diff --git a/org.moreunit.core.test/test/org/moreunit/core/util/ArrayUtilsTest.java b/org.moreunit.core.test/test/org/moreunit/core/util/ArrayUtilsTest.java
new file mode 100644
index 00000000..1b51afc5
--- /dev/null
+++ b/org.moreunit.core.test/test/org/moreunit/core/util/ArrayUtilsTest.java
@@ -0,0 +1,22 @@
+package org.moreunit.core.util;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import org.junit.Test;
+
+public class ArrayUtilsTest
+{
+    @Test
+    public void array_should_return_elements()
+    {
+        String[] strings = ArrayUtils.array("a", "b", "c");
+        assertThat(strings).containsExactly("a", "b", "c");
+    }
+
+    @Test
+    public void array_should_return_empty_array_when_no_arguments()
+    {
+        Object[] objects = ArrayUtils.array();
+        assertThat(objects).isEmpty();
+    }
+}
diff --git a/org.moreunit.core.test/test/org/moreunit/core/util/ExtendedSafeRunnerTest.java b/org.moreunit.core.test/test/org/moreunit/core/util/ExtendedSafeRunnerTest.java
new file mode 100644
index 00000000..772b169a
--- /dev/null
+++ b/org.moreunit.core.test/test/org/moreunit/core/util/ExtendedSafeRunnerTest.java
@@ -0,0 +1,80 @@
+package org.moreunit.core.util;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.util.Arrays;
+import java.util.List;
+
+import org.junit.Test;
+import org.moreunit.core.util.ExtendedSafeRunner.GenericRunnable;
+
+public class ExtendedSafeRunnerTest
+{
+    @Test
+    public void applyTo_should_execute_code_and_return_result()
+    {
+        ExtendedSafeRunner runner = new ExtendedSafeRunner();
+        String result = runner.applyTo("input", new GenericRunnable<String, String>()
+        {
+            @Override
+            public String run(String element) throws Exception
+            {
+                return element.toUpperCase();
+            }
+
+            @Override
+            public void handleException(Throwable throwable, String element)
+            {
+            }
+        });
+
+        assertThat(result).isEqualTo("INPUT");
+    }
+
+    @Test
+    public void applyTo_iterable_should_execute_code_for_each_element()
+    {
+        ExtendedSafeRunner runner = new ExtendedSafeRunner();
+        Iterable<String> results = runner.applyTo(Arrays.asList("a", "b"), new GenericRunnable<String, String>()
+        {
+            @Override
+            public String run(String element) throws Exception
+            {
+                return element.toUpperCase();
+            }
+
+            @Override
+            public void handleException(Throwable throwable, String element)
+            {
+            }
+        });
+
+        assertThat(results).containsExactly("A", "B");
+    }
+
+    @Test
+    public void applyTo_should_handle_exception()
+    {
+        ExtendedSafeRunner runner = new ExtendedSafeRunner();
+        final boolean[] exceptionHandled = { false };
+
+        runner.applyTo("input", new GenericRunnable<String, String>()
+        {
+            @Override
+            public String run(String element) throws Exception
+            {
+                throw new RuntimeException("failed");
+            }
+
+            @Override
+            public void handleException(Throwable throwable, String element)
+            {
+                exceptionHandled[0] = true;
+                assertThat(element).isEqualTo("input");
+                assertThat(throwable.getMessage()).isEqualTo("failed");
+            }
+        });
+
+        assertThat(exceptionHandled[0]).isTrue();
+    }
+}
diff --git a/org.moreunit.core.test/test/org/moreunit/core/util/PreconditionsTest.java b/org.moreunit.core.test/test/org/moreunit/core/util/PreconditionsTest.java
new file mode 100644
index 00000000..00e541f6
--- /dev/null
+++ b/org.moreunit.core.test/test/org/moreunit/core/util/PreconditionsTest.java
@@ -0,0 +1,154 @@
+package org.moreunit.core.util;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.fail;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.List;
+
+import org.junit.Test;
+
+public class PreconditionsTest
+{
+    @Test
+    public void checkNotNull_should_return_reference_when_not_null()
+    {
+        String ref = "abc";
+        assertThat(Preconditions.checkNotNull(ref)).isSameAs(ref);
+    }
+
+    @Test(expected = NullPointerException.class)
+    public void checkNotNull_should_throw_exception_when_null()
+    {
+        Preconditions.checkNotNull(null);
+    }
+
+    @Test
+    public void checkNotNull_with_message_should_return_reference_when_not_null()
+    {
+        String ref = "abc";
+        assertThat(Preconditions.checkNotNull(ref, "error")).isSameAs(ref);
+    }
+
+    @Test
+    public void checkNotNull_with_message_should_throw_exception_with_message_when_null()
+    {
+        try
+        {
+            Preconditions.checkNotNull(null, "custom error");
+            fail("Should have thrown NullPointerException");
+        }
+        catch (NullPointerException e)
+        {
+            assertThat(e.getMessage()).isEqualTo("custom error");
+        }
+    }
+
+    @Test
+    public void checkArgument_should_do_nothing_when_true()
+    {
+        Preconditions.checkArgument(true);
+    }
+
+    @Test(expected = IllegalArgumentException.class)
+    public void checkArgument_should_throw_exception_when_false()
+    {
+        Preconditions.checkArgument(false);
