我有一个fileinput类。它在构造函数中有一个字符串参数来加载提供的文件名。但是，如果文件不存在，它就会退出。如果文件不存在，我希望它输出一条消息 - 但不确定如何....
这是班级：
public class FileInput extends Input {
/**
* Construct <code>FileInput</code> object given a file name.
*/
public FileInput(final String fileName) {
try {
scanner = new Scanner(new FileInputStream(fileName));
} catch (FileNotFoundException e) {
System.err.println("File " + fileName + " could not be found.");
System.exit(1);
}
}
/**
* Construct <code>FileInput</code> object given a file name.
*/
public FileInput(final FileInputStream fileStream) {
super(fileStream);
}
}
及其实施：
private void loadFamilyTree() {
out.print("Enter file name: ");
String fileName = in.nextLine();
FileInput input = new FileInput(fileName);
family.load(input);
input.close();
}
import java.io.Closeable;
import java.io.InputStream;
import java.math.BigDecimal;
import java.math.BigInteger;
import java.util.InputMismatchException;
import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.Scanner;
public class Input implements Closeable, Iterator<String>
{
/**
* A reference to the associated <code>Scanner</code> that supplies all the actual input
* functionality.
* <p/>
* <p>This is protected and not final rather than private and final (as we might have
* expected it to be) so that <code>FileInput</code> can access the variable. This is
* necessary because <code>FileInput</code> needs to capture all exceptions that can happen
* during construction, which means that the <code>super</code> constructor call cannot be
* used. This appears to be something of a misfeature of Java.</p>
*/
protected Scanner scanner;
/**
* The default constructor of an <code>Input</code> that assumes <code>System.in</code> is to
* be the <code>InputStream</code> used.
*/
public Input()
{
this(System.in);
}
/**
* Constructor of an <code>Input</code> object given an <code>InputStream</code> object.
*/
public Input(final InputStream in)
{
scanner = new Scanner(in);
}
/**
* A finalizer to ensure all files are closed if an <code>Input</code> object is garbage
* collected.
*/
public void finalize()
{
close();
}
/**
* Close the file when finished with it.
*/
public void close()
{
scanner.close();
}
/**
* @return <code>true</code> if there is more input, <code>false</code> otherwise.
*/
public boolean hasNext()
{
boolean returnValue = false;
try
{
returnValue = scanner.hasNext();
}
catch (IllegalStateException e)
{
illegalStateExceptionHandler();
}
return returnValue;
}
/**
* @return the next token (sequence of characters terminated by a whitespace) in the input
* stream.
*/
public String next()
{
String returnValue = null;
try
{
returnValue = scanner.next();
}
catch (NoSuchElementException nsee)
{
noSuchElementHandler();
}
catch (IllegalStateException ise)
{
illegalStateExceptionHandler();
}
return returnValue;
}
/**
* This operation is required in order to conform to <code>Iterator<String></code> but is
* not supported. Normally an <code>UnsupportedOperationException</code> would be thrown to
* indicate this situation but the whole point is not to throw exceptions so this is simply a
* "do nothing" method.
*/
public void remove()
{
}
/**
* NB This method currently has a mis-feature in that it returns false incorrectly when there
* is a single end-of-line left in the file.
*
* @return <code>true</code> if there is a <code>char</code> to input, <code>false</code>
* otherwise.
*/
public boolean hasNextChar()
{
// Why doesn't this work, it used to.
//boolean returnValue = false ;
//try { returnValue = scanner.hasNext ( "(?s)." ) ; }
//catch ( IllegalStateException e ) { illegalStateExceptionHandler ( ) ; }
//return returnValue ;
return hasNext();
}
/**
* @return the next <code>char</code> in the input stream.
*/
public char nextChar()
{
char returnValue = '\0';
try
{
returnValue = scanner.findWithinHorizon("(?s).", 1).charAt(0);
}
catch (IllegalArgumentException iae)
{
// This cannot happen as it is clear in the statement that the horizon is 1 which is > 0 and
// this exception only happens for negative horizons.
System.exit(1);
}
catch (IllegalStateException ise)
{
illegalStateExceptionHandler();
}
return returnValue;
}
/**
* @return <code>true</code> if there is an <code>int</code> to input, <code>false</code>
* otherwise.
*/
public boolean hasNextInt()
{
boolean returnValue = false;
try
{
returnValue = scanner.hasNextInt();
}
catch (IllegalStateException e)
{
illegalStateExceptionHandler();
}
return returnValue;
}
/**
* @return the next <code>int</code> in the input stream assumed to be in the default radix
* which is 10.
