added an example as test case for extracting the key type from an EnumMap

java/com.sap.sse.test/src/com/sap/sse/test/EnumMap.java

@@ -0,0 +1,110 @@
+package com.sap.sse.test;
+
+import java.io.ByteArrayInputStream;
+import java.io.ByteArrayOutputStream;
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.ObjectInputStream;
+import java.io.ObjectOutputStream;
+import java.io.ObjectStreamClass;
+
+/**
+ * A serialization-compatible mock for {@link java.util.EnumMap} that uses an equal {@link #serialVersionUID} and a
+ * compatible set of fields. When de-serializing an {@link java.util.EnumMap} instance into an instance of this class,
+ * e.g., by overriding {@link ObjectInputStream}'s {@link ObjectInputStream#resolveClass} method such that it delivers
+ * this class instead of {@link java.util.EnumMap}, the fields are made accessible through getters; in particular,
+ * {@link #getKeyType()} reveals the original {@link java.util.EnumMap}'s key type, even if the map is empty.<p>
+ * 
+ * The {@link EnumMap#getEnumMapKeyType(java.util.EnumMap)} method can be used to determine the key type of any
+ * {@link java.util.EnumMap}, even if it's empty.
+ */
+public class EnumMap<K extends Enum<K>, V> extends java.util.EnumMap<K, V> {
+    private final Class<K> keyType;
+    private transient K[] keyUniverse;
+    private transient Object[] vals;
+    private transient int size = 0;
+    private static final long serialVersionUID = 458661240069192865L;
+    
+    private static class MyObjectInputStream extends ObjectInputStream {
+        public MyObjectInputStream(InputStream in) throws IOException {
+            super(in);
+        }
+
+        @Override
+        protected Class<?> resolveClass(ObjectStreamClass desc) throws IOException, ClassNotFoundException {
+            final Class<?> result;
+            if (desc.getName().equals("java.util.EnumMap")) {
+                result = com.sap.sse.test.EnumMap.class;
+            } else {
+                result = super.resolveClass(desc);
+            }
+            return result;
+        }
+    }
+    
+    public static Class<?> getEnumMapKeyType(java.util.EnumMap<?, ?> enumMap) throws IOException, ClassNotFoundException {
+        final Class<?> result;
+        if (enumMap.isEmpty()) {
+            final ByteArrayOutputStream bos = new ByteArrayOutputStream();
+            final ObjectOutputStream oos = new ObjectOutputStream(bos);
+            oos.writeObject(enumMap);
+            oos.close();
+            final ObjectInputStream ois = new MyObjectInputStream(new ByteArrayInputStream(bos.toByteArray()));
+            final com.sap.sse.test.EnumMap<?, ?> readMap = (com.sap.sse.test.EnumMap<?, ?>) ois.readObject();
+            final Class<?> keyType = readMap.getKeyType();
+            result = keyType;
+        } else {
+            result = enumMap.keySet().iterator().next().getDeclaringClass();
+        }
+        return result;
+    }
+
+    EnumMap(Class<K> c) {
+        super(c);
+        keyType = null;
+    }
+    
+    public K[] getKeyUniverse() {
+        return keyUniverse;
+    }
+
+    public void setKeyUniverse(K[] keyUniverse) {
+        this.keyUniverse = keyUniverse;
+    }
+
+    public Object[] getVals() {
+        return vals;
+    }
+
+    public void setVals(Object[] vals) {
+        this.vals = vals;
+    }
+
+    public int getSize() {
+        return size;
+    }
+
+    public void setSize(int size) {
+        this.size = size;
+    }
+
+    public Class<K> getKeyType() {
+        return keyType;
+    }
+
+    /**
+     * Reconstitute the <tt>EnumMap</tt> instance from a stream (i.e., deserialize it).
+     */
+    private void readObject(java.io.ObjectInputStream s) throws java.io.IOException, ClassNotFoundException {
+        // Read in the key type and any hidden stuff
+        s.defaultReadObject();
+        System.out.println(keyType);
+        // Read in size (number of Mappings)
+        int size = s.readInt();
+        // Read the keys and values, and put the mappings in the HashMap
+        for (int i = 0; i < size; i++) {
+            s.readObject(); // key
+            s.readObject(); // value
+        }
+    }
+}

java/com.sap.sse.test/src/com/sap/sse/test/EnumSetSerializationTest.java

@@ -0,0 +1,28 @@
+package com.sap.sse.test;
+
+import static org.junit.Assert.assertSame;
+
+import java.io.IOException;
+import java.util.EnumMap;
+
+import org.junit.Test;
+
+public class EnumSetSerializationTest {
+    private static enum MyBoolean {
+        TRUE, FALSE
+    };
+
+    private final EnumMap<MyBoolean, Integer> enumMap = new EnumMap<>(MyBoolean.class);
+
+    @Test
+    public void testEmptyEnumMap() throws IOException, ClassNotFoundException {
+        assertSame(MyBoolean.class, com.sap.sse.test.EnumMap.getEnumMapKeyType(enumMap));
+    }
+
+    @Test
+    public void testNonEmptyEnumMap() throws IOException, ClassNotFoundException {
+        enumMap.put(MyBoolean.TRUE, 1);
+        enumMap.put(MyBoolean.FALSE, 0);
+        assertSame(MyBoolean.class, com.sap.sse.test.EnumMap.getEnumMapKeyType(enumMap));
+    }
+}