diff --git a/src/edu/mills/cs180a/StringUtilities.java b/src/edu/mills/cs180a/StringUtilities.java
index 31f03b8..e393ed8 100644
--- a/src/edu/mills/cs180a/StringUtilities.java
+++ b/src/edu/mills/cs180a/StringUtilities.java
@@ -6,50 +6,59 @@
  * Static utility class for testing whether a string is a substring of another string.
  */
 public class StringUtilities {
-  private StringUtilities() {}
-
-  /**
-   * Tests whether the potential substring is in the full string. The empty string is considered a
-   * substring of every full string.
-   *
-   * @param substring the potential substring
-   * @param text the full string
-   * @return true if the substring is contained in the full string, false otherwise
-   * @throws NullPointerException if either argument is null
-   */
-  public static boolean isSubstring(String substring, String text) {
-    Objects.requireNonNull(substring);
-    Objects.requireNonNull(text);
-
-    if (substring.isEmpty()) {
-      return true;
-    }
+    private StringUtilities() {}
+
+    /**
+     * Tests whether the potential substring is in the full string. The empty string is considered a
+     * substring of every full string.
+     *
+     * @param substring the potential substring
+     * @param text the full string
+     * @return true if the substring is contained in the full string, false otherwise
+     * @throws NullPointerException if either argument is null
+     */
+    public static boolean isSubstring(String substring, String text) {
+        Objects.requireNonNull(substring);
+        Objects.requireNonNull(text);
 
-    for (int i = 0; i < text.length(); i++) {
-      // Check if current character of the full string matches start of substring.
-      if (text.charAt(i) == substring.charAt(0)) {
-        // If so, see if the rest of the strings match.
-        if (isSubstringHelper(substring, text, i + 1)) {
-          return true;
+        if (substring.isEmpty()) {
+            return true;
+        }
+
+        for (int i = 0; i < text.length(); i++) {
+            // Check if current character of the full string matches start of substring.
+            if (text.charAt(i) == substring.charAt(0)) {
+                // If so, see if the rest of the strings match.
+
+                if (isSubstringHelper(substring, text, i + 1)) {
+                    return true;
+                }
+            }
+            // If not, keep iterating through the full string.
         }
-      }
-      // If not, keep iterating through the full string.
-    }
-    return false;
-  }
-
-  // check if substring appears at the given offset in text
-  private static boolean isSubstringHelper(String substring, String text, int offset) {
-    // i is used as an index for substring, offset is used for text
-    for (int i = 1; // The character with index 0 has already been tested.
-        i < substring.length() && offset < text.length(); i++, offset++) {
-      if (text.charAt(offset) != substring.charAt(i))
         return false;
     }
-    return true;
-  }
 
-  protected static int getLength(String s) {
-    return s.length();
-  }
+    // check if substring appears at the given offset in text
+    private static boolean isSubstringHelper(String substring, String text, int offset) {
+
+        // check that the substring isn't longer than the remaining text
+        if ((substring.length() - 1) > (text.length() - offset)) {
+            return false;
+        }
+
+        // i is used as an index for substring, offset is used for text
+        for (int i = 1; // The character with index 0 has already been tested.
+                i < substring.length() && offset < text.length(); i++, offset++) {
+            if (text.charAt(offset) != substring.charAt(i)) {
+                return false;
+            }
+        }
+        return true;
+
+    }
+
+    protected static int getLength(String s) {
+        return s.length();
+    }
 }
diff --git a/src/edu/mills/cs180a/StringUtilitiesTester.java b/src/edu/mills/cs180a/StringUtilitiesTester.java
index a38617b..b60c9be 100644
--- a/src/edu/mills/cs180a/StringUtilitiesTester.java
+++ b/src/edu/mills/cs180a/StringUtilitiesTester.java
@@ -1,25 +1,62 @@
 package edu.mills.cs180a;
 
+import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.CsvSource;
 
 class StringUtilitiesTester {
-  @Test
-  void isSubstring_False_null() {
-    assertThrows(NullPointerException.class, () -> StringUtilities.isSubstring(null, "foo"));
-    assertThrows(NullPointerException.class, () -> StringUtilities.isSubstring("foo", null));
-    assertThrows(NullPointerException.class, () -> StringUtilities.isSubstring(null, null));
-  }
-
-  @Test
-  void isSubstring_False_EmptyString() {
-    assertTrue(StringUtilities.isSubstring("", "foo"));
-    // TODO: write rest
-  }
-
-  @Test
-  void isSubstring_True_Length1Substring() {
-    assertTrue(StringUtilities.isSubstring("A", "ABC"));
-  }
+    @Test
+    void isSubstring_False_null() {
+        assertThrows(NullPointerException.class, () -> StringUtilities.isSubstring(null, "foo"));
+        assertThrows(NullPointerException.class, () -> StringUtilities.isSubstring("foo", null));
+        assertThrows(NullPointerException.class, () -> StringUtilities.isSubstring(null, null));
+    }
+
+    @ParameterizedTest
+    @CsvSource({"'',''", "'',' '", "'','   '", "'',XYZ", "'',X Z", "'', XYZ", "'',XYZ "})
+    void isSubstring_True_EmptyString(String emptySubstring, String text) {
+        assertTrue(StringUtilities.isSubstring(emptySubstring, text));
+    }
+
+    @ParameterizedTest
+    @CsvSource({"' ',' '", "' ','   '", "' ','x z'", "' ','xyz '", "' ',' xyz'", "x,xyz", "y,xyz",
+            "z,xyz"})
+    void isSubstring_True_Length1Substring(String substring, String text) {
+        assertTrue(StringUtilities.isSubstring(substring, text));
+    }
+
+    @ParameterizedTest
+    @CsvSource({"' ',''", "X,xyz", "x,XYZ", "x,''", "a,xyz"})
+    void isSubstring_False_Length1Substring(String substring, String text) {
+        assertFalse(StringUtilities.isSubstring(substring, text));
+    }
+
+    @ParameterizedTest
+    @CsvSource({"'  ','  '", "'  ','   '", "xy,xyz", "yz,xyz", "ab, ab", "AB,AB"})
+    void isSubstring_True_Length2Substring(String substring, String text) {
+        assertTrue(StringUtilities.isSubstring(substring, text));
+    }
+
+    @ParameterizedTest
+    @CsvSource({"'  ',''", "'  ',' '", "xz,xyz", "Xy,xyz", "xY,xyz", "XY,xyz", "xy,XYZ",
+            "ba,ab", "ab,AB", "ab,a"})
+    void isSubstring_False_Length2Substring(String substring, String text) {
+        assertFalse(StringUtilities.isSubstring(substring, text));
+    }
+
+    @ParameterizedTest
+    @CsvSource({"'   ','   '", "xyz,xyz", "xyz,' xyz '", "XYZ,XYZ"})
+    void isSubstring_True_Length3Substring(String substring, String text) {
+        assertTrue(StringUtilities.isSubstring(substring, text));
+    }
+
+    @ParameterizedTest
+    @CsvSource({"'   ',''", "'   ',' '", "'   ','  '", "yzx,xyz", "Xyz,xyz", "xYz,xyz", "XYZ,xyz",
+            "xyz,XYZ", "XyZ,xYz", "xyZ,xyz"})
+    void isSubstring_False_Length3Substring(String substring, String text) {
+        assertFalse(StringUtilities.isSubstring(substring, text));
+    }
 }