Refactor common metadata code into base class. Fixes BUKKIT-3624

Implementing the MetadataValue interface is significant work due to
having to provide a large amount of conversion stub methods. This commit
adds a new optional abstract base class to aid in implementation.

src/main/java/org/bukkit/metadata/MetadataValueAdapter.java

@@ -0,0 +1,77 @@
+package org.bukkit.metadata;
+
+import org.apache.commons.lang.Validate;
+import org.bukkit.plugin.Plugin;
+import org.bukkit.util.NumberConversions;
+
+/**
+ * Optional base class for facilitating MetadataValue implementations.
+ *
+ * This provides all the conversion functions for MetadataValue
+ * so that writing an implementation of MetadataValue is as simple
+ * as implementing value() and invalidate().
+ *
+ */
+public abstract class MetadataValueAdapter implements MetadataValue {
+    protected final Plugin owningPlugin;
+
+    protected MetadataValueAdapter(Plugin owningPlugin) {
+        Validate.notNull(owningPlugin, "owningPlugin cannot be null");
+        this.owningPlugin = owningPlugin;
+    }
+
+    public Plugin getOwningPlugin() {
+        return owningPlugin;
+    }
+
+    public int asInt() {
+        return NumberConversions.toInt(value());
+    }
+
+    public float asFloat() {
+        return NumberConversions.toFloat(value());
+    }
+
+    public double asDouble() {
+        return NumberConversions.toDouble(value());
+    }
+
+    public long asLong() {
+        return NumberConversions.toLong(value());
+    }
+
+    public short asShort() {
+        return NumberConversions.toShort(value());
+    }
+
+    public byte asByte() {
+        return NumberConversions.toByte(value());
+    }
+
+    public boolean asBoolean() {
+        Object value = value();
+        if (value instanceof Boolean) {
+            return (Boolean) value;
+        }
+
+        if (value instanceof Number) {
+            return ((Number) value).intValue() != 0;
+        }
+
+        if (value instanceof String) {
+            return Boolean.parseBoolean((String) value);
+        }
+
+        return value != null;
+    }
+
+    public String asString() {
+        Object value = value();
+
+        if (value == null) {
+            return "";
+        }
+        return value.toString();
+    }
+
+}