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.

By: crast <contact@jamescrasta.com>

paper-api/src/test/java/org/bukkit/metadata/MetadataValueAdapterTest.java

@@ -0,0 +1,97 @@
+package org.bukkit.metadata;
+
+import static org.junit.Assert.assertEquals;
+
+import org.bukkit.plugin.Plugin;
+import org.bukkit.plugin.TestPlugin;
+import org.junit.Test;
+
+public class MetadataValueAdapterTest {
+    private TestPlugin plugin = new TestPlugin("x");
+
+    @Test
+    public void testAdapterBasics() {
+        IncrementingMetaValue mv = new IncrementingMetaValue(plugin);
+        // check getOwningPlugin
+        assertEquals(mv.getOwningPlugin(), this.plugin);
+
+        // Check value-getting and invalidation.
+        assertEquals(new Integer(1), mv.value());
+        assertEquals(new Integer(2), mv.value());
+        mv.invalidate();
+        assertEquals(new Integer(1), mv.value());
+    }
+
+    @Test
+    public void testAdapterConversions() {
+        IncrementingMetaValue mv = new IncrementingMetaValue(plugin);
+
+        assertEquals(1, mv.asInt());
+        assertEquals(2L, mv.asLong());
+        assertEquals(3.0, mv.asFloat(), 0.001);
+        assertEquals(4, mv.asByte());
+        assertEquals(5.0, mv.asDouble(), 0.001);
+        assertEquals(6, mv.asShort());
+        assertEquals("7", mv.asString());
+    }
+
+    /** Boolean conversion is non-trivial, we want to test it thoroughly. */
+    @Test
+    public void testBooleanConversion() {
+        // null is False.
+        assertEquals(false, simpleValue(null).asBoolean());
+
+        // String to boolean.
+        assertEquals(true, simpleValue("True").asBoolean());
+        assertEquals(true, simpleValue("TRUE").asBoolean());
+        assertEquals(false, simpleValue("false").asBoolean());
+
+        // Number to boolean.
+        assertEquals(true, simpleValue(1).asBoolean());
+        assertEquals(true, simpleValue(5.0).asBoolean());
+        assertEquals(false, simpleValue(0).asBoolean());
+        assertEquals(false, simpleValue(0.1).asBoolean());
+
+        // Boolean as boolean, of course.
+        assertEquals(true, simpleValue(Boolean.TRUE).asBoolean());
+        assertEquals(false, simpleValue(Boolean.FALSE).asBoolean());
+
+        // any object that is not null and not a Boolean, String, or Number is true.
+        assertEquals(true, simpleValue(new Object()).asBoolean());
+    }
+
+    /** Test String conversions return an empty string when given null. */
+    @Test
+    public void testStringConversionNull() {
+        assertEquals("", simpleValue(null).asString());
+    }
+
+    /** Get a fixed value MetadataValue. */
+    private MetadataValue simpleValue(Object value) {
+        return new FixedMetadataValue(plugin, value);
+    }
+
+    /**
+     * A sample non-trivial MetadataValueAdapter implementation.
+     *
+     * The rationale for implementing an incrementing value is to have a value
+     * which changes with every call to value(). This is important for testing
+     * because we want to make sure all the tested conversions are calling the
+     * value() method exactly once and no caching is going on.
+     */
+    class IncrementingMetaValue extends MetadataValueAdapter {
+        private int internalValue = 0;
+
+        protected IncrementingMetaValue(Plugin owningPlugin) {
+            super(owningPlugin);
+        }
+
+        public Object value() {
+            return ++internalValue;
+        }
+
+        public void invalidate() {
+            internalValue = 0;
+        }
+    }
+}