chapter 14 exception handling. chapter goals to learn how to throw exceptions to be able to design...
TRANSCRIPT
Chapter 14
Exception Handling
Chapter Goals• To learn how to throw exceptions • To be able to design your own exception classes • To understand the difference between checked and unchecked exceptions • To learn how to catch exceptions • To know when and where to catch an exception
Error Codes • Traditional approach to error handling: method returns error code• Example: JOptionPane.showInputDialog returns null if user hits Cancel• Problem: Calling method may forget to check for error code • Problem: Calling method may not know how to fix error--then it
needs to return an error code • Symptom: Programming for success
x.doSomething()
is replaced by programming for failureif (!x.doSomething()) return false;
Exceptions• Can't be overlooked
• Can be handled by a competent handler, not necessarily the calling method
• Throw an exception object to indicate failureif (failure){ XxxException e = new XxxException(. . .); throw e;}
• More conciselythrow new XxxException(. . .);
Exceptions
public class BankAccount { public void withdraw(double amount) { if (amount > balance) throw new IllegalArgumentException(
"Amount exceeds balance"); balance = balance - amount; } ... }
Hierarchy of Exception Classes
Syntax 14.1: Throwing an Exception
throw exceptionObject;
Example: throw new IllegalArgumentException();
Purpose:To throw an exception and transfer control to a handler for this exception type
Checked Exceptions• Compiler checks that you are aware of the exception • Generally used for errors that can happen even in correct programs • IOException and its sublcasses are checked exceptions • NullPointerException, ArrayIndexOutOfBoundsException , ... are unchecked --they are your fault :-) •Virtual machine errors (e.g. OutOfMemoryError) are unchecked• Classification not perfect. For example, Integer.parseInt throws unchecked NumberFormatException • Checked exceptions are subclasses of Exception that are not subclasses of RuntimeException
Checked and Unchecked Exceptions
Exception Specifications
• BufferedReader.readLine may throw IOException
• Tag calling method with throws IOException
public class Coin {
public void read(BufferedReader in) throws IOException
{ value = Double.parseDouble(in.readLine());
name =in.readLine(); }
...}
Exception Specifications • Need to tag caller of Coin.read as well
• Stop at main or with handler (see below)
• Can have multiple exception typespublic void read()
throws IOException, ClassNotFoundException
• throws specifier not a sign of irresponsible programming
• Better to declare exception than to handle it incompetently
Syntax 14.2: Exception Specification
accessSpecifier returnType methodName
(parameterType parameterName, . . .)throws ExceptionClass, ExceptionClass . .
Example: public void read(BufferedReader in) throws
IOException
Purpose: To indicate the checked exceptions that a method can
throw
Designing Your Own Exception Types
• if (amount > balance) throw new InsufficientFundsException(. . .);
• Make it an unchecked exception--programmer could have avoided it by calling getBalance first
• Extend RuntimeException
• Supply two constructors
Designing Your Own Exception Types
public class InsufficientFundsException extends RuntimeException { public InsufficientFundsException() { } public InsufficientFundsException(String reason) {
super(reason); }}
Catching Exceptions try { BufferedReader in = new BufferedReader( new InputStreamReader(System.in)); System.out.println("How old are you?"); String inputLine = in.readLine(); int age = Integer.parseInt(inputLine); age++; System.out.println("Next year,you'll be " + age); } catch (IOException exception) { System.out.println("Input/output error “
+exception);} catch (NumberFormatException exception) { System.out.println("Input was not a number"); }
Catching Exceptions • Statements in try block are executed
• If no exceptions occur, catch clauses are skipped
• If exception of matching type occurs, execution jumps to catch clause
• If exception of another type occurs, it is thrown to the calling method
• If main doesn't catch an exception, the program terminates with a stack trace
Syntax 14.3: General Try Block try { statement
statement ... } catch (ExceptionClass exceptionObject){
statement statement
...} catch (ExceptionClass exceptionObject){
statement statement ...}...
Example: try {
System.out.println("What is your name?"); String name = console.readLine(); System.out.println("Hello,"+name +"!");
} catch (IOException exception) {
exception.printStackTrace(); System.exit(1);
}
Purpose: To execute one or more statements that may generate exceptions. If
an exception of a particular type occurs, then stop executing those statements and instead go to the matching catch clause. If no exception occurs, then skip the catch clauses.
The finally Clause• Exception terminates current method
• Danger: Can skip over essential code
• Example: BufferedReader in; in = new BufferedReader(new FileReader(filename)); purse.read(in); in.close();
• Must execute in.close() even if exception happens
• Use finally clause for code that must be executed "no matter what"
The finally Clause
BufferedReader in = null; try {
in = new BufferedReader(new FileReader(filename));
purse.read(in); } finally {
if (in !=null) in.close(); }
The finally Clause
• Executed when try block comes to normal end
• Executed if a statement in try block throws an exception, before exception is thrown out of try block
• Can also be combined with catch clauses
Syntax 14.4: The finally Clause try{ statement statement ... } finally { statement statement ... }
Example: BufferedReader in = null; try {
in = new BufferedReader(new FileReader(filename));
purse.read(in); }Finally{
if (in !=null) in.close();}
Purpose: To execute one or more statements that may generate
exceptions, and to execute the statements in the finally clause whether or not an exception occured.
A Complete Example • Program
o reads coin descriptions from file o adds coins to purse o prints total
• What can go wrong? o File might not exist o File might have data in wrong format
• Who can detect the faults? o main method of PurseTest interacts with user o main method can report errors o Other methods pass exceptions to caller
The read method of the Coin class Distinguishes between expected and unexpected end of file
public boolean read(BufferedReader in) throws IOException
{ String input =in.readLine(); if (input == null) // normal end of file
return false; value = Double.parseDouble(input); // may throw unchecked NumberFormatException name = in.readLine(); if (name == null) // unexpected end of file throw new EOFException("Coin name expected"); return true; }
The read method of the Purse class • Unconcerned with exceptions • Just passes them to caller
public void read(BufferedReader in) throws IOException
{ boolean done = false; while (!done){
Coin c = new Coin(); if (c.read(in)) add(c);
else done =true; }
}
The readFile method of the Purse class
• finally clause closes files if exception happens public void readFile(String filename) throws IOException {
BufferedReader in = null; try
{ in = new BufferedReader(
new FileReader(filename)); read(in);
} finally
{ if (in != null)
in.close(); }
}
User interaction in main• If an exception occurs, user can specify another file name boolean done = false; String filename = JOptionPane.showInputDialog("Enter file name"); while (!done) {
try {
Purse myPurse = new Purse(); myPurse.readFile(filename); System.out.println("total=" +
myPurse.getTotal()); done =true;
}
catch (IOException exception) { System.out.println("Input/output error " +
exception); } catch (NumberFormatException exception) { exception.printStackTrace(); // error in file format } if (!done) { Filename = JOptionPane.showInputDialog("Try
another file:"); if (filename == null) done =true; } }
Scenario 1. PurseTest.main calls Purse.readFile
2. Purse.readFile calls Purse.read
3. Purse.read calls Coin.read
4. Coin.read throws an EOFException
5. Coin.read has no handler for the exception and terminates immediately.
6. Purse.read has no handler for the exception and terminates immediately
7. Purse.readFile has no handler for the exception and terminates immediately after executing the finally clause and closing the file.
8. PurseTest.main has a handler for an IOException , a superclass of EOFException. That handler prints a message to the user. Afterwards, the user is given another chance to enter a file name. Note that the statement printing the purse total has been skipped.
File PurseTest.java 1 import javax.swing.JOptionPane;
2 import java.io.IOException;
3
4 /**
5 This program prompts the user to enter a file name
6 with coin values. A purse object is filled with
7 the coins specified in the file. In case of an exception,
8 the user can choose another file.
9 */
10 public class PurseTest
11 {
12 public static void main(String[] args)
13 {
14 boolean done = false;
15 String filename
16 = JOptionPane.showInputDialog("Enter file name");
17
18 while (!done)
19 {
20 try
21 {
22 Purse myPurse = new Purse();
23 myPurse.readFile(filename);
24 System.out.println("total=" + myPurse.getTotal());
25 done = true;
26 }
27 catch (IOException exception)
28 {
29 System.out.println("Input/output error " + exception);
30 }
31 catch (NumberFormatException exception)
32 {
33 exception.printStackTrace();
34 }
35
36 if (!done)
37 {
38 filename = JOptionPane.showInputDialog(
39 "Try another file:");
40 if (filename == null) done = true;
41 }
42 }
43 System.exit(0);
44 }
45 }
File Purse.java 1 import java.io.BufferedReader;
2 import java.io.FileReader;
3 import java.io.IOException;
4
5 /**
6 A purse computes the total of a collection of coins.
7 */
8 public class Purse
9 {
10 /**
11 Constructs an empty purse.
12 */
13 public Purse()
14 {
15 total = 0;
16 }
17
18 /**
19 Read a file with coin descriptions and adds the coins
20 to the purse.
21 @param filename the name of the file
22 */
23 public void readFile(String filename)
24 throws IOException
25 {
26 BufferedReader in = null;
27 try
28 {
29 in = new BufferedReader(new FileReader(filename));
30 read(in);
31 }
32 finally
33 {
34 if (in != null) in.close();
35 }
36 }
37
38 /**
39 Read a file with coin descriptions and adds the coins
40 to the purse.
41 @param in the buffered reader for reading the input
42 */
43 public void read(BufferedReader in)
44 throws IOException
45 {
46 boolean done = false;
47 while (!done)
48 {
49 Coin c = new Coin();
50 if (c.read(in))
51 add(c);
52 else
53 done = true;
54 }
55 }
56
57 /**
58 Add a coin to the purse.
59 @param aCoin the coin to add
60 */
61 public void add(Coin aCoin)
62 {
63 total = total + aCoin.getValue();
64 }
65
66 /**
67 Get the total value of the coins in the purse.
68 @return the sum of all coin values
69 */
70 public double getTotal()
71 {
72 return total;
73 }
74
75 private double total;
76 }
77
File Coin.java 1 import java.io.BufferedReader;
2 import java.io.EOFException;
3 import java.io.IOException;
4
5 /**
6 A coin with a monetary value.
7 */
8 public class Coin
9 {
10 /**
11 Constructs a default coin.
12 Use the read method to fill in the value and name.
13 */
14 public Coin()
15 {
16 value = 0;
17 name = "";
18 }
19
20 /**
21 Constructs a coin.
22 @param aValue the monetary value of the coin.
23 @param aName the name of the coin
24 */
25 public Coin(double aValue, String aName)
26 {
27 value = aValue;
28 name = aName;
29 }
30
31 /**
32 Reads a coin value and name.
33 @param in the reader
34 @return true if the data was read,
35 false if the end of the stream was reached
36 */
37 public boolean read(BufferedReader in)
38 throws IOException
39 {
40 String input = in.readLine();
41 if (input == null) return false;
42 value = Double.parseDouble(input);
43 name = in.readLine();
44 if (name == null)
45 throw new EOFException("Coin name expected");
46 return true;
47 }
48
49 /**
50 Gets the coin value.
51 @return the value
52 */
53 public double getValue()
54 {
55 return value;
56 }
57
58 /**
59 Gets the coin name.
60 @return the name
61 */
62 public String getName()
63 {
64 return name;
65 }
66
67 private double value;
68 private String name;
69 }
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