How to use this() in overloaded subclass constructors ( and why) ?

[james dunster]

I wrote the following to demonstrate to myself how super works and that is OK unless someone tells me otherwise. I have now run into the sand on use of this() to invoke overloaded constructors. The book I am reading says I can't use both super and this but it doesn't explain how to use this. I know I don't need it here because the constructors don't do much but I would still like to understnad how to do it. Grateful for a steer.

    

public class tryFormat  {

static int myStaticVar=100;
protected int myInt;
protected String[] myString ={"one","Two","three"};

public tryFormat() {
myInt=65;
}

public tryFormat(int N) {
myInt=N;
}

public tryFormat(int N, String[] sx ){
myInt=N;
myString=sx;
}

public static void main(String[] args)  {

	String[] sy={"four","five","six"};

	
	subFormat sf1 =new subFormat();
	subFormat sf2 =new subFormat(60);
	subFormat sf3 =new subFormat(60,sy);

System.out.println(sf1.myInt + "   "+sf1.myString[2]);
System.out.println(sf2.myInt + "   "+sf2.myString[2]);
System.out.println(sf3.myInt + "   "+sf3.myString[2]);
System.out.println(tryFormat.myStaticVar)
	

}

}

class subFormat extends tryFormat {

public subFormat() {} 		// if you feed it no args then sub takes both super inst vars

public subFormat(int M)  {
super(M);}		                               // this is where sub feeds its M to super, sub has no myInt of its own

public subFormat(int M,String[] arg)  {         // this time feeding both args up to super
super(M, arg);
}

}
    

/*
C:\Documents and Settings\jd>java tryFormat
65 three
60 three
60 six
100
*/

[Campbell Ritchie]

Don't know. Your code is hard to read without the code tags. But I know where you can find an example, the Clock classes in the Deitel Java How to Program book. I can more-or-less remember what it looks like, but I have made some changes

public class Clock
{
    private int hours;
    private int minutes;
    private int seconds;
    /**
     * How many hours there are in a day
     */
    public static final int HOURS_IN_DAY = 24;
    /**
     * Similar
     */
    public static final int HOURS_IN_DAY = 60;
    /**
     * As before
     */
    public static final int HOURS_IN_DAY = 60;
   
    
    /**
     * Set up the Clock object with default values of 0 for hours, minutes and seconds
     */
    public Clock()
    {
        this(0); // Note this calls the 1-arg constructor, and that calls the 3-args constructor.
//      this(0, 0, 0); // That is an alternative: calls 3-args constructor directly.
    }//end no-args constructor
    
    /**
     * Set up the Clock object with default values of 0 for minutes and seconds
     * @param hours The hours to set
     * @throws IllegalArgumentException if hours negative or too high.
     */
    public Clock(int hours)
    {
        this(hours, 0, 0);
    }//end one-arg constructor
    
    /**
     * Set up the Clock object with default value of 0 for seconds
     * @param hours The hours to set
     * @param minutes The minutes to set
     * @throws IllegalArgumentException if hours or minutes negative or too high.
     */
    public Clock(int hours, int minutes)
    {
        this(hours, minutes, 0);
    }//end two-args constructor
    
    /**
     * Set up the Clock object
     * @param hours The hours to set
     * @param minutes The minutes to set
     * @param seconds The seconds to set
     * @throws IllegalArgumentException if hours, minutes, or seconds negative or too high.
     */
    public Clock(int hours, int minutes)
    {
        setHours(hours);
        setMinutes(minutes);
        setSeconds(seconds)
    }//end three-args constructor
    
    /**
     * Set the hours
     * @param hours Must be in the correct range
     * @throws IllegalArgumentException if hours negative or too high.
     */
    public final void setHours(int hours)
    {
        if (hours < 0 || hours >= HOURS_IN_DAY)
        {
            throw new IllegalArgumentException("Hours must be between 0 and " +
                    (HOURS_IN_DAY - 1) + ". " + hours + " passed.");
        }//end if
        this.hours = hours;
    }//end set hours  
        
    /**
     * Set the minutes
     * @param minutes Must be in the correct range
     * @throws IllegalArgumentException if minutes negative or too high.
     */
    public final void setMinutes(int minutes)
    {
        if (minutes < 0 || minutes >= MINUTES_IN_HOUR)
        {
            throw new IllegalArgumentException("Minutes must be between 0 and " +
                    (MINUTES_IN_HOUR - 1) + ". " + minutes + " passed.");
        }//end if
        this.minutes = minutes;
    }//end set minutes  
        
    /**
     * Set the seconds
     * @param seconds Must be in the correct range
     * @throws IllegalArgumentException if seconds negative or too high.
     */
    public final void setMinutes(int minutes)
    {
        if (seconds < 0 || seconds >= SECONDS_IN_MINUTE)
        {
            throw new IllegalArgumentException("Seconds must be between 0 and " +
                    (SECONDS_IN_MINUTE - 1) + ". " + seconds + " passed.");
        }//end if
        this.seconds = seconds;
    }//end set seconds  
}//end class

Note

1 I have gone from few arguments to many arguments.

2 You can go the other direction.

3 Concentrate as much of your code as possible in one of the constructors.

4 Methods called from the constructor are tagged final (or private)

5 The validation of the values entered is all done in one place, the set methods

NB: There was a rather similar subject about 2 weeks ago: look here, particularly noting Sam Mercs' comments.

Your book is correct that you can't use this() and super() together. If you use either, they must be the first statement in the constructor, and you can't have two "first" statements.

[james dunster]

OK, got it thanks. Puzzled by line 57 though.

This works. I guess same can be done with subclasses if I had something more for the constructors to do.

public class tryFormatThis  {

static int myStaticVar=100;
protected int myInt;
protected String[] myString ={"one","Two","three"};

public tryFormatThis() {
//myInt=65;
this(65);
}

public tryFormatThis(int N) {
//myInt=N;
this(N,null);				// tried passing (N, myString) but that's not right
}

public tryFormatThis(int N, String[] sx ){
myInt=N;
myString=sx;
}

public static void main(String[] args)  {   

~ snip

 

[Campbell Ritchie]

james dunster wrote:OK, got it thanks. Puzzled by line 57 though.

Try this version

public Clock(int hours, int minutes, int seconds)

That might work better

And sorry for that mistake.