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How to get a column element from an Instance variable ArrayList

d873d057-8b2e-49e4-a962-aabfa165df4d
edited Apr 23, 2017 12:15PM in Java Programming

I have created a two dimensional ArrayList by inserting one row at a time.  Now I need to find the max value of one column and perform some operations on it and then apply these operations to the other rows in another column.  Is this possible?  I have been Google'ing and Bing'ing for a week now and nothing seems to work.  The array is an Instance variable.

Answers

  • rpc1
    rpc1 Member Posts: 1,503
    edited Apr 23, 2017 6:17AM

    You have to iterate through ArrayList  to specific column. If you need to do the specific operations, may be better implement your own List class.

    This is example for getting maximum col value:

         private static int getMaxColValue(List<int[]> rows, int column) {
              int maxValue = 0;
              for (int[] row : rows) {
                   if (row[column] > maxValue)
                        maxValue = row[column];
              }          return maxValue;
         }
  • Unknown
    edited Apr 23, 2017 12:15PM
    I have created a two dimensional ArrayList by inserting one row at a time.

    No - you haven't - that is NOT possible.

    An ArrayList is a list of single instances.

    Those instances can be another object class. And if that other class implements the Comparable interface then you can use Collections.sort for the list.

    The Java Tutorials has trails that show how to use ALL of the basic Java functionality

    https://docs.oracle.com/javase/tutorial/collections/interfaces/order.html

    Object Ordering

    A List l may be sorted as follows.Collections.sort(l); 

    If the List consists of String elements, it will be sorted into alphabetical order. If it consists of Date elements, it will be sorted into chronological order. How does this happen? String and Date both implement the <a class="APILink" href="https://docs.oracle.com/javase/8/docs/api/java/lang/Comparable.html" target="_blank">Comparable</a> interface. Comparable implementations provide a natural ordering for a class, which allows objects of that class to be sorted automatically. The following table summarizes some of the more important Java platform classes that implement Comparable.

    Implement the Comparable interface in the class that implements those 'rows'

    If you try to sort a list, the elements of which do not implement Comparable, Collections.sort(list) will throw a ClassCastException. Similarly, Collections.sort(list, comparator) will throw a ClassCastException if you try to sort a list whose elements cannot be compared to one another using the comparator.
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