Classes, The Rest of the Story

Transcription

1 Classes, The Rest of the Story 1

2 1. Is the following syntax legal in Java: class MyException extends Exception { class Test { static void f( ) throws MyException { throw MyException(); // (A) public static void main( String[] args ) { try { f(); catch( MyException e ) { System.out.println( "caught MyException" ); 2

3 2. Is the following syntax legal in Java: class MyException extends Exception { class Test { static void f( ) { // (A) public static void main( String[] args ) { try { f(); catch( MyException e ) { System.out.println( "caught MyException" ); 3

4 3. Is the following syntax legal in Java: class MyException extends Exception { class Err extends Exception { class Test { static void f( int j ) throws MyException, Err { if ( j == 1 ) throw new MyException(); if ( j == 2 ) throw new Err(); public static void main( String[] args ) { try { f( 1 ); catch( MyException e ) { catch( Err e ) { try { f( 2 ); catch( MyException e ) { catch( Err e ) { 4

5 4. Is the following syntax legal in Java? class MyException extends Exception { class Err extends Exception { class Test { static void f() throws MyException { throw new MyException(); static void g() throws Err { throw new Err(); static void h() throws MyException, Err { f(); g(); public static void main( String[] args ) { try { h(); catch( Exception e ) { // (A) e.printstacktrace(); 5

6 6

7 5. What is exception specification? Is it always required in C++ and Java? 7

8 6. Is the following syntax legal in Java? class MyException extends Exception { String message; class Test { static void f() throws MyException { throw new MyException( "Hello from f()" ); // (C) public static void main( String[] args ) { try { f(); catch( MyException e ) { System.out.println( e.message ); 8

9 Access Control of Constructors Constructors are not always placed in the public section of a class. Sometimes, it is necessary to place one or more constructors in the private or the protected section. While this forbids certain forms of object creation, it makes possible other behaviors by the class. 9

10 //Singleton.java class X { private int n; private static X unique; private X( int m ){ n = m; //(A) //(B) public static X makeinstanceofx() { if ( unique == null ) unique = new X( 10 ); return unique; //(C) class Test { public static void main( String[] args ) { X xobj_1 = X.makeInstanceOfX(); X xobj_2 = X.makeInstanceOfX(); if ( xobj_1 == xobj_2 ) System.out.println( "The two X objects are the same" ) 10

11 //Singleton.cc #include <iostream> class X { int n; static X* unique; //(A) public: X( int m ){ n = m; //(B) X( const X& ); //(C) X& operator=( const X& ); //(D) ; static X& makeinstanceofx() { if ( unique == 0 ) unique = new X( 10 ); return *unique; //(E) X* X::unique = 0 ; //(F) bool operator==( const X& obj1, const X& obj2 ) { return &obj1 == &obj2? true : false; //(G) 11

12 int main() { X& xobj_1 = X::makeInstanceOfX(); X& xobj_2 = X::makeInstanceOfX(); if ( xobj_1 == xobj_2 ) cout << "The two X objects are the same" << endl; // X xobj_3 = xobj_1 //(H) // xobj_2 = xobj_1; //(I) return 0; 12

13 Limiting the Access to No-Arg Constructor in C++ A constructor that in certain situations is a good candidate for the private or the protected section of a C++ class is the no-arg constructor. 13

14 //NoArgMissing.cc #include <vector> class X { int p; public: X( int m ) : p(m) { ; int main() { X xob; // Error, no-arg constructor missing X arr[ 100 ]; // Error, no-arg constructor missing vector<x> vec1( 100 ); // Error, no-arg constructor missing // OK, no-arg constructor not needed: vector<x> vec2; X xobj( 12 ); vec2.push_back( xobj ); return 0; //(A) 14

15 X(){ X() : p( 0 ) { 15

16 //PrivateConstructor.cc #include <vector> using namespace std; class X { friend class vector<x>; int p; X(){ public: X( int m ) : p(m) { ; //(B) //(C) int main() { // X xob1; // Error, no-arg constructor private // X arr[ 100 ]; // Error, no-arg constructor private vector<x> vec(100); X xob2( 12 ); vec[0] = xob2; return 0; // OK now 16

17 Can Multiple Constructors Help Each Other? When multiple constructors are defined for a class, can they help each other during the construction of an object of the class? The answer to this is yes in Java and no in ISO compatible C++. 17

