@FunctionalInterface
public interface Comparator<T> {

      // @param o1 the first object to be compared.
      // @param o2 the second object to be compared.
      // @return a negative integer, zero, or a positive integer as the
      //       first argument is less than, equal to, or greater than the second.

    int compare(T o1, T o2);
}    

根据源码可知，如果o1大于o2返回正数，如果o1小于o2返回负数，如果o1等于o2返回0

使用stream流进行排序以及比较大小

@Data
@Accessors(chain = true)
public class Stu {
    private Integer age;
    private String name;
}

    @Test
    public void test05() {
        List<Stu> stuList = new ArrayList<>();
        stuList.add(new Stu().setAge(1005));
        stuList.add(new Stu().setAge(1008));
        stuList.add(new Stu().setAge(1004));


        System.out.println("按age默认从小到大排序");
        stuList.stream().sorted(Comparator.comparingInt(Stu::getAge)).collect(Collectors.toList()).forEach(System.out::println);
        System.out.println("按age从大到小排序");
        stuList.stream().sorted(Comparator.comparingInt(stu -> -stu.getAge())).collect(Collectors.toList()).forEach(System.out::println);
        System.out.println("求age最大值");
        stuList.stream().max(Comparator.comparing(Stu::getAge)).ifPresent(System.out::println);
        System.out.println("求age最小值");
        stuList.stream().min(Comparator.comparing(Stu::getAge)).ifPresent(System.out::println);
    }