+    }
+
+    @Test
+    public void checkArgument_with_message_should_do_nothing_when_true()
+    {
+        Preconditions.checkArgument(true, "error");
+    }
+
+    @Test
+    public void checkArgument_with_message_should_throw_exception_with_message_when_false()
+    {
+        try
+        {
+            Preconditions.checkArgument(false, "custom error");
+            fail("Should have thrown IllegalArgumentException");
+        }
+        catch (IllegalArgumentException e)
+        {
+            assertThat(e.getMessage()).isEqualTo("custom error");
+        }
+    }
+
+    @Test
+    public void checkState_should_do_nothing_when_true()
+    {
+        Preconditions.checkState(true, "error");
+    }
+
+    @Test
+    public void checkState_should_throw_exception_with_message_when_false()
+    {
+        try
+        {
+            Preconditions.checkState(false, "custom error");
+            fail("Should have thrown IllegalStateException");
+        }
+        catch (IllegalStateException e)
+        {
+            assertThat(e.getMessage()).isEqualTo("custom error");
+        }
+    }
+
+    @Test
+    public void checkNotNullOrEmpty_should_return_collection_when_not_null_nor_empty()
+    {
+        List<String> list = Arrays.asList("a");
+        assertThat(Preconditions.checkNotNullOrEmpty(list)).isSameAs(list);
+    }
+
+    @Test(expected = NullPointerException.class)
+    public void checkNotNullOrEmpty_should_throw_NPE_when_null()
+    {
+        Preconditions.checkNotNullOrEmpty(null);
+    }
+
+    @Test(expected = IllegalArgumentException.class)
+    public void checkNotNullOrEmpty_should_throw_IAE_when_empty()
+    {
+        Preconditions.checkNotNullOrEmpty(new ArrayList<String>());
+    }
+
+    @Test
+    public void checkNotNullOrEmpty_with_message_should_return_collection_when_not_null_nor_empty()
+    {
+        List<String> list = Arrays.asList("a");
+        assertThat(Preconditions.checkNotNullOrEmpty(list, "error")).isSameAs(list);
+    }
+
+    @Test
+    public void checkNotNullOrEmpty_with_message_should_throw_NPE_with_message_when_null()
+    {
+        try
+        {
+            Preconditions.checkNotNullOrEmpty(null, "custom error");
+            fail("Should have thrown NullPointerException");
+        }
+        catch (NullPointerException e)
+        {
+            assertThat(e.getMessage()).isEqualTo("custom error");
+        }
+    }
+
+    @Test
+    public void checkNotNullOrEmpty_with_message_should_throw_IAE_with_message_when_empty()
+    {
+        try
+        {
+            Preconditions.checkNotNullOrEmpty(new ArrayList<String>(), "custom error");
+            fail("Should have thrown IllegalArgumentException");
+        }
+        catch (IllegalArgumentException e)
+        {
+            assertThat(e.getMessage()).isEqualTo("custom error");
+        }
+    }
+}
diff --git a/org.moreunit.core.test/test/org/moreunit/core/util/StringConstantsTest.java b/org.moreunit.core.test/test/org/moreunit/core/util/StringConstantsTest.java
new file mode 100644
index 00000000..f88953e6
--- /dev/null
+++ b/org.moreunit.core.test/test/org/moreunit/core/util/StringConstantsTest.java
@@ -0,0 +1,18 @@
+package org.moreunit.core.util;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import org.junit.Test;
+
+public class StringConstantsTest
+{
+    @Test
+    public void constants_should_have_expected_values()
+    {
+        assertThat(StringConstants.DOT).isEqualTo(".");
+        assertThat(StringConstants.EMPTY_STRING).isEqualTo("");
+        assertThat(StringConstants.NEWLINE).isEqualTo(System.getProperty("line.separator"));
+        assertThat(StringConstants.SLASH).isEqualTo("/");
+        assertThat(StringConstants.WILDCARD).isEqualTo("*");
+    }
+}
diff --git a/org.moreunit.core.test/test/org/moreunit/core/util/StringLengthComparatorTest.java b/org.moreunit.core.test/test/org/moreunit/core/util/StringLengthComparatorTest.java
index be2f769b..fbb9dd91 100644
--- a/org.moreunit.core.test/test/org/moreunit/core/util/StringLengthComparatorTest.java
+++ b/org.moreunit.core.test/test/org/moreunit/core/util/StringLengthComparatorTest.java
@@ -15,7 +15,7 @@ public void should_return_positive_integer_when_first_has_greater_length_than_se
     @Test
     public void should_return_negative_integer_when_second_has_greater_length_than_first_parameter() throws Exception
     {
-        assertThat(new StringLengthComparator().compare("", "Long")).isLessThanOrEqualTo(0);
+        assertThat(new StringLengthComparator().compare("", "Long")).isLessThan(0);
     }
 
     @Test
diff --git a/org.moreunit.core.test/test/org/moreunit/core/util/StringsTest.java b/org.moreunit.core.test/test/org/moreunit/core/util/StringsTest.java
index ec4b2d97..6e90c0fa 100644
--- a/org.moreunit.core.test/test/org/moreunit/core/util/StringsTest.java
+++ b/org.moreunit.core.test/test/org/moreunit/core/util/StringsTest.java
@@ -127,4 +127,10 @@ public void join_should_join_given_strings_whith_given_separator() throws Except
         assertThat(Strings.join(",", "1", "2", "3")).isEqualTo("1,2,3");
         assertThat(Strings.join(" -> ", "aBc", "2", "DEf")).isEqualTo("aBc -> 2 -> DEf");
     }
+
+    @Test
+    public void emptyArray_should_return_empty_array()
+    {
+        assertThat(Strings.emptyArray()).isEmpty();
+    }
 }