构造器调用顺序-优快云博客

java 代码

//: notes about thinking in java
//c07:Sandwich.java
// Order of constructor calls.
// From 'Thinking in Java, 3rd ed.' (c) Bruce Eckel 2002
// www.BruceEckel.com. See copyright notice in CopyRight.txt.
package c07;
import com.bruceeckel.simpletest.*;
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 {
PortableLunch() { System.out.println("PortableLunch()");}
}
public class Sandwich extends PortableLunch {
private static Test monitor = new Test();
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();
monitor.expect(new String[] {
"Meal()",
"Lunch()",
"PortableLunch()",
"Bread()",
"Cheese()",
"Lettuce()",
"Sandwich()"
});
}
} ///:~
/*例子说明对象调用构造器要遵循以下顺序：
1）调用基类构造器。这个步骤会不断反复递归下去，首先是构造这种层次结构的根，
然后是下一层倒出类，等等，知道最底层的倒出类
2）按声明顺序调用成员的初始化方法
3）调用导出类构造器的主体*/

java 代码

//:notes about thinking in java
//c07:PolyConstructors.java
// Constructors and polymorphism
// don't produce what you might expect.
// From 'Thinking in Java, 3rd ed.' (c) Bruce Eckel 2002
// www.BruceEckel.com. See copyright notice in CopyRight.txt.
import com.bruceeckel.simpletest.*;
abstract class Glyph {
abstract void 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 {
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"
});
}
} ///:~
/*在Glyph中，draw()方法是抽象的，这样设计是为了覆盖该方法。我们确实在RoundGlyph中强制覆盖
draw()。但是Glyph构造器会调用这个方法，结果导致了对RoundGlyph.draw()的调用，这看起来
似乎是我们的目的。但是看到输出结果时我们会发现Glyph的构造起用draw()方法时，radius不是默认
初始值1，而是0。这可能是导致在屏幕上只画了一个点，或者根本什么东西都没有。
这一迷题的关键所在，初始化的实际过程是：
1）在其他任何事务发生之前，将分配给对象的存储空间初始化成二进制的零
2）如前所述那样调用构造器。此时，调用被覆盖后的draw()方法（要在调用RoundGlyph
构造器之前调用），由于步骤1的缘故，我们此时会发现radius的值为0
3）按照声明的顺序调用成员的初始化方法
4）调用导出类的构造器主题*/

java笔记一