Time estimate: 90 min.

7.3. Traversing `ArrayList`s with Loops¶

ArrayLists can be traversed with while loops and both regular and enhanced for loops much the same way we use those constructs to loop over an array.

7.3.1. Enhanced For Each Loop¶

You can use a enhanced for loop to traverse all of the items in an ArrayList, just like you do with an array when you only care about the values in the list and not their indices. An example is shown in the main method below.

Note however that you can’t use the enhanced for loop if you want to add or remove elements while traversing an ArrayList. If an ArrayList is modified, such as by calling the add or remove methods, while it is being looped over, it will cause the loop to throw a ConcurrentModificationException. If you need to modify an ArrayList while looping over it, you’ll need to use a regular while or for loop.

Coding Exercise

What does the following code do? Guess before you run it. Then, add another enhanced for each loop that computes the product of all the elements in myList by multiplying them. Print out the product after the new loop.

import java.util.*; // import all classes in this package.

public class Test1
{
    public static void main(String[] args)
    {
        ArrayList<Integer> myList = new ArrayList<Integer>();
        myList.add(50);
        myList.add(30);
        myList.add(20);
        int total = 0;
        for (Integer value : myList)
        {
            total += value;
        }
        System.out.println("Sum of all elements: " + total);

// Write a for-each loop that computes the product
        // of all the elements in myList and print out the product.

}
}

====
import static org.junit.Assert.*;

import org.junit.*;

import java.io.*;

public class RunestoneTests extends CodeTestHelper
{
    @Test
    public void testExpected() throws IOException
    {
        String output = getMethodOutput("main");
        String expect = "100";
        boolean passed = output.contains(expect);
        getResults(expect, output, "Prints out sum", passed);
        assertTrue(passed);
    }

@Test
    public void testProduct() throws IOException
    {
        String output = getMethodOutput("main");
        String expect = "30000";
        boolean passed = output.contains(expect);
        getResults(expect, output, "Prints out product", passed);
        assertTrue(passed);
    }

@Test
    public void countForLoops()
    {
        String code = removeSpaces(getCode());
        int count = countOccurences(code, "for(Integer");
        boolean passed = count >= 2;
        getResults("2", count + "", "Number of for each loops", passed);
        assertTrue(passed);
    }
}

7.3.2. For Loop¶

You can also use a while loop or a regular for loop to process list elements accessed using an index. ArrayList indices starts at 0 just like array indices, but instead of using the index operator [] to access elements, you use the get(index) method to get the value at the index and set(index,value) to set the element at an index to a new value.

If you try to use an index that is outside of the range of 0 to the number of elements − 1 in an ArrayList, your code will throw an IndexOutOfBoundsException, similar to the ArrayIndexOutOfBoundsException thrown if you use the index operator on an array with an index out of bounds for that array.

Coding Exercise

The following code will throw an IndexOutOfBoundsException. Can you fix it?

7.3.3. While Loop¶

The example below demonstrates a while loop and an object-oriented approach where the list is a field of the current object and you use an instance method rather than a class (static) method to loop through the list.

Coding Exercise

The following code removes a name from a list. Set the found variable to the appropriate true or false values at line 13 and line 20 to make the code work.

import java.util.*;
public class ListWorker
{
   private ArrayList<String> nameList;

public ListWorker(ArrayList<String> nameList)
   {
       this.nameList = nameList;
   }

public boolean removeName(String name)
   {
       boolean found =   // true or false?
       int index = 0;
       while (index < nameList.size())
       {
           if (name.equals(nameList.get(index)))
           {
               nameList.remove(index);
               found =    // true or false?
           }
           else
           {
               index++;
           }
       }
       return found;
    }

public static void main(String[] args)
    {
        ArrayList<String> myList = new ArrayList<String>();
        myList.add("Amun");
        myList.add("Ethan");
        myList.add("Donnie");
        myList.add("Ethan");
        ListWorker listWorker = new ListWorker(myList);
        System.out.println(listWorker.nameList);
        listWorker.removeName("Ethan");
        System.out.println("After removing Ethan: "
                  + listWorker.nameList);
    }
}
====
import static org.junit.Assert.*;

import org.junit.*;

import java.io.*;

public class RunestoneTests extends CodeTestHelper
{
    @Test
    public void testMain() throws IOException
    {
        String output = getMethodOutput("main");
        String expect = "[Amun, Ethan, Donnie, Ethan]\nAfter removing Ethan: [Amun, Donnie]";
        boolean passed = getResults(expect, output, "Expected output from main");
        assertTrue(passed);
    }
}

