在J2ME中从文件中读取内容

Question

I am trying to read the content of a file but it seems not to work. I surfed the net and found different implementations as shown(read(), read2(), readLine()) but each time a run the codes they all give a NullPointer exception. Please what can I do to rectify this problem.

     private String folder;
        static String filename;
        //IMPLEMENTATION 1
        private void readFile(String f) {
            try {
                InputStreamReader reader = new InputStreamReader(getClass().getResourceAsStream(f));
                String line = null;
                while ((line = readLine(reader)) != null) {
                    System.out.println(line);
                }
                reader.close();
            } catch (IOException ex) {
                ex.printStackTrace();
            }
        }
/**
         * Reads a single line using the specified reader.
         * @throws java.io.IOException if an exception occurs when reading the
         * line
         */
        private String readLine(InputStreamReader reader) throws IOException {
            // Test whether the end of file has been reached. If so, return null.
            int readChar = reader.read();
            if (readChar == -1) {
                return null;
            }
            StringBuffer string = new StringBuffer("");
            // Read until end of file or new line
            while (readChar != -1 && readChar != '\n') {
                if (readChar != '\r') {
                    string.append((char) readChar);
                }
                // Read the next character
                readChar = reader.read();
            }
            return string.toString();
        }

        //IMPLEMENTATION 2
        private String read(String file) throws IOException {
            InputStream is = getClass().getResourceAsStream(file);
            StringBuffer sb = new StringBuffer();
            int chars, i = 0;
            while ((chars = is.read()) != -1) {
                sb.append((char) chars);
            }
            return sb.toString();
        }

        //IMPLEMENTATION 3
        private String read2(String file) throws IOException {
            String content = "";
            Reader in = new InputStreamReader(this.getClass().getResourceAsStream(file));
            StringBuffer temp = new StringBuffer(1024);
            char[] buffer = new char[1024];
            int read;
            while ((read = in.read(buffer, 0, buffer.length)) != -1) {
                temp.append(buffer, 0, read);
            }
            content = temp.toString();
                    return content;
        }

        public void execute() throws IOException {
            folder = System.getProperty("fileconn.dir.photos") + "mcast/";
            String path = folder + filename + ".txt";
            FileConnection c = (FileConnection) Connector.open(path, Connector.READ_WRITE);

            try {
                // Checking if the directoy exists or not. If it doesn't exist we create it.
                if (c.exists()) {
            readFile(path);
                    //read(path);
                   // read2(path);
                    System.out.println(read(path));
                } else {
                    System.out.println(filename + ".txt does not exist. Please specify a correct file name");
                }
            } finally {
                c.close();
            }
        }

private String readLine(InputStreamReader reader) throws IOException {
        // Test whether the end of file has been reached. If so, return null.
        int readChar = reader.read();
        if (readChar == -1) {
            return null;
        }
        StringBuffer string = new StringBuffer("");
        // Read until end of file or new line
        while (readChar != -1 && readChar != '\n') {
            // Append the read character to the string. Some operating systems
            // such as Microsoft Windows prepend newline character ('\n') with
            // carriage return ('\r'). This is part of the newline character
            // and therefore an exception that should not be appended to the
            // string.
            if (readChar != '\r') {
                string.append((char) readChar);
            }
            // Read the next character
            readChar = reader.read();
        }
        return string.toString();
    }


    }

Answer 1

ISSUE: Referring to file using incorrect method

getResourceAsStream(...) is for loading resources from the classpath either from your binary package (.jar) or the classpath directory.

So what it essentially means is, to read a file from the binary package use getClass().getResourceAsStream() and to read a file from the device's physical memory use FileConnection API's.

You are trying to create the inputstream from the file schema of the type used in FileConnection hence it will not work. So to solve your problem you have replace the inputstream object initialization in the read(...) , read2(...) and readFile(...) with below code

InputStreamReader reader = new InputStreamReader(in); //here in is the method's input parameter of type InputStream

and in execute(...) pass the file connection inputstream as below

readFile(c.openInputStream()); //here c is object of type FileConnection

.

You might also want to consider these if you are testing your application in emulator / device

1. Make sure the System.getProperty("fileconn.dir.photos") returns NON NULL value
2. The file is stored at appropriate location in the system