java - Truncate a list to a given number of elements

Question

What method truncates a list--for example to the first 100 elements--discarding the others (without iterating through individual elements)?

Answer 1

Use List.subList:

import java.util.*;
import static java.lang.Math.min;

public class T {
  public static void main( String args[] ) {
    List<String> items = Arrays.asList("1");
    List<String> subItems = items.subList(0, min(items.size(), 2));

    // Output: [1]
    System.out.println( subItems );

    items = Arrays.asList("1", "2", "3");
    subItems = items.subList(0, min(items.size(), 2));

    // Output: [1, 2]
    System.out.println( subItems );
  }
}

You should bear in mind that subList returns a view of the items, so if you want the rest of the list to be eligible for garbage collection, you should copy the items you want to a new List:

List<String> subItems = new ArrayList<String>(items.subList(0, 2));

If the list is shorter than the specified size, expect an out of bounds exception. Choose the minimum value of the desired size and the current size of the list as the ending index.

Lastly, note that the second argument should be one more than the last desired index.