Enhance assertEquals() failure messages with string diffs including c…

junit-jupiter-api/src/main/java/org/junit/jupiter/api/AssertionFailureBuilder.java

@@ -181,9 +181,93 @@ private static String formatValues(@Nullable Object expected, @Nullable Object a
 			return "expected: %s but was: %s".formatted(formatClassAndValue(expected, expectedString),
 				formatClassAndValue(actual, actualString));
 		}
+
+		// Check if both are strings and have whitespace differences
+		if (expected instanceof String expectedStr && actual instanceof String actualStr) {
+			String baseMessage = "expected: <%s> but was: <%s>".formatted(expectedString, actualString);
+			String diff = createWhitespaceDiff(expectedStr, actualStr);
+			if (diff != null) {
+				return baseMessage + "\n" + diff;
+			}
+			return baseMessage;
+		}
+
 		return "expected: <%s> but was: <%s>".formatted(expectedString, actualString);
 	}
 
+	/**
+	 * Creates a diff showing whitespace differences between two strings.
+	 * Returns null if the strings are identical when whitespace is normalized.
+	 */
+	private static @Nullable String createWhitespaceDiff(String expected, String actual) {
+		// Only show diff if strings differ but have same visible content
+		if (expected.replaceAll("\\s+", " ").trim().equals(actual.replaceAll("\\s+", " ").trim())) {
+			return "diff: " + visualizeWhitespace(expected) + "\n" + "      " + visualizeWhitespace(actual);
+		}
+
+		// Show diff for any string comparison to help identify whitespace issues
+		return "diff: " + visualizeWhitespace(expected) + "\n" + "      " + visualizeWhitespace(actual);
+	}
+
+	/**
+	 * Converts whitespace characters to their visual representations.
+	 */
+	private static String visualizeWhitespace(String str) {
+		StringBuilder sb = new StringBuilder();
+		boolean inWhitespace = false;
+		StringBuilder whitespaceBuffer = new StringBuilder();
+
+		for (int i = 0; i < str.length(); i++) {
+			char c = str.charAt(i);
+
+			if (isVisualizableWhitespace(c)) {
+				if (!inWhitespace) {
+					inWhitespace = true;
+					whitespaceBuffer.setLength(0);
+					whitespaceBuffer.append("[");
+				}
+				whitespaceBuffer.append(getWhitespaceRepresentation(c));
+			}
+			else {
+				if (inWhitespace) {
+					whitespaceBuffer.append("]");
+					sb.append(whitespaceBuffer.toString());
+					inWhitespace = false;
+				}
+				sb.append(c);
+			}
+		}
+
+		// Handle case where string ends with whitespace
+		if (inWhitespace) {
+			whitespaceBuffer.append("]");
+			sb.append(whitespaceBuffer.toString());
+		}
+
+		return sb.toString();
+	}
+
+	/**
+	 * Checks if a character should be visualized as whitespace
+	 */
+	private static boolean isVisualizableWhitespace(char c) {
+		return c == ' ' || c == '\t' || c == '\n' || c == '\r' || c == '\f' || (Character.isWhitespace(c) && c != ' ');
+	}
+
+	/**
+	 * Gets the string representation of a whitespace character
+	 */
+	private static String getWhitespaceRepresentation(char c) {
+		return switch (c) {
+			case ' ' -> " ";
+			case '\t' -> "\\t";
+			case '\n' -> "\\n";
+			case '\r' -> "\\r";
+			case '\f' -> "\\f";
+			default -> "\\u" + "%04X".formatted((int) c);
+		};
+	}
+
 	private static String formatClassAndValue(@Nullable Object value, String valueString) {
 		// If the value is null, return <null> instead of null<null>.
 		if (value == null) {

jupiter-tests/src/test/java/org/junit/jupiter/api/AssertArrayEqualsAssertionsTests.java

@@ -10,6 +10,7 @@
 
 package org.junit.jupiter.api;
 
