I assume you flipped expected and actual behavior.

This boils down to the question, if the linefeed should be part of the difference or not. I think it should be part of it, since in your example a newline is introduced. The Arrays have different size. If you start with

Arrays.asList("This is a test senctence.","")

you get your expected behavior.

thanks for the reply,I want to compare the difference between two strings 。
String a = "This is a test senctence."
String b="This is a test for diffutils.\nThis is the second line."

               DiffRowGenerator generator = DiffRowGenerator.create()
				.showInlineDiffs(true)
				.inlineDiffByWord(true)
				.oldTag(f -> "~")
				.newTag(f -> "**")
				.build();

		String aa = "This is a test senctence.";
		String bb ="This is a test for diffutils.\nThis is the second line.";
		List<DiffRow> rows = generator.generateDiffRows(
				Arrays.asList(aa.split("\n")),
				Arrays.asList(bb.split("\n")));

		System.out.println("|original|new|");
		System.out.println("|--------|---|");
		for (DiffRow row : rows) {
			System.out.println("|" + row.getOldLine() + "|" + row.getNewLine() + "|");
		}

I changed my mind. Due to visualization it is clear, that there is a linefeed. And because this line diff is generated for visualiization purposes the result should be the one you expected.

The actual result in version 3.0-SNAPSHOT is