我建议使用Java 7中添加的NIO.2。
(Note下面的代码也使用了try-with-resources。

import java.io.BufferedWriter;
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.nio.file.StandardOpenOption;

public class Proj2 {

    public static void main(String[] args) {
        Path path = Paths.get("test.txt");
        try (BufferedWriter bw = Files.newBufferedWriter(path,
                                                         StandardOpenOption.CREATE,
                                                         StandardOpenOption.WRITE)) {
            bw.write("123456789");
        }
        catch (IOException xIo) {
            xIo.printStackTrace();
        }
        try (BufferedWriter bw = Files.newBufferedWriter(path,
                                                         StandardOpenOption.CREATE,
                                                         StandardOpenOption.WRITE)) {
            bw.write("hello");
        }
        catch (IOException xIo) {
            xIo.printStackTrace();
        }
    }
}

字符串
运行上述代码后，test.txt 文件的内容：

hello6789

型
另请参考 javadoc for [enum] StandardOpenOption。