第十四章

例外處理

14.1 丟出例外

錯誤碼

處理錯誤的傳統方法: 傳回錯誤碼, 例如

假如使用者按下 Cancel (取消), 則

    JOptionPane.showInputDialog 

傳回 null

問題: 呼用者可能忘記檢查錯誤碼,
也可能不知道如何改正錯誤--這就需要傳回錯誤碼
徵狀:
正面的程式設計
x.doSomething()改採負面的程式設計
if (!x.doSomething()) return false;

例外

丟出一例外物件以表示失敗
if (failure) { XxxException e = new XxxException(. . .); throw e; }
或用
throw new XxxException(. . .);

例如

public class BankAccount
{
   public void withdraw(double amount)
   {
      if (amount > balance)
         throw new IllegalArgumentException(
               "Amount exceeds balance");
      balance = balance - amount;
   }
   ...
}

例外類別階層

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

14.2 受檢查的例外情形

Compiler 檢查是否注意到例外情形
通常用於甚至在正確的程式中也會發生的錯誤
IOException 及其子類別都是受檢查的例外
NullPointerException, ArrayIndexOutOfBoundsException , . . . 都是未受檢查的例外--這些都是 你的過錯 :-)
虛擬機器的錯誤 (例如 OutOfMemoryError) 也是未受檢查的
這種區分並不完美. 例如, Integer.parseInt 丟出未受檢查的 NumberFormatException
不屬於 RuntimeException 的 Exception 所有子類別都是受檢查的例外

受檢查及未受檢查的例外

註明例外

BufferedReader.readLine 可能丟出 IOException

使用 throws IOException 註明這種情形

例如,

public class Coin
{
   public void read(BufferedReader in) throws IOException
   {
      value = Double.parseDouble(in.readLine());
      name =in.readLine();
   }
   ...
}

可丟出多種例外
public void read() throws IOException, ClassNotFoundException

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

14.3 設計自己的例外型式

有時沒有標準例外型式可以充分描述錯誤情形, 可以使用自行設計的例外型式, 如
if (amount > balance) throw new InsufficientFundsException(. . .);
這種例外應是未受檢查的--在程式中先呼用 getBalance 即可避免
應繼承 RuntimeException 或其子類別

例外類別通常提供兩種 constructors,

public class InsufficientFundsException
   extends RuntimeException
{
   public InsufficientFundsException()
   {
   }
 
   public InsufficientFundsException(String reason)
   {
      super(reason);
   }
}

14.4 接住例外

每一例外都必須處理。另一種例外處理方式是使用 try/catch 敘述。把需要例外處理的程式碼放在 try 方塊內, 而把處理例外的程式碼放在 catch 方塊中。

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");
}

執行 try 方塊內的敘述
假如沒有例外發生, 跳過 catch 子句, 不執行
假如有例外發生, 跳到相對的 catch 子句執行
假如沒有相對的 catch 子句, 則丟給上一層的程式( calling 程式)
假如 main 未接住例外, 中止程式的執行

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.

14.5 `finally` 子句

例外中止正在執行中的方法
危險: 有可能略過重要的程式碼

例如:

BufferedReader in;
in = new BufferedReader(
     new FileReader(filename));
purse.read(in);
in.close();

即使例外發生, 也必須執行 in.close()

這種「無論如何」都必須執行的情形, 就要使用 finally 子句,

BufferedReader in = null;
try
{
   in = new BufferedReader(
        new FileReader(filename));
   purse.read(in);
}
finally 
{
   if (in !=null) in.close();
}

當 try 方塊正常結束時執行。
假如 try 方塊內的敘述丟出例外, 在例外丟出 try 方塊之前執行。
也可以和 catch 子句一起使用

Syntax 14.4: `finally` 子句

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.

14.6 完整的例題

Program

reads coin descriptions from file
adds coins to purse
prints total

What can go wrong?

File might not exist
File might have data in wrong format

Who can detect the faults?

main method of PurseTest interacts with user
main method can report errors
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

PurseTest.main calls Purse.readFile
Purse.readFile calls Purse.read
Purse.read calls Coin.read
Coin.read throws an EOFException
Coin.read has no handler for the exception and terminates immediately.
Purse.read has no handler for the exception and terminates immediately
Purse.readFile has no handler for the exception and terminates immediately after executing the finally clause and closing the file.
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


import javax.swing.JOptionPane;
import java.io.IOException;

/**
   This program prompts the user to enter a file name
   with coin values. A purse object is filled with
   the coins specified in the file. In case of an exception,
   the user can choose another file.
*/
public class PurseTest
{
   public static void main(String[] args)
   {
      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();
         }

         if (!done)
         {
            filename = JOptionPane.showInputDialog(
               "Try another file:");
            if (filename == null) done = true;
         }
      }
      System.exit(0);
   }
}

File Purse.java


import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;

/**
   A purse computes the total of a collection of coins.
*/
public class Purse
{
   /**
      Constructs an empty purse.
   */
   public Purse()
   {
      total = 0;
   }

   /**
      Read a file with coin descriptions and adds the coins
      to the purse.
      @param filename the name of the file
   */
   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();
      }
   }

   /**
      Read a file with coin descriptions and adds the coins
      to the purse.
      @param in the buffered reader for reading the input
   */
   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;
      }
   }

   /**
      Add a coin to the purse.
      @param aCoin the coin to add
   */
   public void add(Coin aCoin)
   {
      total = total + aCoin.getValue();
   }

   /**
      Get the total value of the coins in the purse.
      @return the sum of all coin values
   */
   public double getTotal()
   {
      return total;
   }

   private double total;
}

File Coin.java


import java.io.BufferedReader;
import java.io.EOFException;
import java.io.IOException;

/**
   A coin with a monetary value.
*/
public class Coin
{
   /**
      Constructs a default coin.
      Use the read method to fill in the value and name.
   */
   public Coin()
   {
      value = 0;
      name = "";
   }

   /**
      Constructs a coin.
      @param aValue the monetary value of the coin.
      @param aName the name of the coin
   */
   public Coin(double aValue, String aName)
   {
      value = aValue;
      name = aName;
   }

   /**
      Reads a coin value and name.
      @param in the reader
      @return true if the data was read,
      false if the end of the stream was reached
   */
   public boolean read(BufferedReader in)
      throws IOException
   {
      String input = in.readLine();
      if (input == null) return false;
      value = Double.parseDouble(input);
      name = in.readLine();
      if (name == null)
         throw new EOFException("Coin name expected");
      return true;
   }

   /**
      Gets the coin value.
      @return the value
   */
   public double getValue()
   {
      return value;
   }

   /**
      Gets the coin name.
      @return the name
   */
   public String getName()
   {
      return name;
   }

   private double value;
   private String name;
}

第十四章

例外處理

14.1 丟出例外

錯誤碼

例外

例外類別階層

Syntax 14.1: Throwing an Exception

Example:

Purpose:

14.2 受檢查的例外情形

受檢查及未受檢查的例外

註明例外

Syntax 14.2: Exception Specification

Example:

Purpose:

14.3 設計自己的例外型式

14.4 接住例外

Syntax 14.3: General Try Block

Example:

Purpose:

14.5 finally 子句

Syntax 14.4: finally 子句

Example:

Purpose:

14.6 完整的例題

The read method of the Coin class

The read method of the Purse class

The readFile method of the Purse class

User interaction in main

Scenario

File PurseTest.java

File Purse.java

File Coin.java

14.5 `finally` 子句

Syntax 14.4: `finally` 子句

The `read` method of the `Coin` class

The `read` method of the `Purse` class

The `readFile` method of the `Purse` class

User interaction in `main`