Validate `GsonBuilder.setDateFormat` style arguments & extend tests (#2545)

`DateFormat` validates these arguments as well, but it is currently not
documented, see https://bugs.openjdk.org/browse/JDK-8319628.

Also moves the other tests for `setDateFormat(String)` from
DefaultTypeAdaptersTest to GsonBuilderTest.

gson/src/main/java/com/google/gson/GsonBuilder.java

@@ -593,8 +593,10 @@ public final class GsonBuilder {
    * class. See the documentation in {@link java.text.SimpleDateFormat} for more information on
    * valid date and time patterns.
    *
-   * @param pattern the pattern that dates will be serialized/deserialized to/from
+   * @param pattern the pattern that dates will be serialized/deserialized to/from; can be {@code
+   *     null} to reset the pattern
    * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
+   * @throws IllegalArgumentException if the pattern is invalid
    * @since 1.2
    */
   @CanIgnoreReturnValue
@@ -611,6 +613,14 @@ public final class GsonBuilder {
     return this;
   }
 
+  private static int checkDateFormatStyle(int style) {
+    // Valid DateFormat styles are: 0, 1, 2, 3 (FULL, LONG, MEDIUM, SHORT)
+    if (style < 0 || style > 3) {
+      throw new IllegalArgumentException("Invalid style: " + style);
+    }
+    return style;
+  }
+
   /**
    * Configures Gson to serialize {@code Date} objects according to the style value provided. You
    * can call this method or {@link #setDateFormat(String)} multiple times, but only the last
@@ -623,11 +633,12 @@ public final class GsonBuilder {
    * @param style the predefined date style that date objects will be serialized/deserialized
    *     to/from
    * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
+   * @throws IllegalArgumentException if the style is invalid
    * @since 1.2
    */
   @CanIgnoreReturnValue
   public GsonBuilder setDateFormat(int style) {
-    this.dateStyle = style;
+    this.dateStyle = checkDateFormatStyle(style);
     this.datePattern = null;
     return this;
   }
@@ -645,12 +656,13 @@ public final class GsonBuilder {
    *     to/from
    * @param timeStyle the predefined style for the time portion of the date objects
    * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
+   * @throws IllegalArgumentException if the style values are invalid
    * @since 1.2
    */
   @CanIgnoreReturnValue
   public GsonBuilder setDateFormat(int dateStyle, int timeStyle) {
-    this.dateStyle = dateStyle;
-    this.timeStyle = timeStyle;
+    this.dateStyle = checkDateFormatStyle(dateStyle);
+    this.timeStyle = checkDateFormatStyle(timeStyle);
     this.datePattern = null;
     return this;
   }

gson/src/test/java/com/google/gson/GsonBuilderTest.java

@@ -28,6 +28,8 @@ import java.io.StringWriter;
 import java.lang.reflect.Field;
 import java.lang.reflect.Modifier;
 import java.lang.reflect.Type;
+import java.text.DateFormat;
+import java.util.Date;
 import org.junit.Test;
 
