Chapter 8 Classes and Objects

Chapter 8: Classes and Objects Time Class Case Study //Time1 class declaration public class Time1 { private int hour; // private int minute; // private int second; // public void hour = ( minute = second = } setTime( ( h >= 0 ( ( m >= ( ( s >= maintains the time in 24-hour format 0 - 23 0 - 59 0 - 59 int h, && h < 0 && m 0 && s int m, 24 ) ? < 60 ) < 60 ) int h : ? m ? s s 0 : : ) { ); 0 ); 0 ); public String toUniversalString(){ return String.format( "%02d:%02d:%02d", hour, minute, second ); } public String toString(){ return String.format( "%d:%02d:%02d %s", ( ( hour == 0 || hour == 12 ) ? 12 : hour % 12 ), minute, second, ( hour < 12 ? "AM" : "PM" ) ); } } public class Time1Test{ public static void main( String args[] ){ Time1 time = new Time1(); System.out.println( time.toUniversalString() ); System.out.println( time.toString() ); System.out.println(); time.setTime( 13, 27, 6 ); System.out.println( time.toUniversalString() ); System.out.println( time.toString() ); System.out.println(); time.setTime( 99, 99, 99 ); System.out.println("After attempting invalid settings:" ); System.out.println( time.toUniversalString() ); System.out.println( time.toString() ); } } 88 - Notes: • If a class does not define a constructor the compiler will provide a default constructor • new implicitly invokes Time1’s default constructor since Time1 does not declare any constructors. • Using time object, three methods are called (toString() , toUniversalString() and setTime). These methods are declared public, so they are accessible to the class’s objects. private variables and private methods are not accessible to the class’s objects. • %02d : this specifier indicates that an int value should be formatted as a field of two digits. The 0 flag in the format specifier indicates that values with fewer digits than the field width (2) should begin with a leading 0. • Format method from String class – The format method is similar to printf except that it returns a formatted string instead of displaying it in a command window Example: 1 // Fig. 8.3: MemberAccessTest.java 2 // Private members of class Time1 are not accessible. 3 public class MemberAccessTest 4 { 5 public static void main( String args[] ) 6 { 7 Time1 time = new Time1(); // create and initialize Time1 object 8 9 time.hour = 7; 10 time.minute = 15; // error: minute has private access in Time1 11 12 // error: hour has private access in Time1 time.second = 30; // error: second has private access in Time1 } // end main 13 } // end class MemberAccessTest MemberAccessTest.java:9: hour has private access in Time1 time.hour = 7; // error: hour has private access in Time1 ^ MemberAccessTest.java:10: minute has private access in Time1 time.minute = 15; // error: minute has private access in Time1 ^ MemberAccessTest.java:11: second has private access in Time1 time.second = 30; // error: second has private access in Time1 ^ 3 errors 89 Referring to the Current Object’s Members with the this Reference • The this reference – Any object can access a reference to itself with keyword this. – Non-static methods implicitly use this when referring to the object’s instance variables and other methods – this can be used to access instance variables when they are shadowed by local variables or method parameters – It is often a logic error when a method contains a parameter or local variable that has the same name as a field of the class. In this case, use reference this if you wish to access the field of the class—otherwise, the method parameter or local variable will be referenced. • A .java file can contain more than one class – But only one class in each .java file can be public Example: // ThisTest.java used implicitly and explicitly to refer to // members of an object. public class ThisTest { public static void main( String args[] ) { SimpleTime time = new SimpleTime( 15, 30, 19 ); System.out.println( time.buildString() ); } } // class SimpleTime demonstrates the "this" reference class SimpleTime { private int hour; // 0-23 Constructor parameters shadow private int minute; // 0-59 instance variables private int second; // 0-59 Using this to access the object’s instance variables public SimpleTime( int hour, int minute, int second ){ this.hour = hour; this.minute = minute; Using this explicitly and implicitly to call this.second = second; toUniversalString } public String buildString() { return String.format( "%24s: %s\n%24s: %s", "this.toUniversalString()",this.toUniversalString(), "toUniversalString()", toUniversalString() ); } public String toUniversalString(){ return String.format( "%02d:%02d:%02d", hour, minute, second ); } } 90 Time Class Case Study: Overloaded Constructors • Overloaded constructors – Provide multiple constructor definitions with different signatures • No-argument constructor – A constructor invoked without arguments • The this reference can be used to invoke another constructor – It is in the form: this( ) – Allowed only as the first statement in a constructor’s body. – It is syntax error when a method attempts to invoke a constructor directly via this. • If no constructors are declared: - The compiler will create a default constructor - The default constructor takes no arguments and initializes instance variables to their initial values specified in their declaration or to their default values. • Default values are : - zero for primitive numeric types, false for boolean values and null for references. • • If a class has constructors, but none of the public constructors are no-argument constructors, and a program attempts to call a no-argument constructor to initialize an object of the class, a compilation error occurs. A constructor can be called with no arguments only if the class does not have any constructors (in which case the default constructor is called) or if the class has a public no-argument constructor. 