####錯誤示例1
public void CopyFile() {
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("檔案複製成功");
} catch (IOException e) {
throw new RuntimeException("檔案拷貝操作失敗");
} finally {
try {
fr.close(); // ③
fw.close(); // ④
} catch (IOException e) {
throw new RuntimeException("關閉失敗");
}
}
}```
若①中程式碼出錯,fr根本就沒有初始化,執行③的時候就會報空指標異常。②④同樣是這個道理。
####錯誤示例2
複製程式碼
public void CopyFile() { 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("檔案複製成功");
} catch (IOException e) {
throw new RuntimeException("檔案拷貝操作失敗");
} finally {
try {
if (null != fr) {
fr.close(); // ③
}
if (null != fw) {
fw.close(); // ④
}
} catch (IOException e) {
throw new RuntimeException("關閉失敗"); // ⑤
}
}
}```
複製程式碼
加上是否為空的判斷可以避免空指標異常。但是如果③執行出錯,程式會直接進入⑤而④根本沒有得到執行,導致無法關閉。
####正確關流姿勢:
public void CopyFile() {
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("檔案複製成功");
} catch (IOException e) {
throw new RuntimeException("檔案拷貝操作失敗");
} finally {
try {
if (null != fr) {
fr.close();
}
} catch (IOException e) {
throw new RuntimeException("關閉失敗");
}
try {
if (null != fw) {
fw.close();
}
} catch (IOException e) {
throw new RuntimeException("關閉失敗");
}
}
}
複製程式碼