Chair of Software Engineering. Java and C# in Depth. Prof. Dr. Bertrand Meyer. Exercise Session 7. Nadia Polikarpova

Transcription

1 Chair of Software Engineering Java and C# in Depth Prof. Dr. Bertrand Meyer Exercise Session 7 Nadia Polikarpova

2 Quiz 1: Does it compile? (Java) public class MyException extends Exception { Checked exception... public MyException(String message) { super(message); public class A { public void dosomething() throws MyException { if (badstuffhappened) { dosomething throws a checked throw new MyException( Help! ) exception, but does not declare it } else {... } 2

3 Quiz 1 (cont.): Does it compile? (Java) public class A { public void dosomething() throws MyException {... public class B { public void domore() { A a;... a.dosomething(); domore calls a method, which throws a checked exception, but doesn t itself declare or catch it 3

4 Quiz 2: Exception accumulation (Java) private String readdatafromurl(string url) throws MalformedURLException { // throw MalformedURLException if url is bad. // read data and return it. } private long parselong(string data) throws NumberFormatException { // convert data to long. // throw NumberFormatException if data is not a valid long. } public long readnumberfromurl(string url) throws MalformedURLException, NumberFormatException { String data = readdatafromurl(url); long number = parselong(data); return number; } How to avoid the accumulation of exception declarations up in the call hierarchy? 4

5 Quiz 2: solutions Solution 1: Wrapping public void readnumberfromurl(string url) throws ApplicationException{ try { String data = readdatafromurl(url); long number = parselong(data); } catch (MalformedUrlException e) { throw new ApplicationException(e); } catch (NumberFormatException e) { throw new ApplicationException(e); } } Solution 2: Inheritance public long readnumberfromurl(string url) throws Exception {... } 5

6 Quiz 3: What will happen? (Java) Suppose myfile.txt does not exist. InputStream input = null; Which exception will the client get? try { input = new FileInputStream("myFile.txt"); //do something with the stream } catch(ioexception e) { throw new WrappedException(e); } finally { try { input is null input.close(); } catch(ioexception e) { throw new WrappedException(e); } NullPointerException is thrown inside the } finally block and propagated to the client! 6

7 Quiz 4: Does it compile? (Java) List<Integer> li = new ArrayList<Integer>(); List<Number> ln = li; Generics are not covariant! List<String>[] lsa = new List<String>[10]; List<?>[] lsa = new List<?>[3] Cannot create arrays of generic types This is fine public class DecimalString implements Comparable<Number>, Comparable<String> {... Comparable<Number> and Comparable<String> are the same interface! 7

8 Quiz 4: Does it compile? (C#) List<int> li = new List<int>(); List<Object> lo = li; Generics are not covariant! List<string>[] lsa = new List<string>[10]; This is fine public class DecimalString : Comparable<int>, Comparable<string> {... This is fine public class Comparable2<U, V> : Comparable<U>, Comparable<V> {... U and V might be the same type 8

9 Quiz 5: ArrayList (Java and C#) Does it work? class ArrayList<V> { private V[] storage; public ArrayList() { storage = new V[DEFAULT_SIZE]; How to make it work in Java? class ArrayList<V> { Java: cannot create an array of V C#: it s ok private V[] storage; public ArrayList() { It s a cast, but it works storage = (V[]) new Object[DEFAULT_SIZE]; } } 9

10 Quiz 6: Does it compile? (C#) public T Convert<T>(Object o) where T: class { if (o.gettype() == typeof (T)) { return (T) o; } else { return null; null might not be a valid value of T 10

11 Quiz 7: What is printed? (Java) List<Integer> l1 = new ArrayList<Integer>(); List<String> l2 = new ArrayList<String>(); Class<?> c1 = l1.getclass(); Class<?> c2 = l2.getclass(); System.out.println(c1.equals(c2)); True Both c1 and c2 are ArrayList 11

12 Quiz 7: What is printed (C#) List<int> l1 = new List<int>(10); List<string> l2 = new List<string>(10); Type t1 = l1.gettype(); Type t2 = l2.gettype(); Console.WriteLine(t1.Equals(t2)); False 12

13 Quiz 8: What happens? (Java) package ch.ethz.reflect; public class AClass { private String s; public AClass(String s) { this.s=s; public static void main(string[] args) { Class<?> c=class.forname("ch.ethz.reflect.aclass"); c.newinstance(); InstantiationException at runtime: no default constructor Compilation error: checked exceptions have to be handled 13

14 Quiz 8: A safe solution (Java) public static void main(string[] args) { try { Class<?> c=class.forname("ch.ethz.reflect.aclass"); Class<?>[] types = new Class[] { String.class }; Constructor<?> cons = c.getconstructor(types); Object[] a = new Object[] { "hello" }; Object newinstance = cons.newinstance(a);... } catch (Exception e) {... 14

15 Quiz 8: What happens? (C#) namespace Reflect { class AClass { private string s; public AClass(string s) { this.s = s; public static void Main(string[] args) { Type t = Type.GetType("Reflect.AClass"); Object o = Activator.CreateInstance(t); MissingMethodException at runtime: no default constructor No checked exceptions in C# 15

16 Quiz 8: A safe solution (C#) namespace Reflect { class AClass { private string s; public AClass(string s) { this.s = s; public static void Main(string[] args) { Type t = Type.GetType("Reflect.AClass"); Object[] a = { "hello" }; Object o = Activator.CreateInstance(t, a); 16

17 Questions? 17