//自定义文件过滤器
import java.io.File;

import javax.swing.filechooser.*;

public class JavaChooser extends FileFilter{

    @Override

     public String getDescription()

     {

         return ".java";

     }

    @Override

    public boolean accept(File f) {

        // TODO Auto-generated method stub

        return f.isDirectory() || f.getName().endsWith(".java");

    }

}

//设置文件名
String filename = selectedFile.getName();

int dot = filename.indexOf(".");

FileWriter fw = new FileWriter(new File(filename.substring(0,dot)+".txt"));