当一个复杂的对象被构造时,它的构造函数按下面的顺序被调用(that the order of constructor calls for a complex object is as follows)
1.其基类(base-class)的构造函数被调用,这个步骤以递归的方式重复,所以最底层(the root of
hierarchy)的构造函数首先被执行,然后是它上一层派生类(the next-derived class)...直到最顶层的派生类(the
most-derived class).
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.如果有包含关系(composition),那么它的成员对象按照声明的顺序被构造.
Member initializers are called in the order of declaration.
3.派生类构造函数的内容(body)被执行.
The body of the derived-class constructor is called.
一个实例:
class Cake{
Cake(){System.out.println("Cake()");}
}
class Meal {
Meal() { System.out.println("Meal()"); }
}
class Bread {
Bread() { System.out.println("Bread()"); }
}
class Cheese {
Cheese() { System.out.println("Cheese()"); }
}
class Lettuce {
Lettuce() { System.out.println("Lettuce()"); }
}
class Lunch extends Meal {
Lunch() { System.out.println("Lunch()"); }
}
class PortableLunch extends Lunch {
//if make derived-class object as the menber of the base-class will lead a infinite
//loop and program will stop because of the memory consumed
//private Sandwich s=new Sandwich();
private Cake a=new Cake();
PortableLunch() { System.out.println("PortableLunch()");}
}
public class Sandwich extends PortableLunch
{
private Bread b = new Bread();
private Cheese c = new Cheese();
private Lettuce l = new Lettuce();
public Sandwich() {
System.out.println("Sandwich()");
}
public static void main(String[] args) {
new Sandwich();
}
}
输出:
Meal()
Lunch()
Cake()
PortableLunch()
Bread()
Cheese()
Lettuce()
Sandwich()
main()函数中要构造一个Sandwich的对象,调用(并不是执行)它基类PortableLunch的构造函数,PortableLunch又递
归的调用,然后是Meal,Meal是继承的最底层的基类(不算Object)所以它的构造函数首先被执行,然后按次序返回到Lunch,
PortableLunch,但在PortableLunch的构造函数被执行之前,它的成员对象Cake
a先按照声明的顺序被构造.然后执行PortableLunch(),接着是Sandwich的成员对象,最后是Sandwich().
注:被注释掉的代码,将base-class的对象作为derive-class的成员对象,这样会递归无法结束,最后程序因堆栈耗尽而结束(Exception in thread main java.lang.StackOverflowError).
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