在Java中将一个文本文件的内容复制到另一个文本文件 [英] Copying the Contents of One text file to Another in Java

问题描述

我正在尝试将包含2-3个整数（例如：1 2 3）的一个文本文件（1.txt）的内容复制到另一个文本文件（2.txt）但我得到了编译时出现以下错误

I am trying to copy the contents of one text file ("1.txt") which contains 2-3 integer numbers (ex: 1 2 3) to another text file ("2.txt") but I am getting the following error upon compilation

import java.io.*;
class FileDemo {
    public static void main(String args[]) {
      try {
          FileReader fr=new FileReader("1.txt");
          FileWriter fw=new FileWriter("2.txt");
          int c=fr.read();
          while(c!=-1) {
            fw.write(c);
          }
      } catch(IOException e) {
          System.out.println(e);
      } finally() { 
          fr.close();
          fw.close();
      }
    }
}

命令提示符： -

C:\Documents and Settings\Salman\Desktop>javac FileDemo.java
FileDemo.java:20: error: '{' expected
                finally()
                       ^
FileDemo.java:20: error: illegal start of expression
                finally()
                        ^
FileDemo.java:20: error: ';' expected
                finally()
                         ^
FileDemo.java:27: error: reached end of file while parsing
}
 ^
4 errors

但是在检查代码后，我发现finally（）块已正确关闭。

But upon checking the code, I find that the finally() block is properly closed.

推荐答案

它是最后，而不是 finally（）：

try {
    //...
} catch(IOException e) {
    //...
} finally {
    //...
}

顺便说一下，你有一个无限循环：

By the way, you have an endless loop there:

int c=fr.read();
while(c!=-1) {
    fw.write(c);
}

您必须读取循环内的数据才能完成：

You must read the data inside the loop in order to let it finish:

int c=fr.read();
while(c!=-1) {
    fw.write(c);
    c = fr.read();
}

在 finally 阻止，您的 fr 和 fw 变量无法找到，因为它们是在<$范围内声明的c $ c>尝试阻止。在外面声明它们：

In the finally block, your fr and fw variables can't be found since they're declared in the scope of the try block. Declare them outside:

FileReader fr = null;
FileWriter fw = null;
try {
    //...

现在，因为它们已初始化使用 null 值，您还必须在关闭它们之前执行 null 检查：

Now, since they are initialized with null value, you must also do a null check before closing them:

finally {
    if (fr != null) {
        fr.close();
    }
    if (fw != null) {
        fw.close();
    }
}

关闭方法都可以抛出必须处理的 IOException ：

And the close method on both can throw IOException that must be handled as well:

finally {
    if (fr != null) {
        try {
            fr.close();
        } catch(IOException e) {
            //...
        }
    }
    if (fw != null) {
        try {
            fw.close();
        } catch(IOException e) {
            //...
        }
    }
}

最后，由于您不希望有大量代码来关闭基本流，只需将其移动到处理 可关闭 （请注意， FileReader 和 FileWriter 都实现了此接口）：

In the end, since you don't want to have a lot of code to close a basic stream, just move it into a method that handles a Closeable (note that both FileReader and FileWriter implements this interface):

public static void close(Closeable stream) {
    try {
        if (stream != null) {
            stream.close();
        }
    } catch(IOException e) {
        //...
    }
}

最后，您的代码应如下所示：

In the end, your code should look like:

import java.io.*;
class FileDemo {
    public static void main(String args[]) {
        FileReader fr = null;
        FileWriter fw = null;
        try {
            fr = new FileReader("1.txt");
            fw = new FileWriter("2.txt");
            int c = fr.read();
            while(c!=-1) {
                fw.write(c);
                c = fr.read();
            }
        } catch(IOException e) {
            e.printStackTrace();
        } finally {
            close(fr);
            close(fw);
        }
    }
    public static void close(Closeable stream) {
        try {
            if (stream != null) {
                stream.close();
            }
        } catch(IOException e) {
            //...
        }
    }
}

从Java 7开始，我们有 try-with-resources ，因此可以重写上面的代码喜欢：

Since Java 7, we have try-with-resources, so code above could be rewritten like:

import java.io.*;
class FileDemo {
    public static void main(String args[]) {
        //this will close the resources automatically
        //even if an exception rises
        try (FileReader fr = new FileReader("1.txt");
             FileWriter fw = new FileWriter("2.txt")) {
            int c = fr.read();
            while(c!=-1) {
                fw.write(c);
                c = fr.read();
            }
        } catch(IOException e) {
            e.printStackTrace();
        }
    }
}

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