📓4.9: ArrayList Traversals

Table of Contents

📖 This page is a condensed version of CSAwesome Topic 4.9

ArrayList Traversal

Traversing an ArrayList means visiting each element in the list, often to perform some action or computation on it.

Traversal Methods

You can traverse an ArrayList in several ways:

  1. Enhanced for loop (for-each)
    Useful when you need to look at every element without changing the list.

    for (String name : nameList) {
    System.out.println(name);
}

  2. Regular for loop (with index) Useful when you need to know the index of the element or change elements.

    for (int i = 0; i < nameList.size(); i++) {
    System.out.println(nameList.get(i));
}

  3. While loop with index Gives you more control, especially if you need to adjust the index.

    int i = 0;
while (i < nameList.size()) {
    System.out.println(nameList.get(i));
    i++;
}

Note: Do not change the size of an ArrayList while using an enhanced for loop, as this can cause a ConcurrentModificationException.

Removing Elements While Traversing

When removing elements while traversing an ArrayList, you need special care to avoid skipping elements. If you remove an element at index i, the element after it will shift into index i. If you increment i in the same iteration, you’ll skip that new element.

Example:

int k = 0;
Integer zero = 0;
while (k < nums.size()) {
    if (nums.get(k).equals(zero)) {
        nums.remove(k);
    } else {
        k++;
    }
}

Check Your Understanding

Question: Assume nums initially contains [0, 0, 4, 2, 5, 0, 3, 0]. What will it contain after running numQuest() below?

public void numQuest() {
    int k = 0;
    Integer zero = 0;
    while (k < nums.size()) {
        if (nums.get(k).equals(zero))
            nums.remove(k);
        k++;
    }
}

Choices: a. [0, 4, 2, 5, 3] ✅ b. [3, 5, 2, 4, 0, 0, 0, 0] c. [0, 0, 0, 0, 4, 2, 5, 3] d. [4, 2, 5, 3]

Answer: a — Incrementing k every time skips elements when there are two zeros in a row.

Reading Files with java.nio.file

The java.nio.file package (added in Java 7) has a Files.readAllLines method to read an entire file into a List<String>.

import java.nio.file.*;
...
List<String> lines = Files.readAllLines(Paths.get("data.txt"));

Advantages:

  • Automatically stores lines in a resizable ArrayList.
  • No need to know how many lines in advance.

Coding Exercise: Read Pokémon File

Open a Codespace and create a file ReadData.java:

  1. Import java.nio.file.* and java.util.*.
  2. In main, read "pokemon.csv" into a List<String> lines using Files.readAllLines.
  3. Loop through and print the first 10 Pokémon.
import java.io.*;
import java.nio.file.*;
import java.util.*;

public class ReadData {
    public static void main(String[] args) throws IOException {
        List<String> lines = Files.readAllLines(Paths.get("pokemon.csv"));
        for (int i = 0; i < 10; i++) {
            System.out.println(lines.get(i));
        }
    }
}

ArrayList of Custom Objects

You can store any type of object in an ArrayList.

ArrayList<Student> roster = new ArrayList<>();
roster.add(new Student("Skyler", "skyler@sky.com", 123456));
roster.add(new Student("Ayanna", "ayanna@gmail.com", 789012));

for (Student s : roster) {
    System.out.println(s);
}

class Student {
    private String name, email;
    private int id;

    public Student(String name, String email, int id) {
        this.name = name;
        this.email = email;
        this.id = id;
    }

    public String toString() {
        return id + ": " + name + ", " + email;
    }
}

Groupwork Challenge: FRQ Word Pairs

Create an ArrayList<WordPair> from a given String[] words so that each word is paired with all following words.

  1. Initialize allPairs as an empty ArrayList<WordPair>.
  2. Nested loop: outer loop for i from 0 to length-1, inner loop for j from i+1 to length.
  3. Add each new WordPair(words[i], words[j]) to allPairs.
public class WordPairsList {
    private ArrayList<WordPair> allPairs;

    public WordPairsList(String[] words) {
        allPairs = new ArrayList<>();
        for (int i = 0; i < words.length; i++) {
            for (int j = i + 1; j < words.length; j++) {
                allPairs.add(new WordPair(words[i], words[j]));
            }
        }
    }

    public int numMatches() {
        int count = 0;
        for (WordPair wp : allPairs) {
            if (wp.getFirst().equals(wp.getSecond())) {
                count++;
            }
        }
        return count;
    }

    public String toString() {
        return allPairs.toString();
    }
}

class WordPair {
    private String word1, word2;
    public WordPair(String w1, String w2) { word1 = w1; word2 = w2; }
    public String getFirst() { return word1; }
    public String getSecond() { return word2; }
    public String toString() { return "(" + word1 + ", " + word2 + ")"; }
}

Summary

  • (AP 4.9.A.1) Traversing an ArrayList means visiting each element, usually with a loop.
  • Can use enhanced for, regular for with index, or while loops.
  • (AP 4.9.A.2) When removing during traversal, adjust the index to avoid skipping elements.
  • (AP 4.9.A.3) Accessing an out-of-bounds index throws IndexOutOfBoundsException.
  • (AP 4.9.A.4) Do not add/remove elements during an enhanced for loop — can cause ConcurrentModificationException.

Acknowledgement

Content on this page is adapted from Runestone Academy - Barb Ericson, Beryl Hoffman, Peter Seibel.