FormatExceptionPolicy implementations need to implement Serializable …

…and equals/hashCode with the same constraints as SerializableFileFilter. Refactored the `toBytes()` method into the interface `NoLambda`, and moved `SerializableFileFilter.EqualityBasedOnSerialization` to `NoLambda.EqualityBasedOnSerialization`. Technically a breaking change, but will affect exactly zero real-world client code.

lib/src/main/java/com/diffplug/spotless/FormatExceptionPolicy.java

@@ -20,7 +20,7 @@
 import java.nio.file.Path;
 
 /** A policy for handling exceptions in the format. */
-public interface FormatExceptionPolicy extends Serializable {
+public interface FormatExceptionPolicy extends Serializable, NoLambda {
 	/** Called for every error in the formatter. */
 	void handleError(Throwable e, FormatterStep step, File file, Path rootDir);
 

lib/src/main/java/com/diffplug/spotless/FormatExceptionPolicyLegacy.java

@@ -20,7 +20,7 @@
 import java.util.logging.Level;
 import java.util.logging.Logger;
 
-class FormatExceptionPolicyLegacy implements FormatExceptionPolicy {
+class FormatExceptionPolicyLegacy extends NoLambda.EqualityBasedOnSerialization implements FormatExceptionPolicy {
 	private static final long serialVersionUID = 1L;
 
 	private static final Logger logger = Logger.getLogger(Formatter.class.getName());
@@ -34,9 +34,4 @@ public void handleError(Throwable e, FormatterStep step, File file, Path rootDir
 			logger.log(Level.WARNING, "Unable to apply step '" + step.getName() + "' to '" + rootDir.relativize(file.toPath()), e);
 		}
 	}
-
-	@Override
-	public byte[] toBytes() {
-		return LazyForwardingEquality.toBytes(this);
-	}
 }

lib/src/main/java/com/diffplug/spotless/NoLambda.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.spotless;
+
+import java.io.Serializable;
+import java.util.Arrays;
+
+/**
+ * Marker interface to prevent lambda implementations of
+ * single-method interfaces that require serializability.
+ *
+ * In order for Spotless to support up-to-date checks, all
+ * of its parameters must be {@link Serializable} so that
+ * entries can be written to file, and they must implement
+ * equals and hashCode correctly.
+ *
+ * This interface and its standard implementation,
+ * {@link EqualityBasedOnSerialization}, are a quick way
+ * to accomplish these goals.
+ */
+public interface NoLambda extends Serializable {
+	/**
+	 * Returns a byte array representation of everything inside this `SerializableFileFilter`.
+	 *
+	 * The main purpose of this method is to ensure one can't instantiate this class with lambda
+	 * expressions, which are notoriously difficult to serialize and deserialize properly. (See
+	 * `SerializableFileFilterImpl.SkipFilesNamed` for an example of how to make a serializable
+	 * subclass.)
+	 */
+	public byte[] toBytes();
+
+	/** An implementation of NoLambda in which equality is based on the serialized representation of itself. */
+	public static abstract class EqualityBasedOnSerialization implements NoLambda {
+		private static final long serialVersionUID = 1733798699224768949L;
+
+		@Override
+		public byte[] toBytes() {
+			return LazyForwardingEquality.toBytes(this);
+		}
+
+		@Override
+		public int hashCode() {
+			return Arrays.hashCode(toBytes());
+		}
+
+		@Override
+		public boolean equals(Object otherObj) {
+			if (otherObj == null) {
+				return false;
+			} else if (otherObj.getClass().equals(this.getClass())) {
+				EqualityBasedOnSerialization other = (EqualityBasedOnSerialization) otherObj;
+				return Arrays.equals(toBytes(), other.toBytes());
+			} else {
+				return false;
+			}
+		}
+	}
+}

lib/src/main/java/com/diffplug/spotless/SerializableFileFilter.java

@@ -17,49 +17,11 @@
 
 import java.io.FileFilter;
 import java.io.Serializable;
-import java.util.Arrays;
 
 /** A file filter with full support for serialization. */
-public interface SerializableFileFilter extends FileFilter, Serializable {
+public interface SerializableFileFilter extends FileFilter, Serializable, NoLambda {
 	/** Creates a FileFilter which will accept all files except files with the given name. */
 	public static SerializableFileFilter skipFilesNamed(String name) {
 		return new SerializableFileFilterImpl.SkipFilesNamed(name);
 	}
-
-	/**
-	 * Returns a byte array representation of everything inside this `SerializableFileFilter`.
-	 *
-	 * The main purpose of this method is to ensure one can't instantiate this class with lambda
-	 * expressions, which are notoriously difficult to serialize and deserialize properly. (See
-	 * `SerializableFileFilterImpl.SkipFilesNamed` for an example of how to make a serializable
-	 * subclass.)
-	 */
-	public byte[] toBytes();
-
-	/** An implementation of SerializableFileFilter in which equality is based on the serialized representation. */
-	public static abstract class EqualityBasedOnSerialization implements SerializableFileFilter {
-		private static final long serialVersionUID = 1733798699224768949L;
-
-		@Override
-		public byte[] toBytes() {
-			return LazyForwardingEquality.toBytes(this);
-		}
-
-		@Override
-		public int hashCode() {
-			return Arrays.hashCode(toBytes());
-		}
-
-		@Override
-		public boolean equals(Object otherObj) {
-			if (otherObj == null) {
-				return false;
-			} else if (otherObj.getClass().equals(this.getClass())) {
-				EqualityBasedOnSerialization other = (EqualityBasedOnSerialization) otherObj;
-				return Arrays.equals(toBytes(), other.toBytes());
-			} else {
-				return false;
-			}
-		}
-	}
 }

lib/src/main/java/com/diffplug/spotless/SerializableFileFilterImpl.java

@@ -19,7 +19,7 @@
 import java.util.Objects;
 
 class SerializableFileFilterImpl {
-	static class SkipFilesNamed extends SerializableFileFilter.EqualityBasedOnSerialization {
+	static class SkipFilesNamed extends NoLambda.EqualityBasedOnSerialization implements SerializableFileFilter {
 		private static final long serialVersionUID = 1L;
 
 		private final String nameToSkip;