Java for C++ Programmers Second Night

Overview First Night –Basics –Classes and Objects Second Night –Enumerations –Exceptions –Input/Output –Templates vs. Generics –STL vs. JavaSE API

Second Night Agenda Enumerations, Exceptions, Input/Output – enumeration declaration & usage, exception hierarchy, checked vs. unchecked exceptions, throwing & catching exceptions, scanner class, console I/O, file I/O –Discussion –Lab exercises Break Templates vs. Generics, STL vs. JavaSE API – –Discussion –Lab exercises

Enumerated Values in C++ One way to define a set of enumerated values/constants in C++ is as follows… Example usage… const int CLUBS = 0; const int DIAMONDS = 1; const int HEARTS = 2; const int SPADES = 3; // good invocations DrawSuit(CLUBS); DrawSuit(HEARTS); void DrawSuit(int s) { // draws the suit in a GUI }

Enumerated Values in Java Heres the Java port of that C++ code… Everything looks good, right? public class Suit { public static final int CLUBS = 0; public static final int DIAMONDS = 1; public static final int HEARTS = 2; public static final int SPADES = 3; } // good invocations drawCard(Suit.CLUBS); drawCard(Suit.DIAMONDS); static void drawCard(int s) { // draws the suit in a GUI }

Enumerated Values in Java Well, sort of – as long as the user behaves themselves… Using something unbounded like an int or a String can be problematic, need to restrict the available choices // bad invocations drawCard(Suit.HEARTS * 5); drawCard(-516); static void drawCard(int s) { // draws the suit in a GUI }

Enumerations in Java Thankfully, theres a better way to enumerate a set of values, staring in Java 5 there is an enumerated type –Similar to enum construct in C/C++ –Enums are declared similarly to classes public enum Suit { CLUBS, DIAMONDS, HEARTS, SPADES }

Enumerations in Java Now, each constant is strongly typed When something is expecting a suit, we specify a Suit enum type Invalid usage is now caught at compile time // good invocations drawCard(Suit.CLUBS); drawCard(Suit.DIAMONDS); // compiler errors drawCard(Suit.HEARTS * 5); drawCard(-516); static void drawCard(Suit s) { // draws the symbol for the suit in a GUI }

Enumerations in Java Additionally, we can switch on enums… // draws the symbol for the suit in a GUI static void drawCard(Suit s) { switch(s) { case CLUBS: case SPADES: // switch color to black, then draw break; default: // switch color to red, then draw break; }

Enumerations in Java We can also give enums in Java additional members and methods public enum Planet { VENUS(4.8685e24,6051.8e3), EARTH(5.9736e24,6378.1e3), MARS( e24,3397e3); public static final double G = E-11; final double mass; final double radius; Planet(double mass, double radius) { this.mass = mass; this.radius = radius; } double surfaceGravity() { return G * mass / (radius * radius); } double surfaceWeight(double otherMass) { return otherMass * surfaceGravity(); }

Enumerations Example usage, similar to a class… But we get compiler errors, if we try to construct more… // compiler errors Planet.EARTH = new Planet(1.0, 1.0); Planet p = new Planet(1.0, 1.0); // acceptable usage drawPlanet(Planet.EARTH); System.out.println("Surface gravity on earth: "); System.out.println(Planet.EARTH.surfaceGravity());

Exceptions in C++ C++ allows us to throw anything as an exception Here is a modified factorial function which throws the number back if it is less than 0 int factorial(int n) { // check for exceptional case if(n < 0) { throw n; } // computation for normal case int result = 1; for(int i = n; i > 0; i--) { result *= i; } return result; }

Exceptions in C++ Example catching of that exception in C++… int main(int argc, char** argv) { int num = -4; try { cout << "factorial(" << num << "): " << factorial(num) << endl; } catch(int i) { cerr << i << " is not valid" << endl; } return 0; }