Be careful when you remove items from a list as you loop through it. Notice how the method above only increments the index if an item was not removed from the list. This is because removing an item from a list will shift the remaining items to the left and if you increment the index in all cases you will skip the elements immediately after each element you remove. To see why, consider that those elements will be shifted into the position of the just removed element and if you increment the index, it will move to the next position, skipping the element that used to be at that position. Leaving the index unchanged after a remove allows the shifted-down element to be processed on the next time through the loop.

Check your understanding

7-3-4: Assume that nums has been created as an ArrayList object and it initially contains the following Integer values [0, 0, 4, 2, 5, 0, 3, 0]. What will nums contain as a result of executing numQuest?

ArrayList<Integer> list1 = new ArrayList<Integer>();
private ArrayList<Integer> nums;

// precondition: nums.size() > 0;
// nums contains Integer objects
public void numQuest()
{
   int k = 0;
   Integer zero = new Integer(0);
   while (k < nums.size())
   {
      if (nums.get(k).equals(zero))
         nums.remove(k);
      k++;
   }
}

[0, 4, 2, 5, 3]
Incrementing the index each time through the loop will miss when there are two zeros in a row.
[3, 5, 2, 4, 0, 0, 0, 0]
This would be true if the code moved the zeros to the end, but that is not what it does.
[0, 0, 0, 0, 4, 2, 5, 3]
This would be true if the code moved the zeros to the font, but that is not what it does.
[4, 2, 5, 3]
This would be correct if k was only incremented when an item was not removed from the list.

You can step through the code above by clicking on the following Example.

The following has the correct code for the method getScore plus at least one extra unneeded code statement. This method will calculate and return the score for a word game. The code should loop through all of the elements in wordList and if the length of the current word is 3 it should add one to the score, if the length of the word is 4 it should add 2 to the score, and if the length is greater than 4 it should add 3 to the score. The method should return the score. Drag the needed blocks from the left into the correct order on the right. Check your solution by clicking on the Check button. You will be told if any of the blocks are in the wrong order or if you need to remove one or more blocks. There is one extra block that is not needed in a correct solution.

        public static int getScore(ArrayList wordList)
{
---
  int score = 0;

  for (String word : wordList)
  {
---
    if (word.length() == 3)
---
    {
      score++;
    }
---
    else if (word.length() == 4)
    {
      score = score + 2;
    }
---
    else if (word.length() > 4)
    {
      score = score + 3;
    }
---
  } // end for
---
  return score;

} // end method
---
if (word.length == 3) #distractor

The following has the correct code for a method called insertInOrder plus at least one extra unneeded code statement. This method should add the passed name in alphabetic order to a private list field called nameList. Drag the needed blocks from the left into the correct order on the right. Check your solution by clicking on the Check button. You will be told if any of the blocks are in the wrong order or if you need to remove one or more blocks. There is one extra block that is not needed in a correct solution.

        public void insertInOrder(String name)
{
---
  int index = 0;
---
  while (index < nameList.size() &&
         nameList.get(index).compareTo(name) < 0)
  {
---
    index++;
---
  } // end while
---
  nameList.add(index,name);
---
} // end method
---
nameList.add(name); #distractor

7.3.4. ArrayList of Student Objects¶

Coding Exercise

You can put any kind of objects into an ArrayList. For example, here is an ArrayList of Students. Although the print statement works here, you may want a nicer printout.