*/
public int nextInt()
{
int returnValue = 0;
try
{
returnValue = scanner.nextInt();
}
catch (InputMismatchException ime)
{
inputMismatchExceptionHandler("int");
}
catch (NoSuchElementException nsee)
{
noSuchElementHandler();
}
catch (IllegalStateException ise)
{
illegalStateExceptionHandler();
}
return returnValue;
}
/**
* @param radix the radix of the input.
* @return the next <code>int</code> in the input stream using the radix
* <code>radix</code>.
*/
public int nextInt(final int radix)
{
int returnValue = 0;
try
{
returnValue = scanner.nextInt(radix);
}
catch (InputMismatchException ime)
{
inputMismatchExceptionHandler("int");
}
catch (NoSuchElementException nsee)
{
noSuchElementHandler();
}
catch (IllegalStateException ise)
{
illegalStateExceptionHandler();
}
return returnValue;
}
/**
* @return <code>true</code> if there is a <code>long</code> to input, <code>false</code>
* otherwise.
*/
public boolean hasNextLong()
{
boolean returnValue = false;
try
{
returnValue = scanner.hasNextLong();
}
catch (IllegalStateException e)
{
illegalStateExceptionHandler();
}
return returnValue;
}
/**
* @return the next <code>long</code> in the input stream assumed to be in the default radix
* which is 10.
*/
public long nextLong()
{
long returnValue = 0;
try
{
returnValue = scanner.nextLong();
}
catch (InputMismatchException ime)
{
inputMismatchExceptionHandler("long");
}
catch (NoSuchElementException nsee)
{
noSuchElementHandler();
}
catch (IllegalStateException ise)
{
illegalStateExceptionHandler();
}
return returnValue;
}
/**
* @param radix the radix of the input sequence.
* @return the next <code>long</code> in the input stream using the radix
* <code>radix</code>.
*/
public long nextLong(final int radix)
{
long returnValue = 0;
try
{
returnValue = scanner.nextLong(radix);
}
catch (InputMismatchException ime)
{
inputMismatchExceptionHandler("long");
}
catch (NoSuchElementException nsee)
{
noSuchElementHandler();
}
catch (IllegalStateException ise)
{
illegalStateExceptionHandler();
}
return returnValue;
}
/**
* @return <code>true</code> if there is a <code>BigInteger</code> to input, <code>false</code>
* otherwise.
*/
public boolean hasNextBigInteger()
{
boolean returnValue = false;
try
{
returnValue = scanner.hasNextBigInteger();
}
catch (IllegalStateException e)
{
illegalStateExceptionHandler();
}
return returnValue;
}
/**
* @return the next <code>BigInteger</code> in the input stream assumed to be in the default
* radix which is 10.
*/
public BigInteger nextBigInteger()
{
BigInteger returnValue = new BigInteger("0");
try
{
returnValue = scanner.nextBigInteger();
}
catch (InputMismatchException ime)
{
inputMismatchExceptionHandler("BigInteger");
}
catch (NoSuchElementException nsee)
{
noSuchElementHandler();
}
catch (IllegalStateException ise)
{
illegalStateExceptionHandler();
}
return returnValue;
}
/**
* @param radix the radix of the input sequence.
* @return the next <code>BigInteger</code> in the input stream using the radix
* <code>radix</code.
*/
public BigInteger nextBigInteger(final int radix)
{
BigInteger returnValue = new BigInteger("0");
try
{
returnValue = scanner.nextBigInteger(radix);
}
catch (InputMismatchException ime)
{
inputMismatchExceptionHandler("BigInteger");
}
catch (NoSuchElementException nsee)
{
noSuchElementHandler();
}
catch (IllegalStateException ise)
{
illegalStateExceptionHandler();
}
return returnValue;
}
/**
* @return <code>true</code> if there is a <code>float</code> to input, <code>false</code>
* otherwise.
*/
public boolean hasNextFloat()
{
boolean returnValue = false;
try
{
returnValue = scanner.hasNextFloat();
}
catch (IllegalStateException e)
{
illegalStateExceptionHandler();
}
return returnValue;
}
/**
* @return the next <code>float</code> in the input stream.
