Improve failure message for null vs. null comparisons

Prior to this commit, if the expected and actual values for a failed
assertion both had "null" string representations and if either the
expected or actual value was the null reference, the generated failure
message looked something like the following.

    expected: null<null> but was: java.lang.String@264b3504<null>

This commit improves the assertion failure message in such cases by
representing the null reference as "<null>" instead of "null<null>",
leading to generated failure messages similar to the following.

    expected: <null> but was: java.lang.String@264b3504<null>

This improvement applies to assertEquals(), assertNull, and any other
assertion that internally delegates to format(Object, Object, String) or
formatValues(Object, Object) in AssertionUtils.

See #2523

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

@@ -123,6 +123,10 @@ static String formatValues(Object expected, Object actual) {
 	}
 
 	private static String formatClassAndValue(Object value, String valueString) {
+		// If the value is null, return <null> instead of null<null>.
+		if (value == null) {
+			return "<null>";
+		}
 		String classAndHash = getClassName(value) + toHash(value);
 		// if it's a class, there's no need to repeat the class name contained in the valueString.
 		return (value instanceof Class ? "<" + classAndHash + ">" : classAndHash + "<" + valueString + ">");

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

@@ -550,6 +550,32 @@ void assertEqualsWithObjectVsNull() {
 		}
 	}
 
+	@Test
+	void assertEqualsWithObjectWithNullStringReturnedFromToStringVsNull() {
+		try {
+			assertEquals("null", null);
+			expectAssertionFailedError();
+		}
+		catch (AssertionFailedError ex) {
+			assertMessageStartsWith(ex, "expected: java.lang.String@");
+			assertMessageEndsWith(ex, "<null> but was: <null>");
+			assertExpectedAndActualValues(ex, "null", null);
+		}
+	}
+
+	@Test
+	void assertEqualsWithNullVsObjectWithNullStringReturnedFromToString() {
+		try {
+			assertEquals(null, "null");
+			expectAssertionFailedError();
+		}
+		catch (AssertionFailedError ex) {
+			assertMessageStartsWith(ex, "expected: <null> but was: java.lang.String@");
+			assertMessageEndsWith(ex, "<null>");
+			assertExpectedAndActualValues(ex, null, "null");
+		}
+	}
+
 	@Test
 	void assertEqualsWithNullVsObjectAndMessageSupplier() {
 		try {

junit-jupiter-engine/src/test/java/org/junit/jupiter/api/AssertNullAssertionsTests.java

@@ -74,9 +74,9 @@ private void assertNullWithNonNullObjectWithNullStringReturnedFromToString(Suppl
 		}
 		catch (AssertionFailedError ex) {
 			// Should look something like:
-			// expected: null<null> but was: java.lang.String@264b3504<null>
+			// expected: <null> but was: java.lang.String@264b3504<null>
 			String prefix = (messageSupplier != null ? messageSupplier.get() + " ==> " : "");
-			assertMessageStartsWith(ex, prefix + "expected: null<null> but was: java.lang.String@");
+			assertMessageStartsWith(ex, prefix + "expected: <null> but was: java.lang.String@");
 			assertMessageEndsWith(ex, "<null>");
 			assertExpectedAndActualValues(ex, null, actual);
 		}
@@ -105,10 +105,10 @@ private void assertNullWithNonNullObjectWithNullReferenceReturnedFromToString(Su
 		}
 		catch (AssertionFailedError ex) {
 			// Should look something like:
-			// expected: null<null> but was: org.junit.jupiter.api.AssertNullAssertionsTests$NullToString@4e7912d8<null>
+			// expected: <null> but was: org.junit.jupiter.api.AssertNullAssertionsTests$NullToString@4e7912d8<null>
 			String prefix = (messageSupplier != null ? messageSupplier.get() + " ==> " : "");
 			assertMessageStartsWith(ex,
-				prefix + "expected: null<null> but was: org.junit.jupiter.api.AssertNullAssertionsTests$NullToString@");
+				prefix + "expected: <null> but was: org.junit.jupiter.api.AssertNullAssertionsTests$NullToString@");
 			assertMessageEndsWith(ex, "<null>");
 			assertExpectedAndActualValues(ex, null, actual);
 		}