如何从array / ArrayList获取特定元素？

Question

I am trying to get the last element from the end of each line of my ArrayList /array.

For example i have these records int my list: (don't mind the language, Greek names)

Nikos Pappas : 321/2012999, C,8.53  
Marios Athanasiou : 321/2012001, A,6.89  
Stavroula Markou : 321/2011001, D,7.00  
Giannis Kallogirou : 321/2009005, ST, 6.89  
Nikoletta Stefanou : 321/2010001, D, 7.25  
Stathis Petrou : 321/2011002, E, 8.10  
Kostas Tzovalis : 321/2007667, C, 5.00  
Michalis Kotsis : 321/2009723, D, 7.12  
Maria Katsanou : 321/2012002, C, 5.50  

And I want to take: 8.53, 6.89,....,5.50 (marks of students)

This is what I've made so far (the file is the file which contains the above info)

  public static void main(String[] args)  {

    BufferedReader br = null;
    ArrayList <String> list = new ArrayList <> ();

    try
    {
        br = new BufferedReader(new FileReader("C:/Users/Stefi/Desktop/java_ask3.txt"));
        String str;


        //System.out.print("mpika");

        while((str = br.readLine()) != null)
        {
            list.add(str);
            System.out.println(str);
        }
    }
    catch(IOException e)
    {
        System.out.println("Κάτι πήγε στραβά! Προσπάθησε πάλι.");
    }
    finally
    {
        try
        {
            if (br != null)
            {
                br.close();
            }
        }
        catch(IOException e2)
        {
            System.out.println("Δεν μπορεσα να κλείσω το αρχείο.");
        }

    }

int x = list.size();
    String [] array = new String[x];
    array = list.toArray(array);

    for(int i=0; i<array.length; i++)
    {
        System.out.println(array[i]);
    }

    String [] f = new String[list.size()];
    String vathmos;

     for(int i=0; i<array.length; i++)
    {
        vathmos = list.get(i).replaceAll("\\D+", "");
        f[i] = vathmos;
    }


    System.out.println("\n\n\n Float Print\n");
    for(int i=0; i<array.length; i++)
    {
        System.out.println(f[i]);
    }

}

this is an example of my output:

3212012999853     
3212012001689    
3212011001700    
3212009005689

It takes all the numbers it finds. But I don't know how to take only the last float number. I know it's totally wrong to work like this, and work like this because I'll never get the float i want.

I also tried to convert my list into array and take the last element but didn't work out.

Answer 1

你可以试试这个

String s = line.replaceAll(".*(\\d+\\.\\d+)$", "$1");

Answer 2

Split the line by a comma then get the last element of the resulting array

String[] splitArr = line.split(",");
String lastNumber = splitArr[splitArr.length - 1];

But best solution would be to have a Student class as suggested in comments

Answer 3

If you store your records as Strings, you can use the substring method:

grade = Double.parseDouble(array[i].substring(array[i].length() - 3)); if you are sure the grade is always in x.xx form at the end of each String.

Also consider creating a class that would contain all the information about students in seperate fields, for example:

public class Student{
    public String name;
    public String surname;
    public int age;
    public double grade;
}

Of course this is just an example, you may hide implementation and provide methods you want as well. Then you could create any kind of collection of Student objects, so getting the specified value of any field for any student would be easy.

Answer 4

I can suggest couple String functions to add to your code. This is not tested code though -

Using String.split();

//This will hold all the numbers as String
List<String> masterList = new ArrayList<String>();
while((str = br.readLine()) != null)
{
       String [] lineValues = String.split(",");
       int countElements = lineValues.Length();
       masterList.Add(lineValues[countElements - 1]);
}

Using lastIndexOf();

//This will hold all the numbers as String
List<String> masterList = new ArrayList<String>();
while((str = br.readLine()) != null)
{
       int lastIndex = str.lastIndexOf(",");           
       masterList.Add(str.substring(lastIndex, str.length() - 1));
}

Once you have the masterList populated, you can loop through it and extract the float using

 Float.valueOf(masterList.get(i));