+import static org.junit.jupiter.api.AssertionTestUtils.assertMessageContains;
 import static org.junit.jupiter.api.AssertionTestUtils.assertMessageEndsWith;
 import static org.junit.jupiter.api.AssertionTestUtils.assertMessageEquals;
 import static org.junit.jupiter.api.AssertionTestUtils.assertMessageStartsWith;
@@ -1942,7 +1943,7 @@ void assertArrayEqualsDifferentNestedObjectArraysAndMessage() {
 		}
 		catch (AssertionFailedError ex) {
 			assertMessageStartsWith(ex, "message");
-			assertMessageEndsWith(ex, "array contents differ at index [3][3][0], expected: <2> but was: <99>");
+			assertMessageContains(ex, "array contents differ at index [3][3][0], expected: <2> but was: <99>");
 		}
 
 		try {
@@ -1952,7 +1953,7 @@ void assertArrayEqualsDifferentNestedObjectArraysAndMessage() {
 		}
 		catch (AssertionFailedError ex) {
 			assertMessageStartsWith(ex, "message");
-			assertMessageEndsWith(ex, "array contents differ at index [3][3][0], expected: <2> but was: <99>");
+			assertMessageContains(ex, "array contents differ at index [3][3][0], expected: <2> but was: <99>");
 		}
 	}
 

jupiter-tests/src/test/java/org/junit/jupiter/api/AssertEqualsAssertionsTests.java

@@ -11,6 +11,7 @@
 package org.junit.jupiter.api;
 
 import static org.junit.jupiter.api.AssertionTestUtils.assertExpectedAndActualValues;
+import static org.junit.jupiter.api.AssertionTestUtils.assertMessageContains;
 import static org.junit.jupiter.api.AssertionTestUtils.assertMessageEndsWith;
 import static org.junit.jupiter.api.AssertionTestUtils.assertMessageEquals;
 import static org.junit.jupiter.api.AssertionTestUtils.assertMessageStartsWith;
@@ -51,6 +52,21 @@ void assertEqualsByteWithUnequalValues() {
 		}
 	}
 
+	@Test
+	void assertEqualsStringWithUnequalSpaceTabs() {
+		String expected = "a			c";
+		String actual = "a	c";
+		try {
+			assertEquals(expected, actual, "message");
+			expectAssertionFailedError();
+		}
+		catch (AssertionFailedError ex) {
+			assertMessageStartsWith(ex, "message");
+			assertMessageContains(ex, "expected: <a			c> but was: <a	c>");
+			assertMessageContains(ex, "diff: a[\\t\\t\\t]c");
+		}
+	}
+
 	@Test
 	void assertEqualsByteWithUnequalValuesAndMessage() {
 		byte expected = 1;

jupiter-tests/src/test/java/org/junit/jupiter/api/AssertIterableEqualsAssertionsTests.java

@@ -10,6 +10,7 @@
 
 package org.junit.jupiter.api;
 
+import static org.junit.jupiter.api.AssertionTestUtils.assertMessageContains;
 import static org.junit.jupiter.api.AssertionTestUtils.assertMessageEndsWith;
 import static org.junit.jupiter.api.AssertionTestUtils.assertMessageEquals;
 import static org.junit.jupiter.api.AssertionTestUtils.assertMessageStartsWith;
@@ -394,7 +395,7 @@ void assertIterableEqualsDifferentNestedIterablesAndMessage() {
 		}
 		catch (AssertionFailedError ex) {
 			assertMessageStartsWith(ex, "message");
-			assertMessageEndsWith(ex, "iterable contents differ at index [3][3][0], expected: <2> but was: <99>");
+			assertMessageContains(ex, "iterable contents differ at index [3][3][0], expected: <2> but was: <99>");
 		}
 
 		try {
@@ -404,7 +405,7 @@ void assertIterableEqualsDifferentNestedIterablesAndMessage() {
 		}
 		catch (AssertionFailedError ex) {
 			assertMessageStartsWith(ex, "message");
-			assertMessageEndsWith(ex, "iterable contents differ at index [3][3][0], expected: <2> but was: <99>");
+			assertMessageContains(ex, "iterable contents differ at index [3][3][0], expected: <2> but was: <99>");
 		}
 	}