[英]method overriding Vs class variable overriding in java
I was just trying some sample code for checking class variable overriding behavior in Java. 我只是尝试了一些示例代码来检查Java中的类变量覆盖行为。 Below is the code:
以下是代码:
class A{
int i=0;
void sayHi(){
System.out.println("Hi From A");
}
}
class B extends A{
int i=2;
void sayHi(){
System.out.println("Hi From B");
}
}
public class HelloWorld {
public static void main(String[] args) {
A a= new B();
System.out.println("i->"+a.i); // this prints 0, which is from A
System.out.println("i->"+((B)a).i); // this prints 2, which is from B
a.sayHi(); // method from B gets called since object is of type B
}
}
I am not able to understand whats happening at these two lines below 我无法理解下面这两行发生的事情
System.out.println("i->"+a.i); // this prints 0, which is from A
System.out.println("i->"+((B)a).i); // this prints 2, which is from B
Why does ai
print 0
even if the object is of type B
? 为什么即使对象是
B
类, ai
也会打印0
? And why does it print 2
after casting it to B
? 为什么在将它投射到
B
后打印2
?
i
is not a method - it's a data member. i
不是一个方法 - 它是一个数据成员。 Data members don't override, they hide . 数据成员不会覆盖,他们隐藏 。 So even though your instance is a
B
, it has two data members - i
from A
and i
from B
. 所以,即使你的实例是
B
,它有两个数据成员- i
从A
和i
从B
。 When you reference it through an A
reference you will get the former and when you use a B
reference (eg, by explicitly casting it), you'll get the latter. 当你通过
A
引用引用它时,你会得到前者,当你使用B
引用时(例如,通过明确地引用它),你将得到后者。
Instance methods , on the other hand, behave differently. 另一方面,实例方法的行为也不同。 Regardless of the the type of the reference, since the instance is a
B
instance, you'll get the polymorphic behavior and get the string "Hi From B"
printed. 无论引用的类型如何,因为实例是
B
实例,您将获得多态行为并打印字符串"Hi From B"
。
Even though A is initialized as new B(), the variable is an A. if you say 即使A被初始化为新的B(),如果你说,变量是A.
B a = new B();
you won't have that problem. 你不会有这个问题。
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