3.4. ArrayList

To write an ArrayList version of the program, we will have to introduce several new Java concepts, including the class ArrayList. We will also review different kinds of for loops used in Java.

Before going any further, we suggest you run the ArrayListGradebook program in IntelliJ. You can view this program in java-web-dev-exercises. Once you’ve done that, let’s look at what is happening in the Java source code.

package org.launchcode.java.demos.collections;

import java.util.ArrayList;
import java.util.Scanner;

public class ArrayListGradebook {

   public static void main(String[] args) {

      ArrayList<String> students = new ArrayList<>();
      ArrayList<Double> grades = new ArrayList<>();
      Scanner input = new Scanner(System.in);
      String newStudent;

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

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

         if (!newStudent.equals("")) {
            students.add(newStudent);
         }

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

      // Get student grades
      for (String student : students) {
         System.out.print("Grade for " + student + ": ");
         Double grade = input.nextDouble();
         grades.add(grade);
      }

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

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

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

Here we declare and initialize two objects, students and grades, which appear to be of type ArrayList<String> and ArrayList<Double>, respectively. An ArrayList in Java is very similar to an Array. Like an Array, we must let the compiler know what kind of objects our ArrayList is going to contain. In the case of students, the ArrayList will contain values of type String (representing the names of the students), so we use the ArrayList<String> syntax to inform the compiler that we intend to fill our list with Strings. Similarly, grades will hold exclusively values of type Double and is declared to be of type ArrayList<Double>.

Warning

Notice that we declared grades to be of type ArrayList<Double>, using the wrapper class Double rather than the primitive type double. All values stored in Java collections must be objects, so we’ll have to use object types in those situations.

In lines 10 and 11, we also initialize each list by creating a new, empty list. Note that when we call the ArrayList constructor, as in new ArrayList<>(), we don’t need to specify type (it’s implicit in the left-hand side of the assignment).

Note

You will sometimes see the ArrayList class written as ArrayList<E>, where E represents a placeholder for the type that a programmer will declare a given list to hold. This is especially true in documentation. You can think of E as representing an arbitrary type.

Classes like ArrayList<E> that take another type or class as a parameter are referred to as generic classes or generic types.

3.4.1. ArrayList Iteration

3.4.1.1. do-while

We then use a do-while loop to collect the names of each of the students in the class.

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

   if (!newStudent.equals("")) {
      students.add(newStudent);
   }

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

Recall that a do-while loop is very similar to a while loop, but the execution condition is checked at the end of the loop block. This has the net effect that the code block will always run at least once. In this example, we prompt the user for a name, which Java processes via input.nextLine() when the user hits the enter key. To finish entering names, the user enters a blank line.

For each student that is entered (that is, each non-empty line), we add the new String to the end of our list with students.add(newStudent). The .add() method is provided by the ArrayList Class. There are lots of other ArrayList methods to get familiar with, some of which we will discuss in more detail below.

Note that our program imports java.util.ArrayList to take advantage of this Java provided class.

3.4.1.2. for-each

Below the do-while loop are two different loops that demonstrate two ways you can loop through a list in Java. Here’s the first, which collects the numeric grade for each student:

// Get student grades
for (String student : students) {
   System.out.print("Grade for " + student + ": ");
   Double grade = input.nextDouble();
   grades.add(grade);
}

This, you may recall, is Java’s for-each loop syntax. You may read this in your head, or even aloud, as: for each student in students. As you might expect at this point, we must declare the iterator variable student with its data type.

3.4.1.3. for

The next loop on display prints out each student’s name and grade:

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

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

Here, we introduce the syntax students.size() which utilizes the size() method of ArrayList. This method returns the integer representing the number of items in the list. This is similar to String’s .length() method.

