diff --git a/assertj-core/src/test/java/org/assertj/core/api/AbstractIterableAssertTest.java b/assertj-core/src/test/java/org/assertj/core/api/AbstractIterableAssertTest.java
new file mode 100644
index 0000000000..4a208d7192
--- /dev/null
+++ b/assertj-core/src/test/java/org/assertj/core/api/AbstractIterableAssertTest.java
@@ -0,0 +1,30 @@
+package org.assertj.core.api;
+
+import org.junit.jupiter.api.Test;
+import java.util.List;
+import static org.assertj.core.api.Assertions.*;
+
+public class AbstractIterableAssertTest {
+
+  @Test
+  void shouldFilterListBasedOnCondition() {
+    // Setup
+    List<String> names = List.of("Adam", "Bob", "Chandler");
+
+    // Test
+    ListAssert<String> result = assertThat(names).filteredOn(name -> name.startsWith("A"));
+
+    // Verify
+    result.containsExactly("Adam");
+  }
+
+  @Test
+  void shouldNotContainNullElements() {
+    // Setup
+    List<String> names = List.of("Adam", "Bob", "Chandler");
+
+    // Test and Verify
+    assertThat(names).doesNotContainNull();
+  }
+
+}
diff --git a/assertj-core/src/test/java/org/assertj/core/api/ConditionTest.java b/assertj-core/src/test/java/org/assertj/core/api/ConditionTest.java
new file mode 100644
index 0000000000..9307bee189
--- /dev/null
+++ b/assertj-core/src/test/java/org/assertj/core/api/ConditionTest.java
@@ -0,0 +1,32 @@
+package org.assertj.core.api;
+
+import org.junit.jupiter.api.Test;
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.Mockito.*;
+
+public class ConditionTest {
+
+  @Test
+  void returnTrueWhenConditionIsMet() {
+    Condition<String> mockCondition = mock(Condition.class);
+    when(mockCondition.matches("hello")).thenReturn(true);
+
+    boolean result = mockCondition.matches("hello");
+
+    assertThat(result).isTrue();
+
+    verify(mockCondition).matches("hello");
+  }
+
+  @Test
+  void returnFalseWhenConditionIsNotMet() {
+    Condition<String> mockCondition = mock(Condition.class);
+    when(mockCondition.matches("fail")).thenReturn(false);
+
+    boolean result = mockCondition.matches("fail");
+
+    assertThat(result).isFalse();
+
+    verify(mockCondition).matches("fail");
+  }
+}
diff --git a/assertj-core/src/test/java/org/assertj/core/api/NumberRangeTest.java b/assertj-core/src/test/java/org/assertj/core/api/NumberRangeTest.java
new file mode 100644
index 0000000000..f90ebbbc41
--- /dev/null
+++ b/assertj-core/src/test/java/org/assertj/core/api/NumberRangeTest.java
@@ -0,0 +1,46 @@
+package org.assertj.core.api;
+
+import org.junit.jupiter.api.Test;
+import static org.assertj.core.api.Assertions.assertThat;
+
+public class NumberRangeTest {
+  // typical case
+  @Test
+  void testTypicalNumber(){
+    int number = 50;
+
+    assertThat(number).isBetween(1, 100);
+  }
+
+  // min
+  @Test
+  void testMinNumber(){
+    int number = 1;
+
+    assertThat(number).isBetween(1, 100);
+  }
+
+  // min + 1
+  @Test
+  void testMinPlusOneNumber(){
+    int number = 2;
+
+    assertThat(number).isBetween(1, 100);
+  }
+
+  // max
+  @Test
+  void testMaxNumber(){
+    int number = 100;
+
+    assertThat(number).isBetween(1, 100);
+  }
+
+  // max - 1
+  @Test
+  void testMaxMinusOneNumber(){
+    int number = 99;
+
+    assertThat(number).isBetween(1, 100);
+  }
+}
diff --git a/assertj-core/src/test/java/org/assertj/core/api/StringValidationTest.java b/assertj-core/src/test/java/org/assertj/core/api/StringValidationTest.java
new file mode 100644
index 0000000000..cf430edf9d
--- /dev/null
+++ b/assertj-core/src/test/java/org/assertj/core/api/StringValidationTest.java
@@ -0,0 +1,45 @@
+package org.assertj.core.api;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.Disabled;
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+
+public class StringValidationTest {
+  // Equivalence class: Valid Strings
+  @Test
+  public void shouldPassForValidString() {