Exceptions in C++ We can also throw more complex types, for example, here is a custom exception class which stores an error message… class Exception { private: string m_message; public: Exception(string message) : m_message(message) { }; string GetMessage() { return this->m_message; }; };

Exceptions in C++ Factorial function modified to throw custom exception type… int factorial(int n) { // check for exceptional case if(n < 0) { throw Exception("number must be positive"); } // computation for normal case int result = 1; for(int i = n; i > 0; i--) { result *= i; } return result; }

Exceptions Example catching of that exception in C++… int main(int argc, char** argv) { int num = -4; try { cout << "factorial(" << num << "): " << factorial(num) << endl; } catch(Exception e) { cerr << e.GetMessage() << endl; } return 0; }

Exceptions in Java In Java, we cannot throw primitives or most objects Anything that is thrown must be a Throwable (or a valid subclass) –Though, typically we throw Exceptions (or subclasses) ErrorException RuntimeException Error … … … Throwable Object

Exceptions in Java Heres a Java port of that second example –Were using the built-in Exception class… static int factorial(int n) { // check for exceptional case if(n < 0) { throw new Exception("number must be positive"); } // computation for normal case int result = 1; for(int i = n; i > 0; i--) { result *= i; } return result; }

Exceptions in Java Here is the invoking function in Java, with the try/catch block added… public static void main(String[] args) { int num = -4; try { System.out.println("factorial(" + num + "): " + factorial(num)); } catch(Exception e) { System.err.println(e.getMessage()); }

Exceptions in Java But, if we try to compile this, we get the following error… Why? $ javac.exe Factorial.java Factorial.java:19: unreported exception java.lang.Exception; must be caught or declared to be thrown throw new Exception("number must be positive"); ^ 1 error

Java Exception Hierarchy Java breaks up Exceptions into 2 categories –Unchecked: you do not need to explicitly handle these exceptions –Checked: you must handle these in your code, failure to do so is a compiler error Checked Unchecked ErrorException RuntimeException Error … … … Throwable Object

Handling Checked Exceptions We have one of 2 options… –Handle the exception locally (wrap code in a try/catch block) –Propagate the exception by adding a throws declaration to the method signature

Exceptions in Java The modified factorial function modified to declare an thrown exception… static int factorial(int n) throws Exception { // check for exceptional case if(n < 0) { throw new Exception("number must be positive"); } // computation for normal case int result = 1; for(int i = n; i > 0; i--) { result *= i; } return result; }

Exceptions in Java Other notes about exceptions in Java… –e.printStackTrace() will print out the full stack trace as to where an exception originated –e.getMessage() will give you a detailed message string as to why the exception occurred –Try/catch blocks may also have a finally section with code in it Always executed even if a return or throw is encountered

Console I/O in C++ #include using namespace std; int main() { int n; string s; cout << "What do you want to buy: "; cin >> s; cout << "How many: "; cin >> n; cout << "You want " << n << " " << s << endl; return 0; } Simple C++ class that reads/writes to the console

Console Input/Output in Java Already looked at System.out for print() and println(), though there are many more such as –printf() – C style output (added in Java 5) –write() – byte based output System.err provides the same methods as System.out does, goes to stderr System.in interface is our standard input stream –System.in is an InputStream class and is pretty low level (read bytes) –Typically another class is used to read from System.in

Console Input in Java Scanner class was added in JSE 5 to simplify input… –Has a constructor which takes an InputStream class Scanner(System.in) –Methods are available to read common data types… nextBoolean() nextFloat(), nextDouble() nextShort(), nextInt(), nextLong() next() - reads next String –Corresponding methods to check and see if another value is in stream… i.e. hasNextInt()

Console I/O in Java Java port of our C ++ example… import java.util.Scanner; public class ConsoleIO { public static void main(String[] args) { Scanner console = new Scanner(System.in); System.out.print("What do you want to buy: "); String s = console.next(); System.out.print("How many: "); int n = console.nextInt(); System.out.println("You want " + n + " " + s); }

