Question

I’ve noticed that Java String will reuse char array inside it to avoid creating new char array for a new String instance in method such as subString(). There are several unpublish constructors in String for this purpose, accepting a char array and two int as range to construct a String instance.

But until today I found that split will also reuse the char arr of original String instance. Now I read a loooooong line from a file, split it with “,” and cut a very limit column for real usage. Because every part of the line secretly holding the reference of the looooong char array, I got an OOO very soon.

here is example code:

ArrayList<String> test = new ArrayList<String>(3000000);
BufferedReader origReader = new BufferedReader(new FileReader(new File(
        "G:\\filewithlongline.txt")));
String line = origReader.readLine();
int i = 0;
while ((line = origReader.readLine()) != null) {
    String name = line.split(',')[0];
    test.add(name);
    i++;
    if (i % 100000 == 0) {
        System.out.println(name);
    }
}
System.out.println(test.size());

Is there any standard method in JDK to make sure that every String instance that spitted is a “real deep copy” not “shallow copy”?

Now I am using a very ugly workaround to force creating a new String instance:

ArrayList<String> test = new ArrayList<String>(3000000);
BufferedReader origReader = new BufferedReader(new FileReader(new File(
        "G:\\filewithlongline.txt")));
String line = origReader.readLine();
int i = 0;
while ((line = origReader.readLine()) != null) {
    String name = line.split(',')[0]+"  ".trim(); // force creating a String instance
    test.add(name);
    i++;
    if (i % 100000 == 0) {
        System.out.println(name);
    }
}
System.out.println(test.size());

Answer 1

The simplest approach is to create a new String directly. This is one of the rare cases where its a good idea.

String name = new String(line.split(",")[0]); // note the use of ","

An alternative is to parse the file yourself.

do {
    StringBuilder name = new StringBuilder();
    int ch;
    while((ch = origReader.read()) >= 0 && ch != ',' && ch >= ' ') {
       name.append((char) ch);
    }
    test.add(name.toString());
} while(origReader.readLine() != null);