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Why can't we switch on classes in Java 7+?

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故里飘歌
故里飘歌 2020-12-30 13:35

It seems to me that such a switch statement would make a lot of sense, but it gives a compile error : 

public void m(Class c) {
   switch (c) {
       case S
5条回答
  • 无人及你
    无人及你 (楼主)
    2020-12-30 14:03

    In Java 8 you can create your own "switch-case" using lambdas. Here's a simple example on how to switch on object class (not the Class object itself as you want, but this seems to be more useful):

    import java.util.function.Consumer;

public class SwitchClass {
    private static final SwitchClass EMPTY = new SwitchClass(null) {
        @Override
        public  SwitchClass when(Class subClass,
                Consumer consumer) { return this; }

        @Override
        public void orElse(Consumer consumer) { }
    };

    final T obj;

    private SwitchClass(T obj) {
        this.obj = obj;
    }

    @SuppressWarnings("unchecked")
    public  SwitchClass when(Class subClass,
            Consumer consumer) {
        if (subClass.isInstance(obj)) {
            consumer.accept((S) obj);
            return (SwitchClass) EMPTY;
        }
        return this;
    }

    public void orElse(Consumer consumer) {
        consumer.accept(obj);
    }

    public static  SwitchClass of(T t) {
        return new SwitchClass<>(t);
    }
}



    Usage example:
    


    SwitchClass.of(obj)
    .when(Integer.class, i -> System.out.println("Integer: "+i.intValue()))
    .when(Double.class, d -> System.out.println("Double: "+d.doubleValue()))
    .when(Number.class, n -> System.out.println("Some number: "+n))
    .when(String.class, str -> System.out.println("String of length "+str.length()))
    .orElse(o -> System.out.println("Unknown object: "+o));

    


    If executes only the first matching branch, so for Double object only Double branch will be executed, not Number branch. The neat thing is that typecast is performed automatically.
    
    
    
             
    
                                                        
    
            
            
    
              
                
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