The Java Tutorials have been written for JDK 8. Examples and practices described in this page don't take advantage of improvements introduced in later releases and might use technology no longer available.
See Java Language Changes for a summary of updated language features in Java SE 9 and subsequent releases.
See JDK Release Notes for information about new features, enhancements, and removed or deprecated options for all JDK releases.
Set
, List
, Queue
, and Map
. You're not sure which implementations will work best, so you decide to use general-purpose implementations until you get a better idea how your program will work in the real world. Which implementations are these?Set
: HashSet
List
: ArrayList
Queue
: LinkedList
Map
: HashMap
Set
implementation that provides value-ordered iteration, which class should you use? TreeSet
guarantees that the sorted set is in ascending element order, sorted according to the natural order of the elements or by the Comparator
provided.Collections
class, which provides static methods that operate on or return collections.List
. The program should then print random lines from the file, the number of lines printed to be specified by the second command line argument. Write the program so that a correctly-sized collection is allocated all at once, instead of being gradually expanded as the file is read in. Hint: To determine the number of lines in the file, use
java.io.File.length
to obtain the size of the file, then divide by an assumed size of an average line. List
randomly, we will use ArrayList
. We estimate the number of lines by taking the file size and dividing by 50. We then double that figure, since it is more efficient to overestimate than to underestimate.
import java.util.*; import java.io.*; public class FileList { public static void main(String[] args) { final int assumedLineLength = 50; File file = new File(args[0]); List<String> fileList = new ArrayList<String>((int)(file.length() / assumedLineLength) * 2); BufferedReader reader = null; int lineCount = 0; try { reader = new BufferedReader(new FileReader(file)); for (String line = reader.readLine(); line != null; line = reader.readLine()) { fileList.add(line); lineCount++; } } catch (IOException e) { System.err.format("Could not read %s: %s%n", file, e); System.exit(1); } finally { if (reader != null) { try { reader.close(); } catch (IOException e) {} } } int repeats = Integer.parseInt(args[1]); Random random = new Random(); for (int i = 0; i < repeats; i++) { System.out.format("%d: %s%n", i, fileList.get(random.nextInt(lineCount - 1))); } } }
ArrayList
has little affect on its performance. Specifying an initial capacity in advance is more likely to be useful when your program repeatedly creates large ArrayList
objects without intervening I/O.