法一：枚举所有日期(推荐)

    private static int[] days= {0,31,28,31,30,31,30,31,31,30,31,30,31};
    public static boolean check_valid(int date) {
        int year=date/10000;
        int month=date/100%100;
        int day=date%100;
        if(month<1||month>12)return false;
        if(month!=2&&day>days[month]||day<1)return false;//day要大于0
        if(month==2) {
            int leap=0;
            if(year%4==0&&year%100!=0||year%400==0)
                leap=1;
            if(day>28+leap)return false;
        }
        return true;
    }
    public static void main(String[] args) {
        Scanner scanner=new Scanner(System.in);
        String[] strings=scanner.next().split("/");
        int a=Integer.parseInt(strings[0]);
        int b=Integer.parseInt(strings[1]);
        int c=Integer.parseInt(strings[2]);
        for(int i=19600101;i<=20591231;i++) {//从小到大，已经排好序了
            int year=i/10000;
            int month=i/100%100;
            int day=i%100;
            if(check_valid(i)) {
                if(year%100==a&&month==b&&day==c||
                    month==a&&day==b&&year%100==c||
                    day==a&&month==b&&year%100==c) {
                    System.out.print(year+"-");
                    if(month<10)System.out.print("0");
                    System.out.print(month+"-");
                    if(day<10)System.out.print("0");
                    System.out.println(day);
                }
            }
        }
    }

法二：根据给出的日期进行拼凑(考虑步骤较多)

    private static Map<Integer, String> map=new HashMap<Integer, String>();
    private static int[] days= {0,31,28,31,30,31,30,31,31,30,31,30,31};
    public static boolean check_valid(int date) {
        int year=date/10000;
        int month=date/100%100;
        int day=date%100;
        if(month<1||month>12)return false;
        if(month!=2&&day>days[month]||day<1)return false;//注意可能为00
        if(month==2) {
            int leap=0;
            if(year%4==0&&year%100!=0||year%400==0)
                leap=1;
            if(day<1||day>28+leap)return false;
        }
        return true;
    }
    public static void main(String[] args) {
        Scanner scanner=new Scanner(System.in);
        String string=scanner.next();
        String aString=string.substring(0,2),bString=string.substring(3,5),cString=string.substring(6,8);
        String[] A= {aString,cString,cString};
        String[] B= {bString,aString,bString};
        String[] C= {cString,bString,aString};
        for(int i=0;i<3;i++) {
            int date;
            date=20*100+Integer.parseInt(A[i]);
            date=date*100+Integer.parseInt(B[i]);
            date=date*100+Integer.parseInt(C[i]);
            if(19600101<=date&&date<=20591231&&check_valid(date)) {
                if(!map.containsKey(date))
                    map.put(date, Integer.toString(date));
            }
            date=date-1000000;
            if(19600101<=date&&date<=20591231&&check_valid(date)) {
                if(!map.containsKey(date))
                    map.put(date, Integer.toString(date));
            }
        }
        int[] arr=new int[map.size()];
        int i=0;
        for (Entry<Integer, String> entry : map.entrySet()) {
            arr[i++]=entry.getKey();
        }
        Arrays.sort(arr);
        for(int a:arr) {
            String str=map.get(a);
            System.out.println(str.substring(0,4)+"-"+str.substring(4,6)+"-"+str.substring(6,8));
        }
    }