Java中的向下转换
Java中的向下转换
提问于 2008-12-19 12:11:44
Java中允许向上转换，但是向下转换会导致编译错误。
编译错误可以通过添加强制转换来消除，但无论如何都会在运行时中断。
在这种情况下，如果Java不能在运行时执行，为什么Java允许向下转换？
这个概念有什么实际用途吗？
public class demo {
public static void main(String a[]) {
B b = (B) new A(); // compiles with the cast,
// but runtime exception - java.lang.ClassCastException
}
}
class A {
public void draw() {
System.out.println("1");
}
public void draw1() {
System.out.println("2");
}
}
class B extends A {
public void draw() {
System.out.println("3");
}
public void draw2() {
System.out.println("4");
}
}回答 7
Stack Overflow用户
回答已采纳
发布于 2008-12-19 12:20:14
当存在在运行时成功的可能性时，允许向下转换：
Object o = getSomeObject(),
String s = (String) o; // this is allowed because o could reference a String在某些情况下，这将不会成功：
Object o = new Object();
String s = (String) o; // this will fail at runtime, because o doesn't reference a String当一个类型转换(比如最后一个)在运行时失败时，将抛出一个ClassCastException。
在其他情况下，它将工作：
Object o = "a String";
String s = (String) o; // this will work, since o references a String注意，有些类型转换在编译时是不允许的，因为它们永远不会成功：
Integer i = getSomeInteger();
String s = (String) i; // the compiler will not allow this, since i can never reference a String.Stack Overflow用户
发布于 2008-12-19 13:08:04
使用您的示例，您可以执行以下操作：
public void doit(A a) {
if(a instanceof B) {
// needs to cast to B to access draw2 which isn't present in A
// note that this is probably not a good OO-design, but that would
// be out-of-scope for this discussion :)
((B)a).draw2();
}
a.draw();
}Stack Overflow用户
发布于 2010-03-16 19:25:49
@ Original Poster -查看内联评论。
public class demo
{
public static void main(String a[])
{
B b = (B) new A(); // compiles with the cast, but runtime exception - java.lang.ClassCastException
//- A subclass variable cannot hold a reference to a superclass variable. so, the above statement will not work.
//For downcast, what you need is a superclass ref containing a subclass object.
A superClassRef = new B();//just for the sake of illustration
B subClassRef = (B)superClassRef; // Valid downcast.
}
}
class A
{
public void draw()
{
System.out.println("1");
}
public void draw1()
{
System.out.println("2");
}
}
class B extends A
{
public void draw()
{
System.out.println("3");
}
public void draw2()
{
System.out.println("4");
}
}页面原文内容由Stack Overflow提供。腾讯云小微IT领域专用引擎提供翻译支持
原文链接：
https://stackoverflow.com/questions/380813
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