Is it possible to use the instanceof operator in a switch statement?

Solution 1:

This is a typical scenario where subtype polymorphism helps. Do the following

interface I {
  void do();
}

class A implements I { void do() { doA() } ... }
class B implements I { void do() { doB() } ... }
class C implements I { void do() { doC() } ... }

Then you can simply call do() on this.

If you are not free to change A, B, and C, you could apply the visitor pattern to achieve the same.

Solution 2:

if you absolutely cannot code to an interface, then you could use an enum as an intermediary:

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;

}

Solution 3:

Create a Map where the key is Class<?> and the value is an expression (lambda or similar). Consider:

Map<Class,Runnable> doByClass = new HashMap<>();
doByClass.put(Foo.class, () -> doAClosure(this));
doByClass.put(Bar.class, this::doBMethod);
doByClass.put(Baz.class, new MyCRunnable());

// of course, refactor this to only initialize once

doByClass.get(getClass()).run();

If you need checked exceptions than implement a FunctionalInterface that throws the Exception and use that instead of Runnable.

Here's a real-word before-and-after showing how this approach can simplify code.

The code before refactoring to a map:

private Object unmarshall(
  final Property<?> property, final Object configValue ) {
  final Object result;
  final String value = configValue.toString();

  if( property instanceof SimpleDoubleProperty ) {
    result = Double.parseDouble( value );
  }
  else if( property instanceof SimpleFloatProperty ) {
    result = Float.parseFloat( value );
  }
  else if( property instanceof SimpleBooleanProperty ) {
    result = Boolean.parseBoolean( value );
  }
  else if( property instanceof SimpleFileProperty ) {
    result = new File( value );
  }
  else {
    result = value;
  }

  return result;
}

The code after refactoring to a map:

private final Map<Class<?>, Function<String, Object>> UNMARSHALL = 
Map.of(
  SimpleBooleanProperty.class, Boolean::parseBoolean,
  SimpleDoubleProperty.class, Double::parseDouble,
  SimpleFloatProperty.class, Float::parseFloat,
  SimpleFileProperty.class, File::new
);

private Object unmarshall(
  final Property<?> property, final Object configValue ) {
  return UNMARSHALL
    .getOrDefault( property.getClass(), ( v ) -> v )
    .apply( configValue.toString() );
}

This avoids repetition, eliminates nearly all branching statements, and simplifies maintenance.

Solution 4:

Just in case if someone will read it:

The BEST solution in java is :

public enum Action { 
    a{
        void doAction(...){
            // some code
        }

    }, 
    b{
        void doAction(...){
            // some code
        }

    }, 
    c{
        void doAction(...){
            // some code
        }

    };

    abstract void doAction (...);
}

The GREAT benefits of such pattern are:

1. You just do it like (NO switches at all):

   void someFunction ( Action action ) {
       action.doAction(...);   
   }
   

2. In case if you add new Action called "d" you MUST imlement doAction(...) method

NOTE: This pattern is described in Joshua's Bloch "Effective Java (2nd Edition)"