String str1 = "張三,李四,王五,陳六,呂七,孫八,趙九";
String str2 = "王五,孫八";

String[] arr1 = str1.split(",");
String[] arr2 = str2.split(",");
for (int i = 0; i < arr2.length; i++) {
	for (int j = 0; j < arr1.length; j++) {
		if (arr1[j].equals(arr2[i])) {
			arr1[j] = "";
		}
	}
}

StringBuffer sb = new StringBuffer();
for (int j = 0; j < arr1.length; j++) {
	if (!"".equals(arr1[j])) {
		sb.append(arr1[j] + ",");
	}
}

System.out.println("結果："+ sb.toString().substring(0, sb.toString().length() - 1));