Added the default instance class.

ProtocolLib/src/com/comphenix/protocol/reflect/DefaultInstances.java

@@ -0,0 +1,184 @@
+package com.comphenix.protocol.reflect;
+
+import java.lang.reflect.Array;
+import java.lang.reflect.Constructor;
+import java.util.*;
+
+import javax.annotation.Nullable;
+
+import org.apache.commons.lang.ArrayUtils;
+
+import com.google.common.base.Defaults;
+import com.google.common.base.Function;
+import com.google.gson.internal.Primitives;
+
+/**
+ * Used to construct default instances of any type.
+ * @author Kristian
+ *
+ */
+public class DefaultInstances {
+
+	private static List<Function<Class<?>, Object>> registered = new ArrayList<Function<Class<?>, Object>>();
+	
+	/**
+	 * Default value for Strings.
+	 */
+	public final static String STRING_DEFAULT = "";
+	
+	/**
+	 * The maximum height of the hierachy of creates types. Used to prevent cycles.
+	 */
+	private final static int MAXIMUM_RECURSION = 20;
+	
+	// Provide default registrations
+	static {
+		registered.add(new PrimitiveGenerator());
+		registered.add(new CollectionGenerator());
+	}
+	
+	/**
+	 * Retrieves the default object providers used to generate default values.
+	 * @return Table of object providers.
+	 */
+	public static List<Function<Class<?>, Object>> getRegistered() {
+		return registered;
+	}
+	
+	/**
+	 * Retrieves a default instance or value that is assignable to this type.
+	 * <p>
+	 * This includes, but isn't limited too:
+	 * <ul>
+	 *   <li>Primitive types. Returns either zero or null.</li>
+	 *   <li>Primitive wrappers.</li>
+	 *   <li>String types. Returns an empty string.</li>
+	 *   <li>Arrays. Returns a zero-length array of the same type.</li>
+	 *   <li>Enums. Returns the first declared element.</li>
+	 *   <li>Collection interfaces, such as List and Set. Returns the most appropriate empty container.</li>
+	 *   <li>Any type with a public constructor that has parameters with defaults.</li>
+	 *   </ul>
+	 * </ul>
+	 * @param type - the type to construct a default value.
+	 * @return A default value/instance, or NULL if not possible.
+	 */
+	public static <T> T getDefault(Class<T> type) {
+		return getDefaultInternal(type, 0);
+	}
+	
+	@SuppressWarnings("unchecked")
+	private static <T> T getDefaultInternal(Class<T> type, int recursionLevel) {
+		
+		// Guard against recursion
+		if (recursionLevel > MAXIMUM_RECURSION) {
+			return null;
+		}
+		
+		for (Function<Class<?>, Object> generator : registered) {
+			Object value = generator.apply(type);
+			
+			if (value != null)
+				return (T) value;
+		}
+		
+		Constructor<?> minimum = null;
+		int lastCount = Integer.MAX_VALUE;
+		
+		// Find the constructor with the fewest parameters
+		for (Constructor<?> candidate : type.getConstructors()) {
+			Class<?>[] types = candidate.getParameterTypes();
+			
+			// Note that we don't allow recursive types - that is, types that
+			// require itself in the constructor.
+			if (types.length < lastCount) {
+				if (!ArrayUtils.contains(types, type)) {
+					minimum = candidate;
+					lastCount = types.length;
+					
+					// Don't loop again if we've already found the best possible constructor
+					if (lastCount == 0)
+						break;
+				}
+			}
+		}
+		
+		// Create the type with this constructor using default values. This might fail, though.
+		try {
+			if (minimum != null) {
+				Object[] params = new Object[lastCount];
+				Class<?>[] types = minimum.getParameterTypes();
+				
+				// Fill out 
+				for (int i = 0; i < lastCount; i++) {
+					params[i] = getDefaultInternal(types[i], recursionLevel + 1);
+				}
+				
+				return (T) minimum.newInstance(params);
+			}
+			
+		} catch (Exception e) {
+			// Nope, we couldn't create this type
+		}
+		
+		// No suitable default value could be found
+		return null;
+	}
+	
+	/**
+	 * Provides constructors for primtive types, wrappers, arrays and strings.
+	 * @author Kristian
+	 */
+	private static class PrimitiveGenerator implements Function<Class<?>, Object> {
+		
+		@Override
+		public Object apply(@Nullable Class<?> type) {
+			
+			if (Primitives.isPrimitive(type)) {
+				return Defaults.defaultValue(type);
+			} else if (Primitives.isWrapperType(type)) {
+				return Defaults.defaultValue(Primitives.unwrap(type));
+			} else if (type.isArray()) {
+				Class<?> arrayType = type.getComponentType();
+				return Array.newInstance(arrayType, 0);
+			} else if (type.isEnum()) {
+				Object[] values = type.getEnumConstants();
+				if (values != null && values.length > 0)
+					return values[0];
+			} else if (type.equals(String.class)) {
+				return STRING_DEFAULT;
+			} 
+			
+			// Cannot handle this type
+			return null;
+		}	
+	}
+	
+	/**
+	 * Provides simple constructors for collection interfaces.
+	 * @author Kristian
+	 */
+	private static class CollectionGenerator implements Function<Class<?>, Object> {
+
+		@Override
+		public Object apply(@Nullable Class<?> type) {
+			// Standard collection types
+			if (type.isInterface()) {
+				if (type.equals(Collection.class) || type.equals(List.class))
+					return new ArrayList<Object>();
+				else if (type.equals(Set.class))
+					return new HashSet<Object>();
+				else if (type.equals(Map.class))
+					return new HashMap<Object, Object>();
+				else if (type.equals(SortedSet.class))
+					return new TreeSet<Object>();
+				else if (type.equals(SortedMap.class))
+					return new TreeMap<Object, Object>();
+				else if (type.equals(Queue.class))
+					return new LinkedList<Object>();
+			}
+			
+			// Cannot provide an instance
+			return null;
+		}
+	}
+}