23. What will happen when you attempt to compile and run the following code?
class Base{
int i = 99;
public void amethod(){
System.out.println( "Base.amethod() ");
}
Base(){
amethod();
}
}
public class Derived extends Base{
int i = -1;
public static void main(String argv[]){
Base b = new Derived();
System.out.println(b.i);
b.amethod();
}
public void amethod(){
System.out.println( "Derived.amethod() ");
}
}
A. Derived.amethod()
-1
Derived.amethod()
B. Derived.amethod()
99
Derived.amethod()
C. 99
D. 99
Derived.amethod()
E. compile time error.
B is correct. The reason is that this code creates an instance of the Derived class but assigns it to a reference of a the Base class. In this situation a reference to any of the fields such as i will refer to the value in the Base class, but a call to a method will refer to the method in the class type rather than its reference handle. But note that if the amethod() was not present in the base class then compilation error would be reported as at compile time, when compiler sees the statement like b.amethod(), it checks if the method is present in the base class or not. Only at the run time it decides to call the method from the derived class.
这道题看的我有点不懂，希望能够详细说明，把知识点说一下～～谢谢
------解决方案--------------------
这道题应该是用到的知识点应该有 继承 ,构造方法的调用, 方法的重写
Derived 类 是 Base类的子类
子类构造方法可以调用父类的构造方法,父类构造方法的调用总是先于子类构造方法的掉用
说白了 就是实例话子类对象的时候 先掉用了父类的构造方法,然后才调用子类的构造方法
B是正确的
------解决方案--------------------
主要是编译期间和运行期间的问题
Base b = new Derived();
简单点说就是在编译期间选择的是Base，此时b.i就有值了，属于Base类的 99
第1个输出 调用的是Base（）方法，由于amethod()被重写了，所以调用的是子类的方法，b.amethod();
第2个输出b.amethod()很明显是子类的amethod()
------解决方案--------------------
Base b = new Derived();优先调用父类的构造方法 Base(){amethod();}
而方法被子类Derived的amethod()覆盖了，所以调用
public void amethod(){
System.out.println( "Derived.amethod() ");