Java Programming Unit 7. Error Handling. Collec7ons

Transcription

1 Java Programming Unit 7 Error Handling. Collec7ons

2 Run7me errors An excep7on is an run- 7me error that may stop the execu7on of your program. For example: - someone deleted a file that your program reads - The client calls the server, which doesn t respond - You try to use a variable that should point at the instance of the object but was never ini7alized You should (and oten must) add excep%on handling in your Java programs.

3 Stack Trace 1 class TestStackTrace{ 2 TestStackTrace() 3 { 4 dividebyzero(); int dividebyzero() 8 { 9 return 25/0; sta7c void main(string[]args) 13 { 14 new TestStackTrace(); Run this program and you ll see a stack trace output in the system console. java TestStackTrace Excep7on in thread "main" java.lang.arithme7cexcep7on: / by zero at TestStackTrace.divideByZero(TestStackTrace.java:9) at TestStackTrace.<init>(TestStackTrace.java:4) at TestStackTrace.main(TestStackTrace.java:14) Read it from the bogom up.

4 Walkthrough 1 Download the source code from the Lesson 13 and import it into Eclipse Run the TestStackTrace program and observe the stack trace in the Console view Examine the code of the TestStackTrace2, which handles this excep7on. Run this program

5 Try- catch blocks If you a piece of code may give a run- 7me error, in many cases Java forces you to put it in the try- catch block. For example, reading a file may generate an IOExcep7on: try{ filecustomer.read(); catch (IOExcep7on e){ System.out.println("There seems to be a problem with the file customers. "); Checked excep7ons are the ones that you can foresee, but can t fix inside the program. For example if the file with the customer info is corrupted, there is nothing your program can do about it.

6 Excep7ons Hierarchy Throwable Error Excep7on LoveFailedExcep7on IOExcep7on Subclasses of Excep7on are called checked excep7ons and have to be handled in your code. Subclasses of the class Error are fatal errors and are called unchecked excep%ons.

7 Catching Mul7ple Excep7ons public void getcustomers(){ try{ filecustomers.read(); catch(filenotfoundexcep7on e){ System.out.println( Can not find file Customers ); catch(eofexcep7on e1){ System.out.println( Done with file read ); catch(ioexcep7on e2){ System.out.println( Problem reading file + e2.getmessage()); catch (Excep7on e3){ System.out.println( Something went wrong );

8 Java 7: Mul7catch Star7ng form Java 7 you can catch mul7ple excep7ons in one catch block: public void getcustomers(){ try{ filecustomers.read(); catch(filenotfoundexcep7on EOFExcep7on IOExcep7on e){ System.out.println( Problem reading file +e.getmessage()); catch (Excep7on e1){ System.out.println( Something else went wrong );

9 The throws Keyword class CustomerList{ void getallcustomers() throws IOExcep:on{ // Some other code goes here // Don t use try/catch if you are not handling excep7ons // in this method file.read(); public sta7c void main(string[] args){ System.out.println( Customer List ); Propaga:on of Excep:ons If you are not planning to handle excep7ons in a method, declare that this method may throw and excep7on. Now the caller of getallcustomers() will have top handle it. // Some other code goes here try{ // Since getallcustomers()declared an excep7on, // either handle it over here, or re- throw it //(see the throw keyword explana7on below) getallcustomers(); catch(ioexcep7on e){ System.out.println( Customer List is not available );

10 Walkthrough 2 1. Visit the help page for the class FileInputStream. hgp://download.oracle.com/javase/6/docs/api/java/io/fileinputstream.html 2. Browse the help info on various methods of this class and pay agen7on to the Throws sec7on: 3. Visit the help page for the class IOExcep7on. Take a look at its direct known subclasses: hgp://download.oracle.com/javase/6/docs/api/java/io/ioexcep7on.html

11 The finally keyword If there is a piece of code that must be executed regardless of the success or failure of the code in the try block, put it under the clause finally try{ file.read(); //file.close(); don t close files inside try block catch(excep7on e){ e.printstacktrace(); finally{ file.close(); If a code in a try- catch block uses some external resources, i.e. database connec7ons or open files, you need to close them, and finally is the solu7on in Java 6 and older.

