java: how to use bufferedreader to read specific line

Lets say I have a text file called: data.txt (contains 2000 lines)

How do I read given specific line from: 500-1500 and then 1500-2000 and display the output of specific line?

this code will read whole files (2000 line)

public static String getContents(File aFile) {

        StringBuffer contents = new StringBuffer();

        try {

        BufferedReader input = new BufferedReader(new FileReader(aFile));
        try {
            String line = null; 

            while (( line = input.readLine()) != null){
            contents.append(line);
            contents.append(System.getProperty("line.separator"));
            }
        }
        finally {
            input.close();
        }
        }
            catch (IOException ex){
            ex.printStackTrace();
        }

        return contents.toString();
}

How do I modify above code to read specific line?

I suggest java.io.LineNumberReader. It extends BufferedReader and you can use its LineNumberReader.getLineNumber(); to get the current line number

You can also use Java 7 java.nio.file.Files.readAllLines which returns a List<String> if it suits you better

Note:

1) favour StringBuilder over StringBuffer, StringBuffer is just a legacy class

2) contents.append(System.getProperty("line.separator")) does not look nice use contents.append(File.separator) instead

3) Catching exception seems irrelevant, I would also suggest to change your code as

public static String getContents(File aFile) throws IOException {
    BufferedReader rdr = new BufferedReader(new FileReader("aFile"));
    try {
        StringBuilder sb = new StringBuilder();
        // read your lines
        return sb.toString();
    } finally {
        rdr.close();
    }
}

now code looks cleaner in my view. And if you are in Java 7 use try-with-resources

    try (BufferedReader rdr = new BufferedReader(new FileReader("aFile"))) {
        StringBuilder sb = new StringBuilder();
        // read your lines
        return sb.toString();
    }

so finally your code could look like

public static String[] getContents(File aFile) throws IOException {
    try (LineNumberReader rdr = new LineNumberReader(new FileReader(aFile))) {
        StringBuilder sb1 = new StringBuilder();
        StringBuilder sb2 = new StringBuilder();
        for (String line = null; (line = rdr.readLine()) != null;) {
            if (rdr.getLineNumber() >= 1500) {
                sb2.append(line).append(File.pathSeparatorChar);
            } else if (rdr.getLineNumber() > 500) {
                sb1.append(line).append(File.pathSeparatorChar);
            }
        }
        return new String[] { sb1.toString(), sb2.toString() };
    }
}

Note that it returns 2 strings 500-1499 and 1500-2000