我有一个问题,使用开关情况的instanceof对象:
例如:我的问题可以在Java中重现:
if(this instanceof A)
doA();
else if(this instanceof B)
doB();
else if(this instanceof C)
doC():
如何使用switch…case实现它?
我有一个问题,使用开关情况的instanceof对象:
例如:我的问题可以在Java中重现:
if(this instanceof A)
doA();
else if(this instanceof B)
doB();
else if(this instanceof C)
doC():
如何使用switch…case实现它?
当前回答
不,这是不可能的。然而,您可能想要做的是将多态性作为处理这类问题的一种方法。
其他回答
如果你绝对不能编码到接口,那么你可以使用枚举作为中介:
public A() {
CLAZZ z = CLAZZ.valueOf(this.getClass().getSimpleName());
switch (z) {
case A:
doA();
break;
case B:
doB();
break;
case C:
doC();
break;
}
}
enum CLAZZ {
A,B,C;
}
像这样使用switch语句不是面向对象的方式。相反,您应该使用多态性的力量。简单的写
this.do()
之前已经建立了一个基类:
abstract class Base {
abstract void do();
...
}
它是A, B和C的基类:
class A extends Base {
void do() { this.doA() }
}
class B extends Base {
void do() { this.doB() }
}
class C extends Base {
void do() { this.doC() }
}
不,这是不可能的。然而,您可能想要做的是将多态性作为处理这类问题的一种方法。
下面是在Java 8中使用http://www.vavr.io/来实现这一功能的方法
import static io.vavr.API.*;
import static io.vavr.Predicates.instanceOf;
public Throwable liftRootCause(final Throwable throwable) {
return Match(throwable).of(
Case($(instanceOf(CompletionException.class)), Throwable::getCause),
Case($(instanceOf(ExecutionException.class)), Throwable::getCause),
Case($(), th -> th)
);
}
While it is not possible to write a switch statement, it is possible to branch out to specific processing for each given type. One way of doing this is to use standard double dispatch mechanism. An example where we want to "switch" based on type is Jersey Exception mapper where we need to map multitude of exceptions to error responses. While for this specific case there is probably a better way (i.e. using a polymorphic method that translates each exception to an error response), using double dispatch mechanism is still useful and practical.
interface Processable {
<R> R process(final Processor<R> processor);
}
interface Processor<R> {
R process(final A a);
R process(final B b);
R process(final C c);
// for each type of Processable
...
}
class A implements Processable {
// other class logic here
<R> R process(final Processor<R> processor){
return processor.process(this);
}
}
class B implements Processable {
// other class logic here
<R> R process(final Processor<R> processor){
return processor.process(this);
}
}
class C implements Processable {
// other class logic here
<R> R process(final Processor<R> processor){
return processor.process(this);
}
}
然后在任何需要“开关”的地方,你可以这样做:
public class LogProcessor implements Processor<String> {
private static final Logger log = Logger.for(LogProcessor.class);
public void logIt(final Processable base) {
log.info("Logging for type {}", process(base));
}
// Processor methods, these are basically the effective "case" statements
String process(final A a) {
return "Stringifying A";
}
String process(final B b) {
return "Stringifying B";
}
String process(final C c) {
return "Stringifying C";
}
}