How to Add newline after every 3rd element in arraylist in java?

问题

My program need to add newline after every 3rd element in the arraylist. Here is my input file which contain the following data:

934534519137441534534534366, 0796544345345345348965, 796345345345544894534565, 734534534596544534538965 , 4058991374534534999999, 34534539624, 91953413789453450452, 9137534534482080, 9153453459137482080, 405899137999999, 9653453564564524, 91922734534779797, 0834534534980001528, 82342398534 6356343430001528, 405899137999999, 9191334534643534547423752, 3065345782642564522021, 826422205645345345645621, 40584564563499137999999, 953453345344624, 3063454564345347, 919242353463428434451, 09934634634604641264, 990434634634641264, 40346346345899137999999, 963445636534653452, 919234634643325857953, 91913453453437987385, 59049803463463453455421, 405899137534534999999, 9192273453453453453758434,

and it goes on to multiple lines.

Code:

public class MyFile {

    private static final Pattern ISDN =Pattern.compile("\\s*ISDN=(.*)");

    public List<String> getISDNsFromFile(final String fileName) throws IOException {

        final Path path = Paths.get(fileName);
        final List<String> ret = new ArrayList<>();
        Matcher m;
        String line;
        int index = 0;
        try (
        final BufferedReader reader = Files.newBufferedReader(path, StandardCharsets.UTF_8);) {
            while ((line = reader.readLine()) != null) {
                m = ISDN.matcher(line);
                if (m.matches()) {
                    ret.add(m.group(1));
                    index++;
                    if(index%3==0){
                        if(ret.size() == index){
                            ret.add(m.group(1).concat("\n"));
                        }
                    }
                }
            }
            return ret;
        } 
    }
}

回答1:

I changed your code to read using the "new" Java 7 files I/O and corrected the use of a line separator, along with some formatting. Here I iterate over the list after it was completed. If it is really long you can iterate over it while constructing it.

public class MyFile {

    private static final Pattern ISDN = Pattern.compile("\\s*ISDN=(.*)");
    private static final String LS = System.getProperty("line.separator");

    public List<String> getISDNsFromFile(final String fileName) throws IOException {

        List<String> ret = new ArrayList<>();
        Matcher m;

        List<String> lines = Files.readAllLines(Paths.get(fileName), StandardCharsets.UTF_8);
        for (String line : lines) {
            m = ISDN.matcher(line);
            if (m.matches())
                ret.add(m.group(1));
        }
        for (int i = 3; i < ret.size(); i+=4)
            ret.add(i, LS);
        return ret;
    }
}

回答2:

I think you do not need to compare the size of arraylist with index. Just remove that condition and try with this

if(index%3==0){                         
   ret.add(System.getProperty("line.separator"));
   }

Although I support the comment made by @Peter I would add them all to a List and only add the formatting when you print. Adding formatting in your data usually leads to confusion.

来源：https://stackoverflow.com/questions/22746198/how-to-add-newline-after-every-3rd-element-in-arraylist-in-java

标签

java

arraylist