)->调用RoundGlyph构造器，发现有父代->调用Glyph构造器，顺序执行构造器内的语句，输出Glyph() before draw()；执行到draw()时由于动态绑定执行RoundGlyph类中的draw()方法，输出RoundGlyph.draw(), radius = 0，此时radius并不等于默认的1（和想要的不一样），原因是还没执行到RoundGlyph构造器部分；再执行System.out.println("Glyph() after draw()");输出Glyph() after draw()->调用RoundGlyph构造器，将5传递给radius，输出RoundGlyph.RoundGlyph(), radius = 5。

new RectangularGlyph(2,2)的执行过程与之相似。