Fix bug in AssertionUtils.toString(Object)

Prior to this commit, AssertionUtils.toString(Object) printed an array via
the array's toString() method which resulted in non-user-friendly output.

This commit addresses this issue by printing arrays using Arrays.toString(),
which produces human readable output.

This is a prerequisite for junit-team#961.

Issue: junit-team#1030

.gitignore

@@ -17,6 +17,7 @@ bin/
 *.iws
 *.uml
 .idea/
+*/out/*
 
 # Misc
 *.log

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

@@ -12,6 +12,7 @@
 
 import static java.util.stream.Collectors.joining;
 
+import java.util.Arrays;
 import java.util.Deque;
 import java.util.function.Supplier;
 
@@ -91,7 +92,13 @@ private static String formatClassAndValue(Object value, String valueString) {
 	}
 
 	private static String toString(Object obj) {
-		return (obj instanceof Class ? getCanonicalName((Class<?>) obj) : String.valueOf(obj));
+		if (obj instanceof Class) {
+			return getCanonicalName((Class<?>) obj);
+		}
+		if (obj instanceof Object[]) {
+			return Arrays.toString((Object[]) obj);
+		}
+		return StringUtils.nullSafeToString(obj);
 	}
 
 	private static String toHash(Object obj) {

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

@@ -1867,15 +1867,13 @@ void assertArrayEqualsDifferentNestedObjectArrays() {
 			assertMessageEquals(ex, "array contents differ at index [2][1][1][0], expected: <false> but was: <true>");
 		}
 
-		Object[] differentElement = new Object[] {};
 		try {
 			assertArrayEquals(new Object[] { 1, 2, 3, new Object[] { new Object[] { 4, new Object[] { 5 } } } },
-				new Object[] { 1, 2, 3, new Object[] { new Object[] { 4, new Object[] { differentElement } } } });
+				new Object[] { 1, 2, 3, new Object[] { new Object[] { 4, new Object[] { new Object[] {} } } } });
 			expectAssertionFailedError();
 		}
 		catch (AssertionFailedError ex) {
-			assertMessageEquals(ex,
-				"array contents differ at index [3][0][1][0], expected: <5> but was: <" + differentElement + ">");
+			assertMessageEquals(ex, "array contents differ at index [3][0][1][0], expected: <5> but was: <[]>");
 		}
 	}