File Input in C++ Example reading integers from a file… #include using namespace std; int main() { int num; fstream in("file.txt", ios::in); while(in >> num) { cout << "read " << num << endl; } in.close(); return 0; }

File Input in Java Java port of C++ example… import java.io.File; import java.util.Scanner; public class FileIO { public static void main(String[] args) { try { Scanner in = new Scanner(new File("file.txt")); while(in.hasNextInt()) { System.out.println("read " + in.nextInt()); } in.close(); } catch (Exception e) { e.printStackTrace(); }

File Output in C++ Output to a file in C++… #include using namespace std; int main() { fstream out("file.txt", ios::out); for(int i = 0; i < 10; i++) { out << i << endl; } out.close(); return 0; }

File Output in Java Java port of C++ example (PrintStream is same class as System.out, so same methods available)… import java.io.PrintStream; public class FileIO { public static void main(String[] args) { try { PrintStream out = new PrintStream("file.txt"); for(int i = 0; i < 10; i++) { out.println(i); } out.close(); } catch (Exception e) { e.printStackTrace(); }

Exercises 1.Write an Direction enum for the 4 cardinal directions (North, South, East & West). Then write (in another class) a walk method which takes a Direction enum as an argument 2.Write a class which reads numbers from one file (the 1 st command line arg) and writes out each number squared to a second file (the 2 nd command line arg)

Break

C++ Templates C++ allows us to define generic (template) types which can hold whatever type we specify Frequently people include the C file at the end of the header file Gotchas… –Compilation –Mixing templated and non-templated code –Spaces with respect to nested templates

C++ Templates Example node header file… #ifndef _NODE_H_ #define _NODE_H_ template class Node { private: T m_data; public: Node(T data); T GetData() const; }; #include "node.C" #endif

C++ Templates Corresponding implementation in C++ #ifndef _NODE_C_ #define _NODE_C_ #include "Node.H" template Node ::Node(T data) { this->m_data = data; } template T Node ::GetData() const { return this->m_data; } #endif

C++ Templates Example usage… #include #include "node.H" using namespace std; int main() { Node n("hamburger"); cout << n.GetData() << endl; return 0; }

Java Generics Java introduced Generics in Java 5 which are similar to C++s templates –Syntax is a bit less burdensome Parts of the Java SE API are capable of using generics –Prior to Java 5, all of these things stored Objects and were thus capable of storing any object what-so-ever (as it had to be a subclass of Object)

Java Generics Port of templated Node class to Java… public class Node { private T data; Node(T data) { this.data = data; } T getData() { return this.data; }

Java Generics Example driver… public class NodeTest { public static void main(String[] args) { Node n = new Node ("hamburger"); System.out.println(n.getData()); }

Generics Gotchas Cannot use primitive types as the generic type… –In C++ this is completely valid… –In Java, this is not… –We can still store an int, char, float or boolean, but we need to do something slightly different Node n(516); Node n = new Node (516);

Wrapper Classes For each of the primitive types, Java provides a wrapper class Wrapper class is an object which stores the given primitive type… –Integer – stores an int primitive –Boolean – stores a boolean primitive –Float – stores a float primitive –Etc… We can declare our generic type to be of the wrapper class type

Wrapper Classes & Generics Example using the Integer wrapper class to store ints In Java 5, primitives are automatically converted to and from the corresponding wrapper type as needed –This is known as auto boxing/un-boxing public class NodeTest { public static void main(String[] args) { Node n = new Node (516); System.out.println(n.getData()); }

Generics Gotchas In C++, you can create an array of a templates type… Java does not permit this, you end up with a compiler error… template void Node ::SomeTemplatedFunction() { T array[10]; } void SomeTemplatedFunction() { T array[10]; }

Generics Bounding Allow a method to take anything that subclasses T Allow only classes that implement the Comparable interface… void foo(Collection items) { /*... */ }

