3.5. Array¶

We learned about arrays in Java in a previous lesson, so let’s spend a moment comparing them to ArrayLists. ArrayLists are generally easier to use than Java’s Array. Let’s see why this is.

Why does Java have both Arrays and ArrayLists? The answer is historical, at least in part. Java is a C-style language, and arrays are the most basic data structure in C. Using an Array over an ArrayList might be preferred in some circumstances, primarily for performance reasons (array operations are generally faster than ArrayList operations). Also note that Arrays are of fixed size. You cannot expand or contract an Array after it is created, so you must know exactly how many elements it will need to hold when you create it. This fact is reason enough to use ArrayLists in most scenarios.

To illustrate Array usage, here is a version of the Gradebook program using Arrays instead of ArrayLists:

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package org.launchcode.java.demos.collections;

import java.util.Scanner;

public class ArrayGradebook {

   public static void main(String[] args) {

      // Allow for at most 30 students
      int maxStudents = 30;

      String[] students = new String[maxStudents];
      double[] grades = new double[maxStudents];
      Scanner input = new Scanner(System.in);

      String newStudent;
      int numStudents = 0;

      System.out.println("Enter your students (or ENTER to finish):");

      // Get student names
      do {
         newStudent = input.nextLine();

         if (!newStudent.equals("")) {
            students[numStudents] = newStudent;
            numStudents++;
         }

      } while(!newStudent.equals(""));

      // Get student grades
      for (int i = 0; i < numStudents; i++) {
         System.out.print("Grade for " + students[i] + ": ");
         double grade = input.nextDouble();
         grades[i] = grade;
      }

      // Print class roster
      System.out.println("\nClass roster:");
      double sum = 0.0;

      for (int i = 0; i < numStudents; i++) {
         System.out.println(students[i] + " (" + grades[i] + ")");
         sum += grades[i];
      }

      double avg = sum / numStudents;
      System.out.println("Average grade: " + avg);
   }

}

Note that we have to decide up front how large our Arrays students and grades are going to be. Thus, this program sets an arbitrary maximum amount of students, likely larger than any user will enter. It may seem obvious, then, that Array has no equivalent add() method. The only way to access and alter an element in an Array is with bracket notation, using an explicit index. For example, gradebook defines a counter variable, numStudents. When the first student is entered by the user, the value is stored in newStudent. If the value is not the empty string, then the item in students at position 0, the initial value of numStudents is assigned to the newStudent value. The next time the do-while loop executes, the value of students at position 1 will be assigned. And so on… Because we must always access and assign Array elements using an explicit index, our code can seem littered with Array counter variables (like our friends i and j) and is more difficult to read (not to mention more error-prone).

Like ArrayLists, however, we can loop through an Array using a for-each loop as long as we don’t need to use the index of the current item. If we only wanted to print each student’s name, and not their grade, at the end of our program, we could do the following:

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for (String student : students) {
   System.out.println(student);
}

We’ll use Arrays in Java from time-to-time, but for the most part you should rely on ArrayLists to store collections of values, or ordered data.

int[] randomNumbers = new int[5];