一、错误示例1 public void fileCopy()

{

FileReader fr = null;

FileWriter fw = null;

try

{

fr = new FileReader("c:\\xy1.txt"); // ①

fw = new FileWriter("c:\\xy2.txt"); // ②

char[] charBuffer = new char[1024];

int len = 0;

while ((len = fr.read(charBuffer)) != -1)

{

fw.write(charBuffer, 0, len);

}

System.out.println("文件复制成功");

}

catch (IOException e)

{

throw new RuntimeException("文件拷贝操作失败");

}

finally

{

try

{

fr.close(); // ③

fw.close(); // ④

}

catch (IOException e)

{

throw new RuntimeException("关闭失败");

}

}

}

若①中代码出错，fr根本就没有初始化，执行③的时候就会报空指针异常。②④同样是这个道理。

二、错误示例2 public void fileCopy()

{

FileReader fr = null;

FileWriter fw = null;

try

{

fr = new FileReader("c:\\xy1.txt"); // ①

fw = new FileWriter("c:\\xy2.txt"); // ②

char[] charBuffer = new char[1024];

int len = 0;

while ((len = fr.read(charBuffer)) != -1)

{

fw.write(charBuffer, 0, len);

}

System.out.println("文件复制成功");

}

catch (IOException e)

{

throw new RuntimeException("文件拷贝操作失败");

}

finally

{

try

{

if (null != fr)

{

fr.close(); // ③

}

if (null != fw)

{

fw.close(); // ④

}

}

catch (IOException e)

{

throw new RuntimeException("关闭失败"); // ⑤

}

}

}

加上是否为空的判断可以避免空指针异常。但是如果③执行出错，程序会直接进入⑤而④根本没有得到执行，导致无法关闭。

三、正确示例 public void fileCopy()

{

FileReader fr = null;

FileWriter fw = null;

try

{

fr = new FileReader("c:\\xy1.txt");

fw = new FileWriter("c:\\xy2.txt");

char[] charBuffer = new char[1024];

int len = 0;

while ((len = fr.read(charBuffer)) != -1)

{

fw.write(charBuffer, 0, len);

}

System.out.println("文件复制成功");

}

catch (IOException e)

{

throw new RuntimeException("文件拷贝操作失败");

}

finally

{

try

{

if (null != fr)

{

fr.close();

}

}

catch (IOException e)

{

throw new RuntimeException("关闭失败");

}

try

{

if (null != fw)

{

fw.close();

}

}

catch (IOException e)

{

throw new RuntimeException("关闭失败");

}

}

}