Add a for each loop that prints out each student and then a new line.

import java.util.*;

public class StudentList
{
    // main method for testing
    public static void main(String[] args)
    {
        ArrayList<Student> roster = new ArrayList<Student>();
        roster.add(new Student("Skyler", "skyler@sky.com", 123456));
        roster.add(new Student("Ayanna", "ayanna@gmail.com", 789012));
        // Replace this with a for each loop that prints out each student on a
        // separate line
        System.out.println(roster);
    }
}

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

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

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

====
import static org.junit.Assert.*;

import org.junit.*;

import java.io.*;

public class RunestoneTests extends CodeTestHelper
{
    @Test
    public void testMain() throws IOException
    {
        String output = getMethodOutput("main");
        String expect = "123456: Skyler, skyler@sky.com\n789012: Ayanna, ayanna@gmail.com";
        boolean passed = getResults(expect, output, "Expected output from main");
        assertTrue(passed);
    }

@Test
    public void loopCode()
    {
        boolean passed = checkCodeContains("for loop", "for");
        assertTrue(passed);
    }
}

7.3.5. Programming Challenge : FRQ Word Pairs¶

This challenge is based on the 2018 Free Response Question #2 WordPair. We encourage you to work in pairs on this challenge.

You are given a class called WordPair that can store pairs of words.

class WordPair
{
    private String word1;
    private String word2;

    public WordPair(String word1, String word2)
    {
        this.word1 = word1;
        this.word2 = word2;
    }

    public String getFirst()
    {
        return word1;
    }

    public String getSecond()
    {
        return word2;
    }

    public String toString()
    {
        return "(" + word1 + ", " + word2 + ")";
    }
}

First, see if you can create an ArrayList of WordPair objects below. Look at the StudentList example above for help.

Create an Arraylist of WordPair objects.

import java.util.*;

public class WordPairTest
{
    public static void main(String[] args)
    {
        // Create an ArrayList of WordPair objects called pairs

pairs.add(new WordPair("hi", "there"));
        pairs.add(new WordPair("hi", "bye"));
        System.out.println(pairs);
    }
}

class WordPair
{
    private String word1;
    private String word2;

public WordPair(String word1, String word2)
    {
        this.word1 = word1;
        this.word2 = word2;
    }

public String getFirst()
    {
        return word1;
    }

public String getSecond()
    {
        return word2;
    }

public String toString()
    {
        return "(" + word1 + ", " + word2 + ")";
    }
}

====
 import static org.junit.Assert.*;

import org.junit.*;

import java.io.*;

public class RunestoneTests extends CodeTestHelper
 {
     public RunestoneTests()
     {
         super("WordPairTest");
     }

@Test
     public void testMain() throws IOException
     {
         String output = getMethodOutput("main");
         String expect = "[(hi, there), (hi, bye)]";
         boolean passed = getResults(expect, output, "Expected output from main");
         assertTrue(passed);
     }

@Test
     public void hasArrayList()
     {
         boolean passed = checkCodeContains("ArrayList declaration", "ArrayList<WordPair>");
         assertTrue(passed);
     }
 }

In this FRQ, you are given an array of words and you will create pairs of them by taking the first word and pairing it with all the other words, then taking the second word and pairing it with all but the first one, and so on. For example, if the word array is [“Hi”, “there”, “Tyler”, “Sam”], this figure shows how the word pairs are formed.

In the class WordPairsList below, you will write the constructor which takes the array of words and pairs them up as shown in the figure. You will need nested loops to pair each element with the rest of the elements in the list.

Here is the pseudocode for the constructor method.

Initialize the allPairs list to an empty ArrayList of WordPair objects.
Loop through the words array for the first word in the word pair (for loop from index i = 0 to length-1)
- Loop through the rest of the word array starting from index i + 1 for the second word in the word pair (for loop from index j = i + 1 to length)
  - Add the new WordPair formed from the ith word and the jth word to the allPairs ArrayList.

FRQ WordPairs Challenge: Complete the constructor for WordPairsList below which will add pairs of words from a given array to the ArrayList. Then, complete the method numMatches() as described below this exercise.

import java.util.*;

public class WordPairsList
{
    private ArrayList<WordPair> allPairs;

public WordPairsList(String[] words)
    {
        // WRITE YOUR CODE HERE
        // initialize allPairs to an empty ArrayList of WordPair objects

// nested loops through the words array to add each pair to allPairs

}

public int numMatches()
    {
        // Write the code for the second part described below
        return 0;
    }