12 Java 7: try statement with resources Java 7 introduces the try- with- resources that supports automa7c closing of the resources without the need to use the finally clause. In the below code sample the file stream will be automa7cally closed without finally. InputStream myfileinputstream = null; try (myfileinputstream=new FileInputStream( customers.txt );) { // the code that reads data from customers.txt goes here catch (Excep7on e){ e.printstarktrace(); To see an example of try with automa7c resource management, refer to this blog: hgp://java.dzone.com/ar7cles/java- 7- new- try- resources This works only if the resource implements java.lang.autocloseable or java.io.closeable interface, which declares just one method close(). FileInputStream implements it.

13 The throw keyword class CustomerList{ throws is just for the declara7on, but throw actually throws the object. void getallcustomers() throws Excep7on{ // some other code goes here try{ file.read(); // this line may throw an excep7on catch (IOExcep7on e) { // Log this error here, and re- throw another excep7on throw new Excep:on ("Customer List is not available "+ e.getmessage()); public sta7c void main(string[] args){ System.out.println( Customer List ); try{ // Since the getallcustomers() declares an excep7on, // you have to either handle or re- throw it getallcustomers(); catch(excep7on e){ System.out.println(e.getMessage());

14 Declaring your own excep7ons class TooManyBikesExcep7on extends Excep7on{ TooManyBikesExcep7on (String msgtext){ super(msgtext); class BikeOrder{ sta7c void validateorder(string bikemodel, int quan7ty) throws TooManyBikesExcep7on{ // perform some data valida7on, and if the entered // the quan7ty or model is invalid, do the following: class OrderWindow extends JFrame{ void ac7onperformed(ac7onevent e){ // the user clicked on the Validate Order bugon try{ bikeorder.validateorder( Model- 123, 50); // the next line will be skipped in case of excep7on txtresult.settext( Order is valid ); catch(toomanybikesexcep7on e){ txtresult.settext(e.getmessage()); throw new TooManyBikesExcep7on( Can not ship + quan7ty+ bikes of the model + bikemodels);

15 Adding useful info to Excep7ons class TooManyBikesExcep7on extends Excep7on{ // Declare an applica7on- specific property ShippingErrorInfo shippingerrorinfo; TooManyBikesExcep7on (String msgtext,shippingerrorinfo shippingerrorinfo){ super(msgtext); this.shippingerrorinfo = shippingerrorinfo;

16 Java 7: Final Rethrow Java 7 allows using the final keyword with excep7ons. try{ // some code goes here catch (final Throwable e){ // some excep7on handling code goes here throw e; This is rather confusing feature. Just know that it exists, but don t waste you 7me on trying to use it in your code. If you are s7ll interested in seeing an example of its use, refer to this blog: hgp://alexbtw.livejournal.com/2927.html

17 Arrays Revisited Customer[] cust = new Customer[10]; cust[0] = new Customer("David","Lee ); cust[1] = new Customer("Ringo","Starr ); cust[9] = new Customer("Lucky","Man ); // Now let's give a 15 percent discount to all customers // who spent more than $500 in our online store: for (Customer c: cust){ if (c.gettotalcharges() > 500){ c.setdiscount(15);

18 Java Collec7on Framework Classes located in the packages java.u:l and java.u:l.concurrent are oten called Java collec7ons. ArrayList, Vector, HashMap, Hashtable, Iterator, Proper7es, Collec7ons. Collec7ons store objects no primi7ves allowed

19 Popula7ng an ArrayList ArrayList customers = new ArrayList(); Customer cust1 = new Customer("David","Lee"); customers.add(cust1); Customer cust2 = new Customer("Ringo","Starr"); customers.add(cust2); add() doesn t copy instance of the Customer into the customers collec7on, it just adds the memory address of the Customer being added. You can specify ini7al size of ArrayList by using constructor with the argument: ArrayList customers = new ArrayList(10);

20 Ge{ng objects from an ArrayList The method get() is used to extract a par7cular element (Object) from the ArrayList. You have to cast it to the appropriate type Customer thebestcustomer=(customer) customers.get(1); You are allowed to add objects of different types to the collec7on: ArrayList customers = new ArrayList(); customers.add(new Customer("David","Lee")); Order ord = new Order(123, 500, "IBM"); customers.add(ord); But during run7me without cas7ng you can get IllegaLCastExcep7on int totalelem = customers.size(); for (int i=0; i< totalelem;i++){ Customer currentcust = (Customer) customers.get(i); currentcust.dosomething(); Using generics (we ll learn in next lesson) can do a compile 7me check: ArrayList<Customer> customers = new ArrayList<Customer> (10);

