Programming for Mobile Computing

Transcription

1 1/32 Programming for Mobile Computing EECS 1022 moodle.yorku.ca

2 Drop deadline 2/32 July During this period you can still drop the course but you will receive a W on your transcript. The W will not affect your gpa. contains important dates.

3 Final exam (written part) 3/32 When: Friday August 4, 14:00-15:30 Where: Curtis Lecture Hall E if your last name starts with A-K Curtis Lecture Hall F if your last name starts with L-Z Material: everything covered in the course

4 Final exam (written part) 4/32 No questions are allowed during the test. If a question is not clear, then write down any assumptions made. One page of notes (letter size, double sided) may be used during the test. A non-electronic dictionary may be used during the test.

5 Final exam (written part) 5/32 Preparation Study the material. Prepare your page of notes. Think of a test question. Post your question on the forum at Moodle. Answer questions posted by other students on the forum. Discuss questions and answers on the forum.

6 Extra labs 6/32 Tuesday August 1, 17:30-19:30 Thursday August 3, 17:30-19:30

7 Sources of Crashes 7/32 Enter your choice (1 5): a List<Integer> list =... for (int i = 0; i <= list. size (); i++) { output.println( list.get( i )); import com.cheapbutquestionable.integer;... int value = Integer.parseInt(input.nextInt ()); List<String> list =... while (true) { list.add(new String( Hello ));

8 API 8/32 Which exceptions a method may throw are specified in the API. E get(int index) Returns the element at the specified position in this list. Parameters: index index of the element to return Returns: the element at the specified position in this list Throws: IndexOutOfBoundsException if the index is out of range (index < 0 index >= size())

9 How to Handle Exceptions 9/32 Step 1 Place a try block around the statement(s) that may throw the exception. try {... Step 2 Place a catch block right after the try block. catch (... Exception e) {...

10 Exceptions 10/32 Compiling File file = new File( test.txt ); PrintStream fileoutput = new PrintStream(file); gives rise to the error Client. java :13: unreported exception java. io. FileNotFoundException; must be caught or declared to be thrown PrintStream fileoutput = new PrintStream(file); ˆ 1 error Why?

11 Exceptions 11/32 Answer Because the constructor PrintStream(File) throws a FileNotFoundException if the file object does not denote an existing, writable regular file and a new regular file of that name cannot be created, or if some other error occurs while opening or creating the file (see API).

12 Exceptions 11/32 Answer Because the constructor PrintStream(File) throws a FileNotFoundException if the file object does not denote an existing, writable regular file and a new regular file of that name cannot be created, or if some other error occurs while opening or creating the file (see API). How do we fix a must be caught or declared to be thrown error?

13 Exceptions 11/32 Answer Because the constructor PrintStream(File) throws a FileNotFoundException if the file object does not denote an existing, writable regular file and a new regular file of that name cannot be created, or if some other error occurs while opening or creating the file (see API). How do we fix a must be caught or declared to be thrown error? Answer We can catch the exception.

14 How to Catch Exceptions? 12/32 import java. io. FileNotFoundException;... try{ File file = new File( test.txt ); PrintStream fileoutput = new PrintStream(file); catch (FileNotFoundException e){ output.println( Failed to write to file : + e.getmessage())

15 If all goes well 13/32 import java. io. FileNotFoundException;... try{ File file = new File( test.txt ); PrintStream fileoutput = new PrintStream(file); catch (FileNotFoundException e){ output.println( Failed to write to file : + e.getmessage())

16 If all goes well 14/32 import java. io. FileNotFoundException;... try{ File file = new File( test.txt ); PrintStream fileoutput = new PrintStream(file); catch (FileNotFoundException e){ output.println( Failed to write to file : + e.getmessage())

17 If all goes well 15/32 import java. io. FileNotFoundException;... try{ File file = new File( test.txt ); PrintStream fileoutput = new PrintStream(file); catch (FileNotFoundException e){ output.println( Failed to write to file : + e.getmessage())

18 If all goes well 16/32 import java. io. FileNotFoundException;... try{ File file = new File( test.txt ); PrintStream fileoutput = new PrintStream(file); catch (FileNotFoundException e){ output.println( Failed to write to file : + e.getmessage())

19 If all goes well 17/32 import java. io. FileNotFoundException;... try{ File file = new File( test.txt ); PrintStream fileoutput = new PrintStream(file); catch (FileNotFoundException e){ output.println( Failed to write to file : + e.getmessage())

20 If the file does not exist 18/32 import java. io. FileNotFoundException;... try{ File file = new File( test.txt ); PrintStream fileoutput = new PrintStream(file); catch (FileNotFoundException e){ output.println( Failed to write to file : + e.getmessage())

21 If the file does not exist 19/32 import java. io. FileNotFoundException;... try{ File file = new File( test.txt ); PrintStream fileoutput = new PrintStream(file); catch (FileNotFoundException e){ output.println( Failed to write to file : + e.getmessage())

22 If the file does not exist 20/32 import java. io. FileNotFoundException;... try{ File file = new File( test.txt ); PrintStream fileoutput = new PrintStream(file); catch (FileNotFoundException e){ output.println( Failed to write to file : + e.getmessage())

23 If the file does not exist 21/32 import java. io. FileNotFoundException;... try{ File file = new File( test.txt ); PrintStream fileoutput = new PrintStream(file); catch (FileNotFoundException e){ output.println( Failed to write to file : + e.getmessage())

