javaIO 除了操作位元組，還可以操作字元，從使用情況來看，字元流的使用比例較高些。
Writer類是一個抽象類，它同時實現了Appendable, Closeable, Flushable介面。用於字元流的寫操作。它支援緩衝流，緩衝流的大小看下面的程式碼。直接的子類是OutputStreamWriter，FileWriter繼承了OutputStreamWriter。

    /**
     * Size of writeBuffer, must be >= 1
     */
    private final int writeBufferSize = 1024;

先看FileWriter類，它繼承了OutputStreamWriter，而OutputStreamWriter繼承了Writer。
FileWriter的構造方法，根據檔案路徑構造，或者根據File構造。它們都支援一個boolean型別的引數表示在原檔案基礎上追加或者覆蓋，當引數值為true的時候表示追加。在方法的內部，實際是呼叫了它父類的構造方法。具體些操作也是繼承父類的方法。看下它的原始碼

package java.io;
/**
 * @see OutputStreamWriter
 * @see FileOutputStream
 *
 * @author      Mark Reinhold
 * @since       JDK1.1
 */

public class FileWriter extends OutputStreamWriter {

    /**
     * Constructs a FileWriter object given a file name.
     *
     * @param fileName  String The system-dependent filename.
     * @throws
 
 IOException  if the named 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
     */
    public FileWriter(String fileName) throws IOException {
        super(new FileOutputStream(fileName));
    }

    /**
     * Constructs a FileWriter object given a file name with a boolean
     * indicating whether or not to append the data written.
     *
     * @param
 
 fileName  String The system-dependent filename.
     * @param append    boolean if <code>true</code>, then data will be written
     *                  to the end of the file rather than the beginning.
     * @throws IOException  if the named 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
     */
    public FileWriter(String fileName, boolean append) throws IOException {
        super(new FileOutputStream(fileName, append));
    }

    /**
     * Constructs a FileWriter object given a File object.
     *
     * @param file  a File object to write to.
     * @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
     */
    public FileWriter(File file) throws IOException {
        super(new FileOutputStream(file));
    }

    /**
     * Constructs a FileWriter object given a File object. If the second
     * argument is <code>true</code>, then bytes will be written to the end
     * of the file rather than the beginning.
     *
     * @param file  a File object to write to
     * @param     append    if <code>true</code>, 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));
    }

    /**
     * Constructs a FileWriter object associated with a file descriptor.
     *
     * @param fd  FileDescriptor object to write to.
     */
    public FileWriter(FileDescriptor fd) {
        super(new FileOutputStream(fd));
    }

}

下面使用FileWriter來實現一個最簡單的例子，在檔案中寫入幾句話。
實現思路：
新建一個檔案（該檔案不存在則新建）；
例項化一個 FileWriter，是在原檔案尾部追加內容還是覆蓋，根據需要選擇建構函式；
執行FileWriter的寫操作；
關閉流。

        File file = new File("e://app/fw.txt");
        if(!file.exists()){
            try {
                file.createNewFile();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        Writer out =null;
        String str ="Test write method in class named FileWriter !";
        String res ="result: success!";
        try {
            //會覆蓋原先的內容
            out = new FileWriter(file); 
            //會在原先的檔案尾部追加新的內容
            //out = new FileWriter(file,true);
            out.write(str);
            out.write("\r\n"); //換行
            out.write(res);
        } catch (IOException e1) {
            e1.printStackTrace();
        } finally{
            if(out!=null){
                try {
                    out.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }