java after reading text from file, prints will “null” in the beginning

Question

I\'ve readed from the .txt file this:

Answer 1

private static String str;

is null. You have to initialize it.

private static String str = "";

Answer 2

do this private static String str = "";. By default, your str variable is null so while concatenating null is used.

Initialize it to a blank string as I've shown.

Answer 3

When you concatenate a null reference with a string, it is converted to "null" string, and then concatenation is performed.

This is specified in JLS - Section 5.1.11:

If the reference is null, it is converted to the string "null" (four ASCII characters n, u, l, l).

Now, when you declare an instance or static reference field, without initializing to any value, it will be initialized to default value - null, as in your declaration:

private static String str;  // This is `null`.

Then for the first iteration of the loop:

str += sCurrentLine;

is equivalent to:

str = null + sCurrentLine;  // Hence the result.

As for your str.indexOf() query, I don't know where you are testing the index, because it should return the index of null that is 0, provided, you used that method after the loop, as shown in the below snippet:

String str = null;
String str2 = str + "someString";
System.out.println("Index of null in : " + str2 + " : " + str2.indexOf("null"));

This will output to you:

Index of null in : nullsomeString : 0

---

Also, while performing string concatenation inside such a loop, which you can't control, when it ends, and even in general, you should use StringBuilder instance, to avoid creating intermediate string instances:

StringBuilder stringBuilder = new StringBuilder();
stringBuilder.append(sCurrentLine);

Answer 4

Consider the very first iteration of your loop. The value of str is null, and the value of sCurrentLine is "<?xml version="1.0"?>".

In string concatenation, a null reference is converted into a string of "null". For example:

String x = null;
String y = "a";
String z = x + y; // "nulla"

So on your first iteration, when you execute:

str += sCurrentLine;

the value of str will be "null<?xml version="1.0"?>"

You could just initialize str to ""... but I wouldn't.

There's no reason why it should be a static variable anyway - would you really want to keep the old value hanging around even if the method is called again? That would be very odd. It should be a local variable.

Also, you shouldn't use string concatenation in a loop like this - it's horrible in terms of performance, because it needs to keep copying the old data. Use a StringBuilder instead.

I wouldn't catch exceptions like that either - do you really want the caller not to know that anything went wrong? Just declare that your method can throw IOException.

Additionally, I wouldn't use FileReader myself - it gives you no control over the encoding used to read the file - it will always use the platform default encoding, which may be inappropriate. I'd use a FileInputStream wrapped in an InputStreamReader. You can then use a try-with-resources statement in Java 7 to close the FileInputStream automatically.

So, with all those changes, your code would become something like this:

public static String read(String file) throws IOException {
    StringBuilder builder = new StringBuilder();
    try (InputStream input = new FileInputStream(file)) {
        BufferedReader reader = new BufferedReader(
            new InputStreamReader(input, "UTF-8"));
        String line;
        while ((line = reader.readLine()) != null) {
            builder.append(line);
        }
    }
    return builder.toString();
}

Note that this will convert a multi-line file into a single line of text - use builder.append("\n") or something similar in the loop if you want to preserve the number of lines (but don't care about line separators). If you want to preserve the exact line endings, don't use readLine - just read characters into a buffer instead.

Also, consider using a third party library such as Guava to do all this instead - for example, you could use Files.toString().