// File:  TimeTester.java

// Shows uses of "this" - the invisible "implicit parameter"
// passed to all "instance" methods (i.e. non-static methods).  
// this is a reference to the object for which the method was called.

// 1. explicit (but unnecessary) use of this to access instance
//    variables of an object and call methods for it 
// 2. to prevent "shadowing" (i.e., where a local variable or
//    method parameter has the same name as an instance variable
//    and would otherwise "hide" the instance variable)
// 3. to enable a method to return a reference to the object for which
//    it was called, which allows "chaining" of method calls
// 4. the convenience of enabling one constructor of a class
//    to call other constructors of the same class (saves code)

class Time 
{
  // instance vars store time in military format
  private int hour ;			// 0..23
  private int minute ;		// 0..59
  private int second ;		// 0..59
  
  public Time(int hr, int min, int sec)	// 3-argument constructor
  {
  		// eliminate duplicate code by passing param's to "set" methods
  		this.setHour(hr) ;
  		this.setMinute(min) ;
  		this.setSecond(sec) ;
  }

  public Time(int hr, int min)		// 2-argument constructor
  {
  		// call 3-arg constructor, passing param's hr and min, and 0
  		this(hr, min, 0) ;
  }

  public Time(int hr)				// 1-argument constructor
  {
  		// call 3-arg constructor, passing param hr, and two 0's
  		this(hr, 0, 0) ;
  }

  public Time()	// "default" (i.e., no-argument) constructor
  {
  		// call 3 arg constructor with all 0's
  		this(0, 0, 0) ;
  }

  // return time as String  hh:mm:ss A.M./P.M.
  public String toString()	
  { 
   	String out = "" ;	// the "null" string
   	
   	if (hour == 0)
  	  		out += "12" ;
  		else if (hour <= 12)
  	  		out += hour ;
  		else
  	  		out += hour % 12 ;
  	
  		// assure two-digit minute
  		if (minute <= 9)
  	  		out += ":0" + minute ;
  		else 
  	  		out += ":" + minute ;
  	
  		// assure two-digit minute
  		if (second <= 9)
  	  		out += ":0" + second ;
  		else 
  	  		out += ":" + second ;
  	  
  		if (hour <= 11)
  	  		out += " A.M." ;
  		else 
  	  		out += " P.M." ;
  	  
  		return out ;
  }
  
  /**
   * Set a Time object to hr:min:sec and return a reference to that object
   * @return a reference to the object for which the method was called
   */
  public Time setTime(int hr, int min, int sec) 
  {
  		// pass parameters to "set" methods to save code
  		// shows explicit (but unnecessary) use of this to call other methods
  	
  		this.setHour(hr) ;		// same as: setHour(hr)
  		this.setMinute(min) ;			
  		this.setSecond(sec) ;		
  		return this ;				// returns reference
  }

  /**
   * Reset the hour part of a Time obj and return a reference to that obj
   * @return a reference to the object for which the method was called
   */
  public Time setHour(int hour) 
  {
  		// uses this to avoid "shadowing" of instance var "hour" by 
  		// parameter "hour"
  		this.hour = (hour >= 0 && hour <= 23) ? hour : 0 ;
  		return this ;				// returns reference
  }
  
  /**
   * Reset the minute part of a Time obj and return a reference to that obj
   * @return a reference to the object for which the method was called
   */
  public Time setMinute(int minute)
  {
  		// uses this to avoid "shadowing" of instance var minute
  		this.minute = (minute >= 0 && minute <= 59) ? minute : 0 ;
  		return this ;			// returns reference
  }
  
  /**
   * Reset the second part of a Time obj and return a reference to that obj
   * @return a reference to the object for which the method was called
   */
  public Time setSecond(int second)
  {
  		// uses this to avoid "shadowing" of instance var second
  		this.second = (second >= 0 && second <= 59) ? second : 0 ;
  		return this ;			// returns reference
  }
  
}
  	
public class TimeTester
{
  public static void main(String[] args) 
  {
    	Time a = new Time(10,11,12) ;			// 10:11:12 am
    	Time b = new Time(13,14);				// 1:14:00 pm
    	Time c = new Time(12) ;					// 12:00:00 pm (noon)
    	final Time MIDNIGHT = new Time() ;	// a constant, set to 00:00:00
    	
    	System.out.println("Initial times (a,b,c, and MIDNIGHT) are:\n") ;
    	System.out.println(a + "\n" + b + "\n" + c + "\n" + MIDNIGHT) ;
	
	 	// reset "a" to 5:00:00 PM and print via "chaining" of method calls
		System.out.println
	          ("\nExecuting:   a.setHour(17).setMinute(0).setSecond(0) ;") ;
		a.setHour(17).setMinute(0).setSecond(0) ;
    	System.out.println("Time is now " + a) ;
	
		// reset "a" to 6:25:00 PM and printing via "chaining"
		System.out.println
			("\nExecuting:  System.out.println( a.setTime(18,25,0) ) ;") ;

    	System.out.println( "Time is now " + a.setTime(18,25,0) ) ;
  }
}

/* program output:
 
Initial times (a,b,c, and MIDNIGHT) are:

10:11:12 A.M.
1:14:00 P.M.
12:00:00 P.M.
12:00:00 A.M.

Executing:   a.setHour(17).setMinute(0).setSecond(0) ;
Time is now 5:00:00 P.M.

Executing:  System.out.println( a.setTime(18,25,0) ) ;
Time is now 6:25:00 P.M.

*/