+    // These tests are expected to pass
+    assertThat("Hello").isNotBlank();
+    assertThat("Hello World!").isNotBlank();
+    assertThat("12345").isNotBlank();
+    assertThat("!!!").isNotBlank();
+  }
+
+  // Equivalence class: Empty Strings
+  @Disabled // remove to test if the build fails
+  @Test
+  public void shouldFailForEmptyString() {
+    // This test is expected to fail
+    assertThatThrownBy(() -> assertThat("").isNotBlank())
+      .isInstanceOf(AssertionError.class)
+      .hasMessageContaining("blank");
+  }
+
+  // Equivalence class: Whitespace Strings
+  @Disabled // remove to test if the build fails
+  @Test
+  public void shouldFailForWhitespaceString() {
+    // This test is expected to fail
+    assertThat("       ").isNotBlank();
+  }
+
+  // Equivalence class: Null Strings
+  @Disabled // remove to test if the build fails
+  @Test
+  public void shouldFailForNullString() {
+    // This test is expected to fail
+    String nullString = null;
+    assertThat(nullString).isNotBlank();
+  }
+}
diff --git a/assertj-core/src/test/java/org/assertj/core/util/ListsTest.java b/assertj-core/src/test/java/org/assertj/core/util/ListsTest.java
new file mode 100644
index 0000000000..a3cce90c0d
--- /dev/null
+++ b/assertj-core/src/test/java/org/assertj/core/util/ListsTest.java
@@ -0,0 +1,40 @@
+package org.assertj.core.util;
+
+import org.junit.jupiter.api.Test;
+import java.util.List;
+import static org.junit.jupiter.api.Assertions.*;
+
+public class ListsTest {
+  // No elements in list, tests to make sure it returns an empty list
+  @Test
+  void testNewEmptyList() {
+    List<String> emptyList = Lists.newArrayList();
+
+    // Checks if list is not null
+    assertNotNull(emptyList);
+    // List should be true if it is empty
+    assertTrue(emptyList.isEmpty());
+  }
+
+  // Single element list test, makes sure list knows the size and the value of the element
+  @Test
+  void testSingleElementList() {
+    List<Integer> singleElementList = Lists.newArrayList(15);
+    // Tests the size of the list
+    assertEquals(1, singleElementList.size());
+    // Tests the value of the element in the list
+    assertEquals(15, singleElementList.get(0));
+  }
+
+  // Multiple element list test, makes sure list knows the size and the values of each element
+  @Test
+  void testMultipleElementsList() {
+    List<String> multipleElementsList = Lists.newArrayList("a", "b", "c");
+    // Tests the size of the list
+    assertEquals(3, multipleElementsList.size());
+    // Tests the values of each element in the list
+    assertEquals("a", multipleElementsList.get(0));
+    assertEquals("b", multipleElementsList.get(1));
+    assertEquals("c", multipleElementsList.get(2));
+  }
+}
diff --git a/assertj-core/src/test/java/org/assertj/core/util/StringEmptyOrNullTest.java b/assertj-core/src/test/java/org/assertj/core/util/StringEmptyOrNullTest.java
new file mode 100644
index 0000000000..63eb505ce3
--- /dev/null
+++ b/assertj-core/src/test/java/org/assertj/core/util/StringEmptyOrNullTest.java
@@ -0,0 +1,42 @@
+package org.assertj.core.util;
+
+import static org.junit.jupiter.api.Assertions.*;
+import org.junit.jupiter.api.Test;
+
+public class StringEmptyOrNullTest {
+  @Test
+  void returnTrueIfStringIsNull() {
+    // setup
+    String input = null;
+
+    // test
+    boolean result = Strings.isNullOrEmpty(input);
+
+    //verify
+    assertTrue(result);
+  }
+
+  @Test
+  void returnTrueIfStringIsEmpty() {
+    // setup
+    String input = "";
+
+    // test
+    boolean result = Strings.isNullOrEmpty(input);
+
+    // verify
+    assertTrue(result);
+  }
+
+  @Test
+  void returnFalseIfStringIsNotEmpty() {
+    // setup
+    String input = "Hello World!";
+
+    // test
+    boolean result = Strings.isNullOrEmpty(input);
+
+    // verify
+    assertFalse(result);
+  }
+}