Cast a raw map to a generic map using a method, cleanly and safely in a fail early manner

问题

Casting, instanceof, and @SuppressWarnings("unchecked") are noisy. It would be nice to stuff them down into a method where they won't need to be looked at. CheckedCast.castToMapOf() is an attempt to do that.

castToMapOf() is making some assumptions:

• (1) The map can't be trusted to be homogeneous
• (2) Redesigning to avoid need for casting or instanceof is not viable
• (3) Ensuring type safety in an fail early manner is more important than the performance hit
• (4) Returning Map<String,String> is sufficient (rather than returning HashMap<String, String>)
• (5) The key and value type args are not generic (like HashMap<String, ArrayList<String>>)

(1), (2) and (3) are symptoms of my work environment, beyond my control. (4) and (5) are compromises I've made because I haven't found good ways to overcome them yet.

(4) Is difficult to overcome because even if a HashMap.class was passed into a Class<M> I haven't been able to figure out how to return a M<K, V>. So I return a Map<K, V>.

(5) Is probably an inherent limitation of using Class<T>. I'd love to hear alternative ideas.

Despite those limitations can you see any problems with this code? Am I making any assumptions I haven't identified? Is there a better way to do this? If I'm reinventing the wheel please point me to the wheel. :)

public class CheckedCast {

    public static final String LS = System.getProperty("line.separator");

    /** Check all contained items are claimed types and fail early if they aren't */
    public static <K, V> Map<K, V> castToMapOf( 
            Class<K> clazzK,    
            Class<V> clazzV,
            Map<?, ?> map) {

        for ( Map.Entry<?, ?> e: map.entrySet() ) {
            checkCast( clazzK, e.getKey() );            
            checkCast( clazzV, e.getValue() );            
        }

        @SuppressWarnings("unchecked")
        Map<K, V> result = (Map<K, V>) map;        
        return result; 
    }

    /** Check if cast would work */
    public static <T> void checkCast(Class<T> clazz, Object obj) {
        if ( !clazz.isInstance(obj) ) {
            throw new ClassCastException(
                LS + "Expected: " + clazz.getName() +
                LS + "Was:      " + obj.getClass().getName() +
                LS + "Value:    " + obj
            );
        }
    }

    public static void main(String[] args) {

        // -- Raw maps -- //

        Map heterogeneousMap = new HashMap();
        heterogeneousMap.put("Hmm", "Well");
        heterogeneousMap.put(1, 2); 

        Map homogeneousMap = new HashMap();
        homogeneousMap.put("Hmm", "Well");

        // -- Attempts to make generic -- //

        //Unsafe, will fail later when accessing 2nd entry
        @SuppressWarnings("unchecked") //Doesn't check if map contains only Strings
        Map<String, String> simpleCastOfHeteroMap = 
                    (Map<String, String>) heterogeneousMap;  

        //Happens to be safe.  Does nothing to prove claim to be homogeneous.
        @SuppressWarnings("unchecked") //Doesn't check if map contains only Strings
        Map<String, String> simpleCastOfHomoMap = 
                    (Map<String, String>) homogeneousMap;  

        //Succeeds properly after checking each item is an instance of a String
        Map<String, String> checkedCastOfHomoMap = 
                    castToMapOf(String.class, String.class, homogeneousMap);

        //Properly throws ClassCastException
        Map<String, String> checkedCastOfHeteroMap = 
                    castToMapOf(String.class, String.class, heterogeneousMap); 
        //Exception in thread "main" java.lang.ClassCastException: 
        //Expected: java.lang.String
        //Was:      java.lang.Integer
        //Value:    1
        //    at checkedcast.CheckedCast.checkCast(CheckedCast.java:14)
        //    at checkedcast.CheckedCast.castToMapOf(CheckedCast.java:36)
        //    at checkedcast.CheckedCast.main(CheckedCast.java:96)

    }
}

Some reading I found helpful:

Generic factory with unknown implementation classes

Generic And Parameterized Types

I'm also wondering if a TypeReference / super type tokens might help with (4) and (5) and be a better way to approach this problem. If you think so please post an example.

回答1:

The code looks good, but I would add an assumption: (6) the raw reference will never be used anymore. Because if you cast your Map to a Map<String, String>, then put an integer to the raw map, you may get surprises.

Map raw = new HashMap();
raw.put("Hmm", "Well");
Map<String, String> casted = castToMapOf(String.class, String.class, raw); // No error
raw.put("one", 1);
String one = casted.get("one"); // Error

Instead of casting the map, I would create a new one (maybe a LinkedHashMap to preserve order), casting each object as you add them to the new map. This way, the ClassCastException would be thrown naturally, and the old map reference can still be modified without affecting the new one.

来源：https://stackoverflow.com/questions/25618452/cast-a-raw-map-to-a-generic-map-using-a-method-cleanly-and-safely-in-a-fail-ear