类型PrintStream的方法printf（String，Object ...）不适用于参数（String，void）

Question

It gives me an error message saying

The method printf(String, Object...) in the type PrintStream is not applicable for the arguments (String, void)

public class project {

        public static void main(String[] args){

        duo duoObject = new duo();
        duo duoObject1 = new duo(5);
        duo duoObject2 = new duo(2,6);
        duo duoObject3 = new duo(3,7,5);
        System.out.printf("%s\n", duoObject.militaryTime());
        System.out.printf("%s\n", duoObject1.militaryTime());
        System.out.printf("%s\n", duoObject2.militaryTime());
        System.out.printf("%s\n", duoObject3.militaryTime());
        }

    }


    public class duo {
        private int hour;
        private int minute;
        private int second;

        public duo(){

        }
        public duo(int h){
            setHour(h);
        }
        public duo(int h, int m){
            setHour(h);
            setHour(m);
        }
        public duo(int h, int m, int s){
            setHour(h);
            setHour(m);
            setHour(s);
        }
        public void setHour(int h){
            hour = ((h>=0 && h<24)? h : 0);
        }
        public void setMinute(int h){
            minute = ((h>=0 && h<60)? h : 0);
        }
        public void setSecond(int h){
            second = ((h>=0 && h<60)? h : 0);
        }
        public int getHour(){
            return hour;
        }
        public int getMinute(){
            return minute;
        }
        public int getSecond(){
            return second;
        }
        public void militaryTime(){
            System.out.printf("%02d:%02d:%02d", getHour(), getMinute(), getSecond());
        }
    }

Answer 1

militaryTime returns void . void represents nothing and can't be printed.

Perhaps you want militaryTime to return a string representing the military time. In that case try defining the method as follows:

public String militaryTime() {
    return String.format("%02d:%02d:%02d", getHour(), getMinute(), getSecond());
}