Вы можете попробовать что-то вроде этого:
//your input
String numbers = "-25+26+78-21";
//split lookahead by + or - and store them in array of strings
//you can do with it afterwards whatever you like, turn it into ints for example
String[] tokens = numbers.split("(?=\\-)|\\+");
System.out.println(Arrays.asList(tokens));