 /**
@@ -305,4 +307,88 @@ public class GsonBuilderTest {
     // But registering type hierarchy adapter for Object should be allowed
     gsonBuilder.registerTypeHierarchyAdapter(Object.class, NULL_TYPE_ADAPTER);
   }
+
+  @Test
+  public void testSetDateFormatWithInvalidPattern() {
+    GsonBuilder builder = new GsonBuilder();
+    String invalidPattern = "This is an invalid Pattern";
+    IllegalArgumentException e =
+        assertThrows(IllegalArgumentException.class, () -> builder.setDateFormat(invalidPattern));
+    assertThat(e)
+        .hasMessageThat()
+        .isEqualTo("The date pattern '" + invalidPattern + "' is not valid");
+  }
+
+  @Test
+  public void testSetDateFormatWithValidPattern() {
+    GsonBuilder builder = new GsonBuilder();
+    String validPattern = "yyyy-MM-dd";
+    // Should not throw an exception
+    builder.setDateFormat(validPattern);
+  }
+
+  @Test
+  public void testSetDateFormatNullPattern() {
+    GsonBuilder builder = new GsonBuilder();
+    @SuppressWarnings("JavaUtilDate")
+    Date date = new Date(0);
+    String originalFormatted = builder.create().toJson(date);
+
+    String customFormatted = builder.setDateFormat("yyyy-MM-dd").create().toJson(date);
+    assertThat(customFormatted).isNotEqualTo(originalFormatted);
+
+    // `null` should reset the format to the default
+    String resetFormatted = builder.setDateFormat(null).create().toJson(date);
+    assertThat(resetFormatted).isEqualTo(originalFormatted);
+  }
+
+  /**
+   * Tests behavior for an empty date pattern; this behavior is not publicly documented at the
+   * moment.
+   */
+  @Test
+  public void testSetDateFormatEmptyPattern() {
+    GsonBuilder builder = new GsonBuilder();
+    @SuppressWarnings("JavaUtilDate")
+    Date date = new Date(0);
+    String originalFormatted = builder.create().toJson(date);
+
+    String emptyFormatted = builder.setDateFormat("    ").create().toJson(date);
+    // Empty pattern was ignored
+    assertThat(emptyFormatted).isEqualTo(originalFormatted);
+  }
+
+  @Test
+  public void testSetDateFormatValidStyle() {
+    GsonBuilder builder = new GsonBuilder();
+    int[] validStyles = {DateFormat.FULL, DateFormat.LONG, DateFormat.MEDIUM, DateFormat.SHORT};
+
+    for (int style : validStyles) {
+      // Should not throw an exception
+      builder.setDateFormat(style);
+      builder.setDateFormat(style, style);
+    }
+  }
+
+  @Test
+  public void testSetDateFormatInvalidStyle() {
+    GsonBuilder builder = new GsonBuilder();
+
+    IllegalArgumentException e =
+        assertThrows(IllegalArgumentException.class, () -> builder.setDateFormat(-1));
+    assertThat(e).hasMessageThat().isEqualTo("Invalid style: -1");
+
+    e = assertThrows(IllegalArgumentException.class, () -> builder.setDateFormat(4));
+    assertThat(e).hasMessageThat().isEqualTo("Invalid style: 4");
+
+    e =
+        assertThrows(
+            IllegalArgumentException.class, () -> builder.setDateFormat(-1, DateFormat.FULL));
+    assertThat(e).hasMessageThat().isEqualTo("Invalid style: -1");
+
+    e =
+        assertThrows(
+            IllegalArgumentException.class, () -> builder.setDateFormat(DateFormat.FULL, -1));
+    assertThat(e).hasMessageThat().isEqualTo("Invalid style: -1");
+  }
 }

gson/src/test/java/com/google/gson/functional/DefaultTypeAdaptersTest.java

@@ -16,7 +16,6 @@
 package com.google.gson.functional;
 
 import static com.google.common.truth.Truth.assertThat;
-import static org.junit.Assert.assertThrows;
 import static org.junit.Assert.fail;
 
 import com.google.gson.Gson;
@@ -493,6 +492,19 @@ public class DefaultTypeAdaptersTest {
     assertThat(cal.get(Calendar.SECOND)).isEqualTo(23);
   }
 
+  @Test
+  public void testDateSerializationWithStyle() {
+    int style = DateFormat.SHORT;
+    Date date = new Date(0);
+    String expectedFormatted = DateFormat.getDateTimeInstance(style, style, Locale.US).format(date);
+
+    Gson gson = new GsonBuilder().setDateFormat(style, style).create();
+    String json = gson.toJson(date);
+    assertThat(json).isEqualTo("\"" + expectedFormatted + "\"");
+    // Verify that custom style is not equal to default style
+    assertThat(json).isNotEqualTo(new Gson().toJson(date));
+  }
+
   @Test
   public void testDateSerializationWithPattern() {
     String pattern = "yyyy-MM-dd";
@@ -738,24 +750,6 @@ public class DefaultTypeAdaptersTest {
     assertThat(sb.toString()).isEqualTo("abc");
   }
 
-  @Test
-  public void testSetDateFormatWithInvalidPattern() {
-    GsonBuilder builder = new GsonBuilder();
-    String invalidPattern = "This is a invalid Pattern";
-    assertThrows(
-        IllegalArgumentException.class,
-        () -> {
-          builder.setDateFormat(invalidPattern);
-        });
-  }
-
-  @Test
-  public void testSetDateFormatWithValidPattern() {
-    GsonBuilder builder = new GsonBuilder();
-    String validPattern = "yyyy-MM-dd";
-    builder.setDateFormat(validPattern);
-  }
-
   private static class MyClassTypeAdapter extends TypeAdapter<Class<?>> {
     @Override
     public void write(JsonWriter out, Class<?> value) throws IOException {