*/
public float nextFloat()
{
float returnValue = 0;
try
{
returnValue = scanner.nextFloat();
}
catch (InputMismatchException ime)
{
inputMismatchExceptionHandler("float");
}
catch (NoSuchElementException nsee)
{
noSuchElementHandler();
}
catch (IllegalStateException ise)
{
illegalStateExceptionHandler();
}
return returnValue;
}
/**
* @return <code>true</code> if there is a <code>double</code> to input, <code>false</code>
* otherwise.
*/
public boolean hasNextDouble()
{
boolean returnValue = false;
try
{
returnValue = scanner.hasNextDouble();
}
catch (IllegalStateException e)
{
illegalStateExceptionHandler();
}
return returnValue;
}
/**
* @return the next <code>double</code> in the input stream.
*/
public double nextDouble()
{
double returnValue = 0;
try
{
returnValue = scanner.nextDouble();
}
catch (InputMismatchException ime)
{
inputMismatchExceptionHandler("double");
}
catch (NoSuchElementException nsee)
{
noSuchElementHandler();
}
catch (IllegalStateException ise)
{
illegalStateExceptionHandler();
}
return returnValue;
}
/**
* @return <code>true</code> if there is a <code>BigDecimal</code> to input,
* <code>false</code> otherwise.
*/
public boolean hasNextBigDecimal()
{
boolean returnValue = false;
try
{
returnValue = scanner.hasNextBigDecimal();
}
catch (IllegalStateException e)
{
illegalStateExceptionHandler();
}
return returnValue;
}
/**
* @return the next <code>BigDecimal</code> in the input stream.
*/
public BigDecimal nextBigDecimal()
{
BigDecimal returnValue = new BigDecimal("0");
try
{
returnValue = scanner.nextBigDecimal();
}
catch (InputMismatchException ime)
{
inputMismatchExceptionHandler("BigDecimal");
}
catch (NoSuchElementException nsee)
{
noSuchElementHandler();
}
catch (IllegalStateException ise)
{
illegalStateExceptionHandler();
}
return returnValue;
}
/**
* @return <code>true</code> if there is more input including an end of line marker,
* <code>false</code> otherwise.
*/
public boolean hasNextLine()
{
boolean returnValue = false;
try
{
returnValue = scanner.hasNextLine();
}
catch (IllegalStateException e)
{
illegalStateExceptionHandler();
}
return returnValue;
}
/**
* @return all the characters in the input stream up to and including the next end of line
* marker in the input stream.
*/
public String nextLine()
{
String returnValue = null;
try
{
returnValue = scanner.nextLine();
}
catch (NoSuchElementException nsee)
{
noSuchElementHandler();
}
catch (IllegalStateException ise)
{
illegalStateExceptionHandler();
}
return returnValue;
}
/**
* The method to handle an <code>IllegalStateException</code>.
*/
private void illegalStateExceptionHandler()
{
System.err.println("Input has been closed.");
System.exit(1);
}
/**
* The method to handle an <code>InputMismatchException</code>.
*/
private void inputMismatchExceptionHandler(final String type)
{
System.err.println("Input did not represent " +
(type.equals("int") ? "an" : "a") + " " + type + " value.");
System.exit(1);
}
/**
* The method to handle an <code>NoSuchElementException</code>.
*/
private void noSuchElementHandler()
{
System.err.println("No input to be read.");
System.exit(1);
}
}
答案 0 :(得分：1)
看起来确实输出了一条消息。
System.err.println("File " + fileName + " could not be found.");
但你真的不应该退出这样的失败。 这种事情使得编写测试常见故障的单元测试成为不可能。
如何使用有用的错误消息抛出FileNotFoundException？
如果您的项目未使用特定的日志记录框架，则通过java.util.logging进行日志记录是一种很好的开始方式。
import java.util.logging.Logger;
...
private static final Logger LOGGER = Logger.getLogger(FileInput.class.getName());
...
LOGGER.severe("Tried to create a FileInput from a non-existant/unreadable file " + fileName);
如果您想对Java日志记录框架进行一些研究，请参阅https://stackoverflow.com/questions/85605/can-anyone-recommend-a-simple-java-logging-framework
答案 1 :(得分：0)
您可以在代码中添加以下内容：
public FileInput(final String fileName) {
try {
java.io.File f = new File(filename);
if(!f.exists()){
System.out.println("file: "+f.getPath()+ " does not exist.");
return;
}
scanner = new Scanner(new FileInputStream(fileName));
} catch (FileNotFoundException e) {
System.err.println("File " + fileName + " could not be found.");
System.exit(1);
}
}