91 // Time2.java - class declaration with overloaded constructors. public class Time2{ private int hour; // 0 - 23 private int minute; // 0 - 59 private int second; // 0 - 59 public Time2() { this( 0, 0, 0 ); } // end Time2 no-argument constructor public Time2( int h ) { this( h, 0, 0 ); } // end Time2 one-argument constructor public Time2( int h, int m ) { this( h, m, 0 ); } // end Time2 two-argument constructor public Time2( int h, int m, int s ) { setTime( h, m, s ); } // end Time2 three-argument constructor public Time2( Time2 time ){ this( time.getHour(), time.getMinute(),time.getSecond() ); } // end Time2 constructor with a Time2 object argument public void setTime( int h, int m, int s ) { setHour( h ); setMinute( m ); setSecond( s ); } // end method setTime public void setHour( int h ) { hour = ( ( h >= 0 && h < 24 ) ? h : 0 ); } // end method setHour public void setMinute( int m ) { minute = ( ( m >= 0 && m < 60 ) ? m : 0 ); } // end method setMinute public void setSecond( int s ) { second = ( ( s >= 0 && s < 60 ) ? s : 0 ); } // end method setSecond public int getHour() { return hour; } public int getMinute() { return minute; } public int getSecond() { return second; } public String toUniversalString() { return String.format( "%02d:%02d:%02d", getHour(), getMinute(), getSecond() ); } // end method toUniversalString public String toString(){ return String.format( "%d:%02d:%02d %s", ((getHour() == 0 || getHour() == 12) ? 12 : getHour()%12 ), getMinute(), getSecond(),( getHour() < 12 ? "AM": "PM") ); }} // end class Time2 92 // Time2Test.java public class Time2Test{ public static void main( String args[] Time2 t1 = new Time2(); Time2 t2 = new Time2( 2 ); Time2 t3 = new Time2( 21, 34 ); Time2 t4 = new Time2( 12, 25, 42 ); Time2 t5 = new Time2( 27, 74, 99 ); Time2 t6 = new Time2( t4 ); ) { // 00:00:00 // 02:00:00 // 21:34:00 // 12:25:42 // 00:00:00 // 12:25:42 System.out.println( "t1: all arguments defaulted" ); System.out.printf( " %s\n", t1.toUniversalString() ); System.out.printf( " %s\n", t1.toString() ); System.out.println( "t2: hour specified; minute and second defaulted" ); System.out.printf( " %s\n", t2.toUniversalString() ); System.out.printf( " %s\n", t2.toString() ); System.out.println( "t3: hour and minute specified; second defaulted" ); System.out.printf( " %s\n", t3.toUniversalString() ); System.out.printf( " %s\n", t3.toString() ); System.out.println("t4:hour,minute and second specified"); System.out.printf( " %s\n", t4.toUniversalString() ); System.out.printf( " %s\n", t4.toString() ); System.out.println( "t5: all invalid values specified" ); System.out.printf( " %s\n", t5.toUniversalString() ); System.out.printf( " %s\n", t5.toString() ); System.out.println( "t6: Time2 object t4 specified" ); System.out.printf( " %s\n", t6.toUniversalString() ); System.out.printf( " %s\n", t6.toString() ); } // end main } // end class Time2Test 93 Garbage Collection and method finalize  JVM performs automatic garbage collection to reclaim the memory occupied by objects that are no longer in use (i.e. there are no references to the object)  JVM executes its garbage collector method to retrieve memory, and the garbage collector calls the finalize method before the object's memory is reclaimed.  Method finalize is declared protected without parameters and has return type void. (protected void finalize())  To ask for garbage collection explicitly, call the static method gc() of the class System: System.gc();  Note: The garbage collector is not guaranteed to execute at a specified time. Static and instance variables • Static variables are shared by all the objects of the class. • Instance variables – Can be initialized when they are declared or in a constructor • Printing an object using only its name will implicitly call toString method of Object class 94 - Example: //Increment.java final instance variable and static variable public class Increment { public static int count =0; private int total = 0; private final int INCREMENT; public Increment( int incrementValue ) { INCREMENT = incrementValue; count++; } public void addIncrementToTotal() { total += INCREMENT; } public String toString() { return String.format( "total = %d", total ); }} // IncrementTest.java public class IncrementTest { public static void main( String args[] ) { Increment value1 = new Increment( 5 ); System.out.printf("Beforeincrementing:%s\n\n", value1.toString()); value1.addIncrementToTotal(); System.out.printf("After increment %s\n", value1.toString()); System.out.printf("The count is %d\n",value1.count ); Increment value2 = new Increment( 3 ); System.out.printf( "Before incrementing: %s\n\n", value2 ); value2.addIncrementToTotal(); System.out.printf( "After increment %s\n", value2 ); System.out.printf( "The count is : %d\n", value2.count ); }} 95