24 If the file does not exist 22/32 import java. io. FileNotFoundException;... try{ File file = new File( test.txt ); PrintStream fileoutput = new PrintStream(file); catch (FileNotFoundException e){ output.println( Failed to write to file: + e.getmessage())

25 If the file does not exist 23/32 import java. io. FileNotFoundException;... try{ File file = new File( test.txt ); PrintStream fileoutput = new PrintStream(file); catch (FileNotFoundException e){ output.println( Failed to write to file : + e.getmessage())

26 If the file name is null 24/32 import java. io. FileNotFoundException;... try{ File file = new File(null); PrintStream fileoutput = new PrintStream(file); catch (FileNotFoundException e){ output.println( Failed to write to file : + e.getmessage())

27 If the file name is null 25/32 import java. io. FileNotFoundException;... try{ File file = new File(null); PrintStream fileoutput = new PrintStream(file); catch (FileNotFoundException e){ output.println( Failed to write to file : + e.getmessage())

28 If the file name is null 26/32 import java. io. FileNotFoundException;... try{ File file = new File(null); PrintStream fileoutput = new PrintStream(file); catch (FileNotFoundException e){ output.println( Failed to write to file : + e.getmessage()) Since a NullPointerException is not a FileNotFoundException, the app crashes.

29 Exceptions 27/32 May the method charat(int) of the class String throw an exception?

30 Exceptions 27/32 May the method charat(int) of the class String throw an exception? Answer Yes.

31 Exceptions 27/32 May the method charat(int) of the class String throw an exception? Answer Yes. Which type of exception?

32 Exceptions 27/32 May the method charat(int) of the class String throw an exception? Answer Yes. Which type of exception? Answer An IndexOutOfBoundsException.

33 Exceptions 28/32 String word =...; output.println(word.charat(2)); Why does the above snippet not give rise to a must be caught or declared to be thrown error?

34 Exceptions 28/32 String word =...; output.println(word.charat(2)); Why does the above snippet not give rise to a must be caught or declared to be thrown error? Answer The must be caught or declared to be thrown rule is only applicable to checked exceptions and an IndexOutOfBoundsException is not checked.

35 What are Checked Exceptions? 29/32 Definition An exception is checked if it is Exception or any of its subclasses, and it is not RuntimeException or any of its subclasses. Is NullPointerException checked?

36 What are Checked Exceptions? 29/32 Definition An exception is checked if it is Exception or any of its subclasses, and it is not RuntimeException or any of its subclasses. Is NullPointerException checked? Answer No.

37 What are Checked Exceptions? 29/32 Definition An exception is checked if it is Exception or any of its subclasses, and it is not RuntimeException or any of its subclasses. Is NullPointerException checked? Answer No. Is InvalidPropertiesFormatException checked?

38 What are Checked Exceptions? 29/32 Definition An exception is checked if it is Exception or any of its subclasses, and it is not RuntimeException or any of its subclasses. Is NullPointerException checked? Answer No. Is InvalidPropertiesFormatException checked? Answer Yes.

39 What are Checked Exceptions? 30/32 Definition An exception is checked if it is Exception or any of its subclasses, and it is not RuntimeException or any of its subclasses. Is Exception checked?

40 What are Checked Exceptions? 30/32 Definition An exception is checked if it is Exception or any of its subclasses, and it is not RuntimeException or any of its subclasses. Is Exception checked? Answer Yes.

41 What are Checked Exceptions? 30/32 Definition An exception is checked if it is Exception or any of its subclasses, and it is not RuntimeException or any of its subclasses. Is Exception checked? Answer Yes. Is RuntimeException checked?

42 What are Checked Exceptions? 30/32 Definition An exception is checked if it is Exception or any of its subclasses, and it is not RuntimeException or any of its subclasses. Is Exception checked? Answer Yes. Is RuntimeException checked? Answer No.

43 Errors 31/32 Why are Errors exempt from the must be caught or declared to be thrown rule?

44 Errors 31/32 Why are Errors exempt from the must be caught or declared to be thrown rule? Answer Errors represent conditions that are so abnormal the reliability of the whole environment is suspect and, hence, the code in the catch block may not run properly either.

45 Errors 31/32 Why are Errors exempt from the must be caught or declared to be thrown rule? Answer Errors represent conditions that are so abnormal the reliability of the whole environment is suspect and, hence, the code in the catch block may not run properly either. Why are RuntimeExceptions exempt from the must be caught or declared to be thrown rule?

46 Errors 31/32 Why are Errors exempt from the must be caught or declared to be thrown rule? Answer Errors represent conditions that are so abnormal the reliability of the whole environment is suspect and, hence, the code in the catch block may not run properly either. Why are RuntimeExceptions exempt from the must be caught or declared to be thrown rule? Answer RuntimeExceptions represent conditions that can be validated by the programmer.

47 Throwing Exceptions 32/32 How can we throw an exception?

48 Throwing Exceptions 32/32 How can we throw an exception? Answer throw new...exception(...);

49 Throwing Exceptions 32/32 How can we throw an exception? Answer throw new...exception(...); Why would a programmer ever throw an exception?

50 Throwing Exceptions 32/32 How can we throw an exception? Answer throw new...exception(...); Why would a programmer ever throw an exception? Answer For example, the programmer may want to separate the error handling code from the rest.