C++ STL The C++ STL provides numerous built-in data types that are used by programmers Some of the more commonly used ones are… –vector –iterators –deque –list –map

Java SE API One thing you will notice about Java is that the Java SE API is huge –Java SE 6 has Nearly 3800 classes, interfaces & enums in the built-in API Bookmark the javadocs, they are your friend… –

Java Collections Framework Java has a robust collections (containers) framework –With Java 5, these have been generified to support arbitrary types with strong typing –There are interfaces provided for many ADTs List, Set, Map, etc… –There are also multiple concrete implementations of many of the data structures, each providing their own advantages… ArrayList, LinkedList, Stack, Vector, etc… –Typically you instantiate a concrete implementation, but maintain a handle on it based on its interface Interface = ImplementationOfInterface ();

C++ Vector Sample C++ Vector usage… #include using namespace std; int main() { vector v; v.push_back(1); v.push_back(20); v.push_back(2009); for(int i = 0, n = v.size(); i < n; i++) { cout << v[i] << endl; } for(vector ::iterator itr = v.begin(); itr != v.end(); itr++) { cout << *itr << endl; } return 0; }

Java Vector Port of the C++ vector example… import java.util.Vector; public class VectorTest { public static void main(String[] args) { Vector v = new Vector (); v.add(1); v.add(20); v.add(2009); for(int i = 0, n = v.size(); i < n; i++) { System.out.println(v.get(i)); } for(int tmp : v) { System.out.println(tmp); }

C++ List Sample C++ Vector usage… #include using namespace std; int main() { list l; l.push_front(1); l.push_front(2); l.push_back(3); l.push_back(4); list ::iterator itr; for (itr = l.begin(); itr != l.end(); itr++) { cout << *itr << endl; } cout << l.front() << endl; l.pop_front(); cout << l.front() << endl; }

Java List Port of the C++ list example… import java.util.LinkedList; import java.util.List; public class ListTest { public static void main(String[] args) { List l = new LinkedList (); l.add(1); l.add(0, 2); l.add(l.size(), 3); l.add(l.size(), 4); for(int i : l) { System.out.println(i); } System.out.println(l.get(0)); l.remove(0); System.out.println(l.get(0)); }

C++ Map Sample C++ map usage… #include using namespace std; int main() { map courses; courses[201] = "Computer Science I"; courses[202] = "Computer Science II"; courses[341] = "Data Structures"; map ::iterator itr; for (itr = courses.begin(); itr != courses.end(); itr++) { cout first second << endl; } return 0; }

Java Map Port of the C++ map example… import java.util.Iterator; import java.util.Map; import java.util.TreeMap; public class MapTest { public static void main(String[] args) { Map courses = new TreeMap (); courses.put(201, "Computer Science I"); courses.put(202, "Computer Science II"); courses.put(341, "Data Structures"); Iterator itr; for (itr = courses.keySet().iterator(); itr.hasNext();) { int num = itr.next(); System.out.println(num + ": " + courses.get(num)); }

C++ Deque Sample C++ deque usage… #include using namespace std; int main() { deque d; d.push_front(1); d.push_front(2); d.push_back(3); d.push_back(4); deque ::iterator itr; for (itr = d.begin(); itr != d.end(); itr++) { cout << *itr << endl; } cout << d.front() << endl; d.pop_front(); cout << d.front() << endl; return 0; }

Java Deque Port of the C++ deque example… import java.util.ArrayDeque; import java.util.Deque; public class DequeTest { public static void main(String[] args) { Deque d = new ArrayDeque (); d.addFirst(1); d.addFirst(2); d.addLast(3); d.addLast(4); for (int i : d) { System.out.println(i); } System.out.println(d.getFirst()); d.removeFirst(); System.out.println(d.getFirst()); }

Exercises 1.Create, compile and run a program which puts several Person objects into a Vector and iterates over them and prints each out The members in a Person are up to you, but there should be at least one, and toString should overridden