当我必须使用文件阅读器Java时，如何从txt文件填充数组？

Question

I have to populate an array with integers that are in a text file, i need the file reader to take an integer from each line and put into an array but it cant put duplicates in to the array aswell which is making it even more complex, and the duplicates, i have to write them to another text file eg:sorted.txt , i cant figure it out how to do all that im only in my first year i college. if someone could help would be gratefully appreciated. Thank you in advance

here is what i got so far in my Method

public static void readFromfile()throws IOException {
    List<String> lines = new ArrayList<String>();
    BufferedReader reader = null;
    try {
     reader = new BufferedReader(new FileReader("file.txt"));
     String line = null;
     while ((line = reader.readLine()) != null) {
        lines.add(line);
    }
} finally {
    reader.close();
}
int[] array = lines.toArray();// i keep getting incopatible type error in this line
awell

for the last 6 days iv bein doing this and thats how far i got :(

Answer 1

 int[] array = lines.toArray();// i keep getting incopatible type error in this line 

Of course you do, List<String>#toArray returns an Object[] , not an int[] . :-)

Ideally, you'd be able to do this by declaring your list as List<int> (or List<long> if the numbers will be really big). Unfortunately, at least in Java 6, you can't do that, you have to use List<Integer> / List<Long> instead. So that's your starting point.

Then parse the numbers from the strings (eg, from line ) as you go. Integer.parseInt (or Long.parseLong ) can do the parsing for you. Their results are int and long respectively, but they'll get auto-boxed when added to the list.

Alternately, you might look at the Scanner class , which is "...A simple text scanner which can parse primitive types and strings using regular expressions..."

To get the final array of int[] (for instance) from the List<Integer> list is a bit of a pain. If you can use an Integer[] instead (and you mostly can, thanks to autoboxing / unboxing), that's easy: Integer[] numbers = yourList.toArray(new Integer[yourList.size()]);

If you really need an int[] instead, you'll have to write a loop to copy it, or use something like the Apache Commons toPrimitive method .

Answer 2

Using the Scanner class will make things easier.

List<Integer> numbers = new ArrayList<Integer>();
Scanner s = new Scanner(new FileInputStream("file.txt"));

while (s.hasNextInt()) {
   numbers.add(s.nextInt());
}

Answer 3

Your problem is that you have a List of Strings and your trying to turn that into an int array .

As TJ Crowder pointed out, BUT you cannot have a List<int> - you must use the wrapper class Integer .

So change your list to List<Integer> , and then it will be lines.add(Integer.parseInt(line));

Answer 4

I recommend to use Scanner class, it is easier than what you are doing. The error is due to assigning object to integer type.