Fixup the FormatterStep api.

Author: nedtwigg

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

@@ -127,6 +127,28 @@ static <RoundtripState extends Serializable, EqualityState extends Serializable>
 		return new FormatterStepSerializationRoundtrip(name, roundtripInit, equalityFunc, formatterFunc);
 	}
 
+	/**
+	 * @param name
+	 *             The name of the formatter step.
+	 * @param roundTrip
+	 *             The roundtrip serializable state of the step.
+	 * @param equalityFunc
+	 * 		       A pure serializable function (method reference recommended) which takes the result of `roundTrip`,
+	 * 		       and returns a serializable object whose serialized representation will be used for `.equals` and
+	 * 		       `.hashCode` of the FormatterStep.
+	 * @param formatterFunc
+	 * 		       A pure serializable function (method reference recommended) which takes the result of `equalityFunc`,
+	 * 		       and returns a `FormatterFunc` which will be used for the actual formatting.
+	 * @return A FormatterStep which can be losslessly roundtripped through the java serialization machinery.
+	 */
+	static <RoundtripState extends Serializable, EqualityState extends Serializable> FormatterStep create(
+			String name,
+			RoundtripState roundTrip,
+			SerializedFunction<RoundtripState, EqualityState> equalityFunc,
+			SerializedFunction<EqualityState, FormatterFunc> formatterFunc) {
+		return createLazy(name, () -> roundTrip, equalityFunc, formatterFunc);
+	}
+
 	/**
 	 * @param name
 	 *             The name of the formatter step

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

@@ -29,7 +29,7 @@
  * and such without interfering with buildcache keys.
  */
 @SuppressFBWarnings("SE_TRANSIENT_FIELD_NOT_RESTORED")
-public abstract class FormatterStepEqualityOnStateSerialization<State extends Serializable> implements FormatterStep, Serializable {
+abstract class FormatterStepEqualityOnStateSerialization<State extends Serializable> implements FormatterStep, Serializable {
 	private static final long serialVersionUID = 1L;
 
 	protected abstract State stateSupplier() throws Exception;
@@ -80,11 +80,7 @@ private State state() throws Exception {
 
 	private byte[] serializedState() {
 		if (serializedStateInternal == null) {
-			try {
-				serializedStateInternal = LazyForwardingEquality.toBytes(state());
-			} catch (Exception e) {
-				throw new RuntimeException(e);
-			}
+			serializedStateInternal = ThrowingEx.get(() -> LazyForwardingEquality.toBytes(state()));
 		}
 		return serializedStateInternal;
 	}

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

@@ -42,7 +42,7 @@ public String getName() {
 
 	@Override
 	protected EqualityState stateSupplier() throws Exception {
-		if (roundtripStateInternal != null) {
+		if (roundtripStateInternal == null) {
 			roundtripStateInternal = initializer.get();
 		}
 		return equalityStateExtractor.apply(roundtripStateInternal);

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

@@ -18,4 +18,8 @@
 import java.io.Serializable;
 
 @FunctionalInterface
-interface SerializedFunction<T, R> extends Serializable, ThrowingEx.Function<T, R> {}
+public interface SerializedFunction<T, R> extends Serializable, ThrowingEx.Function<T, R> {
+	static <T> SerializedFunction<T, T> identity() {
+		return t -> t;
+	}
+}