21 Walkthrough 3 (start) 1. Download and import the source code for Lesson 14 into Eclipse 2. Add the following code to the end of the method main() in the class Test: Order ord = new Order(); customers.add(ord); int totalelem = customers.size(); for (int i=0; i< totalelem;i++){ Customer currentcust =(Customer) customers.get(i); 3. Run Test and observe the run7me excep7on 4. Put the breakpoint (right- click Toggle breakpoint) on the line for (int i=0; i< totalelem;i++){ 5. Debug the program. Observe the content of the variable customers. con:nued

22 Walkthrough 3 (end) 6. Modify the class Customer to look like this: public class Customer { String firstname; String lastname; public Customer (String a, String b){ firstname=a; lastname=b; 7. Add the following line to the end of method main() in class Test: System.out.println("The current customer is " + currentcust.lastname); Why the program doesn t compile? 8. Move the prin7ng line inside the for- loop and rung the program. 9. Observe the output on the console and explain it: The current customer is Lee The current customer is Starr Excep7on in thread "main" java.lang.classcastexcep7on: Order cannot be cast to Customer at Test.main(Test.java:23)

23 Hashtable and Hashmap are for key- value pairs Customer cust = new Customer("David", "Lee"); Order ord = new Order(123, 500, "IBM"); Por~olio port = new Por~olio(123); Hashtable data = new Hashtable(); data.put("customer", cust); data.put("order", ord); data.put("por~olio", port); Ge{ng the object by key: Order myorder = (Order) data.get( Order ); Hashtable is synchronized, but Hashmap is not. You ll understand the difference ater learning about threads and concurrent access explained in Lessons 20 and 21. Hashtable is not used very oten. It has beger replacements in the java.concurrent package (see Lesson 21 of textbook).

24 Class Proper7es Class Proper7es is a special case allowing to store key- value pairs of String type Proper7es prop=new Proper7es(); FileInputStream in =null; try{ in = new FileInputStream ("mailman.proper7es"); prop.load(in); catch(excep7on e){ finally{ in.close(); File mailman.proper7es: SmtpServer=mail.xyz.com to=abc@xyz.com cc=mary@xyz.com from=yakov@xyz.com String from = prop.getproperty("from"); String mailserver=prop.getproperty("smtpserver");

25 Enumera7on interface Enumera7on allows traversing its elements sequen7ally, without even knowing size of the collec7on. Obtain the enumera7on of all elements and use the methods hasmoreelements() and nextelement(). ArrayList customers = new ArrayList(); Enumera7on enum = Collec7ons.enumera7on(customers); //returns Enumera7on while(enum.hasmoreelements()){ Customer currentcust = (Customer) enum.nextelement()); currentcust.dosomething(); Hashtable data = new Hashtable(); // Get the keys Enumera7on enumkeys = data.keys(); while(enum.hasmoreelements()){ // Get the elements Enumera7on enumelements = data.elements();

26 Iterator Interface Iterator is a newer class than Enumera7on. Iterator icust = customers.iterator(); while (icust.hasnext()) { System.out.println( icust.next() ); Enumera7on can only read the elements of the collec7on but Iterator can also remove them from collec7on.

27 LinkedList LinkedList is a collec7on that s useful when you oten need to insert/remove elements of the collec7on. Each element contains a reference to the next one (aka node). Inser7on of a new object inside the list comes down to a simple update of two references. public class TestLinkedList { public sta7c void main(string[] args) { LinkedList passengerlist = new LinkedList(); passengerlist.add("alex Smith"); passengerlist.add("mary Lou"); passengerlist.add("sim Monk"); // Get the list iterator and print every element of the list ListIterator iterator = passengerlist.listiterator(); System.out.println(iterator.next()); System.out.println(iterator.next()); System.out.println(iterator.next());

28 Homework Complete the assignments from the Try It sec7ons for Lessons 13 and 14 Addi:onal read: How to select the right collec7on: hgp://javabeans.asia/ar7cles/cheat- sheet- for- selec7ng- maplistset- in- java.html Oracle s tutorial on Excep7ons: hgp://download.oracle.com/javase/tutorial/essen7al/excep7ons/index.html Using Java 7 AutoCloseable interface: hgp:// 7- try- with- resources- explained.html