java基础：IO流之FileReader和FileWriter

FileReader使用

read()

项目根目录下有一个hello.txt

FileReader的read方法：

    /**
     * Reads a single character.
     *
       读取字符，当流结束的时候，返回-1
     * @return The character read, or -1 if the end of the stream has been
     *         reached
     *
     * @exception  IOException  If an I/O error occurs
     */
    public int read() throws IOException {
        return sd.read();
    }

由此我们可以写出：

    @Test
    public void test1() throws IOException {
        //实例化File类的对象，指明要操作的文件
        File file = new File("hello.txt");
        //提供具体的流
        FileReader fileReader = new FileReader(file);
        int data;
        while((data = fileReader.read()) != -1){
            System.out.print((char)data);
        }
        //关闭流
        fileReader.close();
    }

优化上述代码：

    @Test
    public void test1(){
        //实例化File类的对象，指明要操作的文件
        File file = new File("hello.txt");
        //提供具体的流
        //读入的文件一定要存在，否则报异常
        try(FileReader fileReader = new FileReader(file)){
            int data;
            while((data = fileReader.read()) != -1){
                System.out.print((char)data);
            }
        }catch (IOException e){
            e.printStackTrace();
        }
    }

read(char cbuf[])

将读取的字符放入到传入的数组中。返回值为int，返回每次读入到cbuf数组中的字符个数，到达文件末尾，返回-1。

当文件很大时，效率要比无参的read方法要高。

注意到read(char cbuf[])内部还调用了read(char cbuf[], int off, int len)方法，该方法实际工作用到可能性很小，这里不介绍。

    /**
     * Reads characters into an array.  This method will block until some input
     * is available, an I/O error occurs, or the end of the stream is reached.
     *
     * @param       cbuf  Destination buffer
     *
     * @return      The number of characters read, or -1
     *              if the end of the stream
     *              has been reached
     *
     * @exception   IOException  If an I/O error occurs
     */
    public int read(char cbuf[]) throws IOException {
        return read(cbuf, 0, cbuf.length);
    }

测试：

//实例化File类的对象，指明要操作的文件
File file = new File("hello.txt");
//提供具体的流,读入操作
try(FileReader fileReader = new FileReader(file)){
    char[] cBuf = new char[5];
    int len;
    while ((len = fileReader.read(cBuf))!=-1){
        for(int i =0 ;i<len;i++){
            System.out.print(cBuf[i]);
        }
    }
}catch (IOException e){
    e.printStackTrace();
}

改进写法：

        //实例化File类的对象，指明要操作的文件
        File file = new File("hello.txt");
        //提供具体的流,读入操作
        try(FileReader fileReader = new FileReader(file)){
            char[] cBuf = new char[5];
            int len;
            while ((len = fileReader.read(cBuf))!=-1){
                String str = new String(cBuf, 0, len);
                System.out.print(str);
            }
        }catch (IOException e){
            e.printStackTrace();
        }

FileWriter使用

File file = new File("hello1.txt");
//提供FileWriter的对象，用于写出操作
try(FileWriter fw = new FileWriter(file)){
    //写出操作
    fw.write("hello FileWriter!
");
    fw.write("hello world!");
}catch (IOException e){
    e.printStackTrace();
}

如果使用FileWriter(File file)构造器，如果文件不存在的话，会自动创建文件；如果文件存在的话，会覆盖原来文件，写出新的内容。

此外还有一个构造器传入boolean参数的：如果传入true，则追加到文件末尾不覆盖

    /**
     * Constructs a {@code FileWriter} given the {@code File} to write and
     * a boolean indicating whether to append the data written, using the platform's
     * {@linkplain java.nio.charset.Charset#defaultCharset() default charset}.
     *
     * @param file  the {@code File} to write
     * @param     append    if {@code true}, then bytes will be written
     *                      to the end of the file rather than the beginning
     * @throws IOException  if the file exists but is a directory rather than
     *                  a regular file, does not exist but cannot be created,
     *                  or cannot be opened for any other reason
     * @since 1.4
     */
    public FileWriter(File file, boolean append) throws IOException {
        super(new FileOutputStream(file, append));
    }

测试：

File file = new File("hello1.txt");
//提供FileWriter的对象，用于写出操作
try(FileWriter fw = new FileWriter(file, true)){
    //写出操作
    fw.write("hello FileWriter!
");
    fw.write("hello world!");
}catch (IOException e){
    e.printStackTrace();
}

实现文件复制

使用FileReader和FileWriter可以实现，代码也很简单

File src = new File("hello1.txt");
File dest = new File("hello2.txt");

try(FileReader fr = new FileReader(src);
    FileWriter fw = new FileWriter(dest)){

    char[] cBuf = new char[5];
    int len;
    while ((len = fr.read(cBuf))!=-1){
        fw.write(cBuf, 0 ,len);
    }
}catch (IOException e){
    e.printStackTrace();
}

字符流不能处理图片

测试代码：

@Test
public void test5(){
    File src = new File("a.jpg");
    File dest = new File("b.jpg");

    try(FileReader fr = new FileReader(src);
        FileWriter fw = new FileWriter(dest)){

        char[] cBuf = new char[5];
        int len;
        while ((len = fr.read(cBuf))!=-1){
            fw.write(cBuf, 0 ,len);
        }
    }catch (IOException e){
        e.printStackTrace();
    }
}

发现生成的b.jpg打不开：

所以字符流不能处理图片等字节数据。