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

public static void main(String[] args)
    {
        String[] words = {"Hi", "there", "Tyler", "Sam"};
        WordPairsList list = new WordPairsList(words);
        System.out.println(list);
        // For second part below, uncomment this test:
        // System.out.println("The number of matched pairs is: " +
        // list.numMatches());
    }
}

class WordPair
{
    private String word1;
    private String word2;

public WordPair(String word1, String word2)
    {
        this.word1 = word1;
        this.word2 = word2;
    }

public String getFirst()
    {
        return word1;
    }

public String getSecond()
    {
        return word2;
    }

public String toString()
    {
        return "(" + word1 + ", " + word2 + ")";
    }
}

====
 import static org.junit.Assert.*;

import org.junit.*;

import java.io.*;

public class RunestoneTests extends CodeTestHelper
 {
     public RunestoneTests()
     {
         super("WordPairsList");
     }

@Test
     public void test1()
     {
         String output = getMethodOutput("main");
         String expect =
                 "[(Hi, there), (Hi, Tyler), (Hi, Sam), (there, Tyler), (there, Sam), (Tyler, Sam)]";

boolean passed = output.contains(expect);

String[] lines = output.split("\n");
         if (lines.length > 1) output = lines[0];

getResults(expect, output, "Part 1 - Add all word pairs from main()", passed);
         assertTrue(passed);
     }

/* //remove test because instructions say to add a matched pair.
     @Test
     public void test2()
     {
         String output = getMethodOutput("main");
         String expect = "The number of matched pairs is: 0";

boolean passed = output.contains(expect);
         String[] lines = output.split("\n");
         if (lines.length > 1) output = lines[1];

getResults(expect, output, "Part 2 - call to numMatches from main()", passed);
         assertTrue(passed);
     }
     */

@Test
     public void test3()
     {
         String[] words = {"Hi", "Hi", "Test", "Test"};
         WordPairsList list = new WordPairsList(words);
         String output = list.toString();
         String expect = "[(Hi, Hi), (Hi, Test), (Hi, Test), (Hi, Test), (Hi, Test), (Test, Test)]";

boolean passed =
                 getResults(
                         expect,
                         output,
                         "Part 1 - Add all word pairs with {\"Hi\", \"Hi\", \"Test\", \"Test\"}");
         assertTrue(passed);
     }

@Test
     public void test4()
     {
         String[] words = {"Hi", "Hi", "Test", "Test"};
         WordPairsList list = new WordPairsList(words);
         String output = "The number of matched pairs is: " + list.numMatches();
         String expect = "The number of matched pairs is: 2";

boolean passed =
                 getResults(
                         expect,
                         output,
                         "Part 2 - numMatches() with {\"Hi\", \"Hi\", \"Test\", \"Test\"}");
         assertTrue(passed);
     }
 }

In the next part of the FRQ challenge, you are asked to write a method called numMatches that counts and returns the number of pairs where the first word is the same as the second word. For example, if the word array is ["hi","bye","hi"], the pairs generated would be ["hi","bye"], ["hi","hi"], and ["bye","hi"]. In the second pair ["hi","hi"], the first word is the same as the second word, so numMatches would return 1.

For this method, you will need a loop that goes through the ArrayList allPairs and for each WordPair in allPairs, it checks to see if its first word (using the getFirst method) equals the second word (using the getSecond method). If there is a match, it increments a counter which it returns at the end of the method. To test this method, add another “there” into the words array and then uncomment the call to numMatches.

7.3.6. Summary¶

ArrayLists can be traversed with an enhanced for loop, a while loop, or a regular for loop using an index.
Deleting elements during a traversal of an ArrayList requires using special techniques to avoid skipping elements, since remove moves all the elements above the removed index down.
Since the indices for an ArrayList start at 0 and end at the number of elements − 1, accessing an index value outside of this range will result in an IndexOutOfBoundsException being thrown.
Changing the size of an ArrayList while traversing it using an enhanced for loop can result in a ConcurrentModificationException being thrown. Therefore, when using an enhanced for loop to traverse an ArrayList, you should not add or remove elements.

You have attempted of activities on this page

7.3. Traversing ArrayLists with Loops¶

7.3.1. Enhanced For Each Loop¶

7.3.2. For Loop¶

7.3.3. While Loop¶

7.3.4. ArrayList of Student Objects¶

7.3.5. Programming Challenge : FRQ Word Pairs¶

7.3.6. Summary¶

7.3. Traversing `ArrayList`s with Loops¶