In this for loop, we use a loop index to define the starting point, ending point, and increment for iteration. It may be helpful for you to consider this kind of construction as something like, for integer i in the range of the number of items in students.... The first statement inside the parenthesis declares and initializes a loop index variable i. The second statement is a Boolean expression that is our exit condition. In other words, we will keep looping as long as this expression evaluates to true. The third statement is used to increment the value of the loop index variable at the end of iteration through the loop.

Again, the syntax i++ is Java shorthand for i = i + 1. Java also supports the shorthand i-- to decrement the value of i. We can also write i += 2 as shorthand for i = i + 2.

In the final lines of the program, we compute the average grade for all students:

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

3.4.2. ArrayList Methods

Let’s gather up a few of the ArrayList methods that we’ve encountered so far, along with a few new ones. While these will be the most common methods and properties that you use with this class, they by no means represent a complete list. Refer to the official documentation on the ArrayList class for such a list, and for more details.

To demonstrate the use of these methods, we’ll create a new ArrayList called planets.

ArrayList<String> planets = new ArrayList<>();

Ok, we’ve got an empty ArrayList. We need to use the class’s .add() method to populate this collection with items.

Note

There are other means to declare and initialize an ArrayList in fewer lines. These require knowledge of other collections types, so we’ll stick with .add() for the time being.

Using .add() to populate planets:

planets.add("Mercury");
planets.add("Venus");
planets.add("Earth");
planets.add("Mars");
planets.add("Jupiter");
planets.add("Saturn");
planets.add("Uranus");
planets.add("Neptune");

Thus, the first item in this table:

ArrayList methods in Java

Java Syntax	Description	Example
add()	Adds an item to the ArrayList	planets.add("Pluto") adds Pluto to planets
size()	Returns the number of items in an ArrayList, as an int	planets.size() returns 9
contains()	Checks to see if the ArrayList contains a given item, returning a Boolean	planets.contains("Earth") returns true
indexOf()	Looks for an item in an ArrayList, returns the index of the first occurrence of the item if it exists, returns -1 otherwise	planets.indexOf("Jupiter") returns 4

Here’s a couple more methods that require slightly longer descriptions:

Collections.sort()

Java Syntax	Description	Example
Collections.sort()	Rearranges the elements of a Collection into ascending order.	Collections.sort(planets) produces ["Earth", "Jupiter", "Mars", "Mercury", "Neptune", "Pluto", "Saturn", "Uranus", "Venus"]

This method is technically used on Java’s Collections class and thus requires a different import statement:

import java.util.Collections;

Collections is itself a member of the collections framework but not all members of the framework are instances of this class. We include this method here because, should you be in the market for a sorting method, this is a helpful one to know.

toArray()

Java Syntax	Description	Example
toArray()	Returns an Array containing the elements of the ArrayList	planets.toArray(planetsArray) returns {"Earth", "Jupiter", "Mars", "Mercury", "Neptune", "Pluto", "Saturn", "Uranus", "Venus"}

Perhaps you recall that in Java, you must know the size of the Array when you create it. So we’ll need to first define the new Array before we can use toArray().

String[] planetsArray = new String[planets.size()];
planets.toArray(planetsArray);

Speaking of Arrays, let’s see the Array version of Gradebook next.

3.4.3. References

• Java Collections (docs.oracle.com)

• ArrayList Class (docs.oracle.com)

3.4.4. Check Your Understanding

Question

The number of entries in an ArrayList may not be modified.

1. True

2. False

Question

Create an ArrayList called charStars containing a, b, and c.

1. ArrayList<String> charStars = new ArrayList<>();
   charStars.add('a');
   charStars.add('b');
   charStars.add('c');
   

2. ArrayList<Char> charStars = new ArrayList<>();
   charStars.add('a');
   charStars.add('b');
   charStars.add('c');
   

3. ArrayList<char> charStars = new ArrayList<char>('a', 'b', 'c');
   

4. ArrayList<String> charStars = new ArrayList<>();
   charStars.add("a");
   charStars.add("b");
   charStars.add("c");