#459: Add test and fix.

src/main/java/com/cronutils/converter/CronConverter.java

@@ -25,26 +25,22 @@
 
 public class CronConverter {
 
-	private static final Logger LOGGER = LoggerFactory
-			.getLogger(CronConverter.class);
+	private static final Logger LOGGER = LoggerFactory.getLogger(CronConverter.class);
 
 	private static final String CRON_FIELDS_SEPARATOR = " ";
-
 	private String[] cronParts;
-
 	private Calendar fromCalendar;
-
 	private String sourceCron;
-
 	private ZoneId sourceZoneId;
-
 	private ZoneId targetZoneId;
 
-	@Setter
-	CronToCalendarTransformer toCalendarConverter = new CronToCalendarTransformer();
+	private CronToCalendarTransformer toCalendarConverter;
+	private CalendarToCronTransformer toCronConverter;
 
-	@Setter
-	CalendarToCronTransformer toCronConverter = new CalendarToCronTransformer();
+	public CronConverter(CronToCalendarTransformer toCalendarConverter, CalendarToCronTransformer toCronConverter){
+		this.toCalendarConverter = toCalendarConverter;
+		this.toCronConverter = toCronConverter;
+	}
 
 	public CronConverter using(String cronExpression) {
 		this.sourceCron = cronExpression;

src/main/java/com/cronutils/model/field/expression/FieldExpressionFactory.java

@@ -18,6 +18,7 @@
 import com.cronutils.model.field.value.IntegerFieldValue;
 import com.cronutils.model.field.value.SpecialChar;
 import com.cronutils.model.field.value.SpecialCharFieldValue;
+import com.cronutils.utils.Preconditions;
 
 public class FieldExpressionFactory {
     private FieldExpressionFactory() {
@@ -48,6 +49,7 @@ public static On on(SpecialChar specialChar) {
     }
 
     public static On on(int time) {
+        Preconditions.checkPositive(time, "Time must not be negative");
         return new On(new IntegerFieldValue(time));
     }
 

src/main/java/com/cronutils/model/field/expression/On.java

@@ -18,6 +18,7 @@
 import com.cronutils.model.field.value.SpecialCharFieldValue;
 import com.cronutils.utils.Preconditions;
 import com.cronutils.utils.StringUtils;
+import org.apache.commons.lang3.Validate;
 
 import static com.cronutils.utils.Preconditions.checkArgument;
 

src/main/java/com/cronutils/utils/Preconditions.java

@@ -92,8 +92,7 @@
  * @since 2.0 (imported from Google Collections Library)
  */
 public class Preconditions {
-    private Preconditions() {
-    }
+    private Preconditions() {}
 
     /**
      * Ensures the truth of an expression involving one or more parameters to the calling method.
@@ -109,6 +108,12 @@ public static void checkArgument(final boolean expression, final Object errorMes
         }
     }
 
+    public static void checkPositive(final int value, final Object errorMessage) {
+        if (value < 0) {
+            throw new IllegalArgumentException(String.valueOf(errorMessage));
+        }
+    }
+
     /**
      * Ensures the truth of an expression involving one or more parameters to the calling method.
      *

src/test/java/com/cronutils/Issue459Test.java

@@ -0,0 +1,22 @@
+package com.cronutils;
+
+import com.cronutils.builder.CronBuilder;
+import com.cronutils.model.definition.CronDefinitionBuilder;
+import org.junit.Test;
+
+import static com.cronutils.model.CronType.UNIX;
+import static com.cronutils.model.field.expression.FieldExpression.always;
+import static com.cronutils.model.field.expression.FieldExpressionFactory.on;
+
+public class Issue459Test {
+    @Test(expected = RuntimeException.class)
+    public void testNegativeValuesNotAllowed() {
+        CronBuilder.cron(CronDefinitionBuilder.instanceDefinitionFor(UNIX))
+                .withDoM(always())
+                .withMonth(always())
+                .withDoW(always())
+                .withHour(on(-1))
+                .withMinute(on(5))
+                .instance();
+    }
+}

src/test/java/com/cronutils/converter/CronConverterTest.java

@@ -18,7 +18,6 @@
 import java.util.Arrays;
 import java.util.Collection;
 import java.time.ZoneId;
-import java.time.ZonedDateTime;
 
 import org.junit.Assert;
 import org.junit.Test;
@@ -28,13 +27,13 @@
 @RunWith(Parameterized.class)
 public class CronConverterTest {
 
-	CronConverter cronConverter = spy(new CronConverter());
-
 	private String timezone;
-
 	private String inputCronExpression;
-
 	private String expectedCronExpression;
+	private CronConverter cronConverter = spy(new CronConverter(
+			new CronToCalendarTransformer(),
+			new CalendarToCronTransformer()
+	));
 
 	public CronConverterTest(String timezone, String inputCronExpression, String expectedCronExpression) {
 		this.timezone = timezone;
@@ -56,8 +55,6 @@ public static Collection cronExpressions() {
 
 	@Test
 	public void testCronConverterBuilder() {
-		cronConverter.setToCalendarConverter(new CronToCalendarTransformer());
-		cronConverter.setToCronConverter(new CalendarToCronTransformer());
 		Assert.assertEquals(expectedCronExpression,
 				cronConverter.using(inputCronExpression)
 						.from(ZoneId.of(timezone)).to(ZoneId.of("UTC"))