First cut at ForwardingEquality.

Author: nedtwigg

build.gradle

@@ -82,6 +82,7 @@ dependencies {
 	compile "org.eclipse.jgit:org.eclipse.jgit:${VER_JGIT}"
 	testCompile "junit:junit:${VER_JUNIT}"
 	testCompile "org.assertj:assertj-core:${VER_ASSERTJ}"
+	testCompile "com.diffplug.durian:durian-testlib:${VER_DURIAN}"
 
 	// add the eclipse jars to the embedded configuration
 	eclipseDeps.each { embeddedJars "p2:${it}:+" }

src/main/java/com/diffplug/gradle/spotless/ForwardingEquality.java

@@ -0,0 +1,71 @@
+/*
+ * Copyright 2016 DiffPlug
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.diffplug.gradle.spotless;
+
+import java.io.IOException;
+import java.io.ObjectInputStream;
+import java.io.ObjectOutputStream;
+import java.io.ObjectStreamException;
+import java.io.Serializable;
+import java.util.Objects;
+
+/**
+ * Implements equality, hashcode, and serialization entirely in terms
+ * of the given key.  It is appropriate for subclasses of this class
+ * to use `@SuppressWarnings("serial")`.
+ */
+@SuppressWarnings("serial")
+public abstract class ForwardingEquality<T extends Serializable> implements Serializable {
+	private T key;
+
+	ForwardingEquality(T key) {
+		this.key = Objects.requireNonNull(key);
+	}
+
+	protected T key() {
+		return key;
+	}
+
+	private void writeObject(ObjectOutputStream out) throws IOException {
+		out.writeObject(key);
+	}
+
+	@SuppressWarnings("unchecked")
+	private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException {
+		key = (T) Objects.requireNonNull(in.readObject());
+	}
+
+	@SuppressWarnings("unused")
+	private void readObjectNoData() throws ObjectStreamException {
+		throw new UnsupportedOperationException();
+	}
+
+	@Override
+	public boolean equals(Object other) {
+		if (other == null) {
+			return false;
+		} else if (getClass().equals(other.getClass())) {
+			return key.equals(((ForwardingEquality<?>) other).key);
+		} else {
+			return false;
+		}
+	}
+
+	@Override
+	public int hashCode() {
+		return key.hashCode();
+	}
+}

src/test/java/com/diffplug/gradle/spotless/ForwardingEqualityTest.java

@@ -0,0 +1,63 @@
+/*
+ * Copyright 2016 DiffPlug
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.diffplug.gradle.spotless;
+
+import org.junit.Test;
+
+import com.diffplug.common.testing.EqualsTester;
+import com.diffplug.common.testing.SerializableTester;
+
+public class ForwardingEqualityTest {
+	static Str s(String key) {
+		return new Str(key);
+	}
+
+	@SuppressWarnings("serial")
+	static class Str extends ForwardingEquality<String> {
+		Str(String key) {
+			super(key);
+		}
+	}
+
+	static Int i(int key) {
+		return new Int(key);
+	}
+
+	@SuppressWarnings("serial")
+	static class Int extends ForwardingEquality<Integer> {
+		Int(int key) {
+			super(key);
+		}
+	}
+
+	@Test
+	public void testEquality() {
+		new EqualsTester()
+				.addEqualityGroup(s("hello"), s("hello"), s("h" + "ello"))
+				.addEqualityGroup(s("world"), s("world"), s("wor" + "ld"))
+				.addEqualityGroup(i(3), i(3), i(1 + 2))
+				.addEqualityGroup(i(-6), i(-6), i(1 - 7))
+				.testEquals();
+	}
+
+	@Test
+	public void testSerialization() {
+		SerializableTester.reserializeAndAssert(s("hello"));
+		SerializableTester.reserializeAndAssert(s("world"));
+		SerializableTester.reserializeAndAssert(i(4));
+		SerializableTester.reserializeAndAssert(i(-6));
+	}
+}