18 //MultiConstructors.java class X { int x; int y; X() { x = 5; //(A) X( int m ) { //(B) this(); //(C) y = m; public static void main( String[] args ) { X xobj = new X( 100 ); System.out.println ( "x: " + xobj.x + " y: " + xobj.y ); 18

19 Static Members in C++ A static member of a class is global to all objects of the class. A static member is also sometimes called a class variable. A static member does not belong to any particular object of that class. There is exactly one copy per class of such a member, as opposed to one copy per object for non-static members. 19

20 //Static1.cc #include <iostream> using namespace std; class X { public: int m; static int n; static const int p; static const int q = 51; X( int mm ) { m = mm; ; int X::n = 100; const int X::p = 101; //(A) //(B) int main() { cout << "static n: " << X::n << endl; // 100 //(C) X xobj_1( 20 ); cout << xobj_1.m << " " << xobj_1.n << endl; // X xobj_2( 40 ); cout << xobj_2.m << " " << xobj_2.n << endl; //

21 X::n = 1000; cout << xobj_1.m << " " << xobj_1.n << endl; // cout << xobj_2.m << " " << xobj_2.n << endl; // return 0; 21

22 //Static2.cc #include <iostream> using namespace std; class X { static int n; public: int m; X( int mm ) { m = mm; ; static int getn() { return n; //(A) static void setn( int nn ) { n = nn; //(B) int X::n = 100; //(C) int main() { cout << "static n: " << X::getn() << endl; // 100 X xobj_1( 20 ); cout << xobj_1.m << " " << xobj_1.getn() << endl; // X xobj_2( 40 ); cout << xobj_2.m << " " << xobj_2.getn() << endl; // X::setn( 1000 ); 22

23 cout << xobj_1.m << " " << xobj_1.getn() << endl; // cout << xobj_2.m << " " << xobj_2.getn() << endl; // return 0; 23

24 //Date.cc #include <iostream> using namespace std; class Date { int d, m, y; static Date today; public: Date( int dd = 0, int mm = 0, int yy = 0 ); static void settoday(int, int, int); static Date gettoday() { return today; ; void print() { cout << "day: " << d << " month: " << m << " year: " << y << endl; ; //(A) //(B) Date::Date( int dd, int mm, int yy ) { d = dd? dd : today.d; m = mm? mm : today.m; y = yy? yy : today.y; void Date::setToday( int dd, int mm, int yy ) { today = Date(dd, mm, yy); //(C) 24

25 Date Date::today( 31, 10, 2001 ); //(D) int main() { Date d1( 1, 1, 1970); d1.print(); // day: 1 month: 1 year: 1970 Date d2( 2 ); d2.print(); // day: 2 month: 10 year: 2001 Date::setToday(3, 4, 2000); // (E) Date::getToday().print(); // day: 3 month: 4 year: 2000 Date d3( 7 ); d3.print(); // day: 7 month: 4 year: 2000 Date d4 = Date(); d4.print(); // day: 3 month: 4 year: 2000 return 0; 25

26 //Robot.cc #include <iostream> #include <string> using namespace std; class Robot { int idnum; static int nextidnum; // (A) string owner; public: int getidnum(); int whatisnextidnum(); // (B) string getowner(); Robot( string ownername); void print() { cout << idnum << " " << owner << endl; ; int Robot::nextIdNum = 1; // (C) int Robot::whatIsNextIdNum() { return nextidnum++; int Robot::getIdNum() { return idnum; string Robot::getOwner() { return owner; Robot::Robot( string name ) { idnum = whatisnextidnum(); owner = name; // (D) // (E) 26

27 int main() { Robot r1( "ariel" ); r1.print(); Robot r2( "mauriel" ); r2.print(); Robot r3( "mercurial" ); r3.print(); return 0; // 1 ariel // 2 mauriel // 3 mercurial 27

28 Initialization and Destruction of Static Objects in C++ One of the more interesting aspects of C++ is that there is usually more to the main of a program than meets the eye, especially if the program includes static class members and their initializations in global scope. 28

29 //StaticInit.cc #include <iostream> class X { public: X() { cout << "X s constructor invoked" << endl; ~X() { cout << "X s destructor invoked" << endl; ; class Y { public: static X xobj; ; X Y::xobj; //(A) //(B) int main() { cout << "This is the first executable statement of main" << endl; //(C) return 0; 29