Nicer failure messages on partial adapters.

Closes https://github.com/square/moshi/issues/59

moshi/src/main/java/com/squareup/moshi/AdapterMethodsFactory.java

@@ -39,9 +39,18 @@ final class AdapterMethodsFactory implements JsonAdapter.Factory {
     final AdapterMethod fromAdapter = get(fromAdapters, type, annotations);
     if (toAdapter == null && fromAdapter == null) return null;
 
-    final JsonAdapter<Object> delegate = toAdapter == null || fromAdapter == null
+    final JsonAdapter<Object> delegate;
-        ? moshi.nextAdapter(this, type, annotations)
+    if (toAdapter == null || fromAdapter == null) {
-        : null;
+      try {
+        delegate = moshi.nextAdapter(this, type, annotations);
+      } catch (IllegalArgumentException e) {
+        String missingAnnotation = toAdapter == null ? "@ToJson" : "@FromJson";
+        throw new IllegalArgumentException("No " + missingAnnotation + " adapter for "
+            + type + " annotated " + annotations);
+      }
+    } else {
+      delegate = null;
+    }
 
     return new JsonAdapter<Object>() {
       @Override public void toJson(JsonWriter writer, Object value) throws IOException {

moshi/src/test/java/com/squareup/moshi/AdapterMethodsTest.java

@@ -264,6 +264,44 @@ public final class AdapterMethodsTest {
     }
   }
 
+  @Test public void adapterDoesToJsonOnly() throws Exception {
+    Object shapeToJsonAdapter = new Object() {
+      @ToJson String shapeToJson(Shape shape) {
+        throw new AssertionError();
+      }
+    };
+
+    Moshi toJsonMoshi = new Moshi.Builder()
+        .add(shapeToJsonAdapter)
+        .build();
+    try {
+      toJsonMoshi.adapter(Shape.class);
+      fail();
+    } catch (IllegalArgumentException e) {
+      assertThat(e).hasMessage("No @FromJson adapter for interface "
+          + "com.squareup.moshi.AdapterMethodsTest$Shape annotated []");
+    }
+  }
+
+  @Test public void adapterDoesFromJsonOnly() throws Exception {
+    Object shapeFromJsonAdapter = new Object() {
+      @FromJson Shape shapeFromJson(String shape) {
+        throw new AssertionError();
+      }
+    };
+
+    Moshi fromJsonMoshi = new Moshi.Builder()
+        .add(shapeFromJsonAdapter)
+        .build();
+    try {
+      fromJsonMoshi.adapter(Shape.class);
+      fail();
+    } catch (IllegalArgumentException e) {
+      assertThat(e).hasMessage("No @ToJson adapter for interface "
+          + "com.squareup.moshi.AdapterMethodsTest$Shape annotated []");
+    }
+  }
+
   static class Point {
     final int x;
     final int y;
@@ -281,4 +319,8 @@ public final class AdapterMethodsTest {
       return x * 37 + y;
     }
   }
+
+  interface Shape {
+    String draw();
+  }
 }

moshi/src/test/java/com/squareup/moshi/JsonQualifiersTest.java

@@ -332,7 +332,7 @@ public final class JsonQualifiersTest {
       moshi.adapter(StringAndFooString.class);
       fail();
     } catch (IllegalArgumentException expected) {
-      assertThat(expected).hasMessage("No JsonAdapter for class java.lang.String "
+      assertThat(expected).hasMessage("No @FromJson adapter for class java.lang.String "
           + "annotated [@com.squareup.moshi.JsonQualifiersTest$FooPrefix()]");
     }
   }
@@ -353,7 +353,7 @@ public final class JsonQualifiersTest {
       moshi.adapter(StringAndFooString.class);
       fail();
     } catch (IllegalArgumentException expected) {
-      assertThat(expected).hasMessage("No JsonAdapter for class java.lang.String "
+      assertThat(expected).hasMessage("No @ToJson adapter for class java.lang.String "
           + "annotated [@com.squareup.moshi.JsonQualifiersTest$FooPrefix()]");
     }
   }