Question

I have 2 classes . The one is about a 24 hour clock and the second one is a subclass of the first and is a 12 hour clock.
To convert the new time in second class i need to change the value of the first class. But i can’t do that.
I am talking about “h” variable
Specifically…

    class Clock{
    public int h, m , s;
    public String a,b,c;

    public void setHour(int hour){
        this.h = hour;
    }
    public void setMin(int min){
        this.m = min;
    }
    public void setSec(int sec){
        this.s = sec;
    }
    public void tick(){ 
        if(h != 23){
            if(m == 59 && s==59){
                m = 0;
                s=0;
                h++;
            }
            else if(m != 59 && s == 59){
                m++;
                s=0;
            }
            else if ( m != 59 && s != 59){
                s++;
            }
            else if( m == 59 && s!=59){
                s++;
            }
        }

        else if(h == 23 && m == 59 && s !=59){
            s++;
        }
        else if(h == 23 && m!=59 && s == 59){
            s=0;
            s++;
            m++;
        }
        else if(h == 23 && m!=59 && s!=59){
            s++;
        } 
        else if(h == 23 && m == 59 && s == 59){
            s = 0;
            m =0;
            h = 0;
        }
    }
    public String toString(){ 
        a = "";
        b = "";
        c = "";

        if (h < 10)
            a = "0";
        if (m <10 )
            b = "0";
        if (s <10)
            c = "0";

        return a+h+":"+b+m+":"+c+s;
    }
}

   class AMPMClock extends Clock{
    Clock clock2 = new Clock();
    public void setAMPM(boolean yes){
        if(yes == true){
            **clock2.h = clock2.h - 12**;
        }
    }
}

Answer 1

Your AMPMClock should either just extend Clock or just use it. But you are trying to combine them both as follows:

class AMPMClock extends Clock {
    Clock clock2 = new Clock();

Either extend:

class AMPMClock extends Clock {

Because, the public accesor methods are inherited, you can invoke getter/setter methods on this. e.g. this.getHour().

Or use it but don’t extend:

class AMPMClock {
    Clock clock2 = new Clock();

Here, instead of this use clock2. e.g. clock2.getHour().

But, you need to make all the instance members private (they should not to be public). And make corresponding public getter/setter for all of them.

As suggested by dasblinkenlight:

It would be better to have an abstract class:

public abstract class BaseClock {
    private int h, m, s;
    //getter/setter
}

And two different concrete classes:

public class TwelveHourClock extends BaseClock { }

and

public class TwentyFourHourClock extends BaseClock { }