ArraysList Overview

Important Vocabulary

Array is a data structure used to implement a list of primitive or object data Element is a single value within the array The index of an element is the position of the element within the array - For Java, the first element is at index 0 The length of an array is the number of elements in the array - length is a public final data member of an array - Since length is public, we can access it in any class - Since length is final, we cannot change an array’s length after it has been created

In Java, the last element of an array list is at index list.length - 1

2017 FRQ #1

View FRQ here

Syntax to remember

  • This part importantly declares an array and references the ‘.add’ function

Remember the syntax:

// declare the array list
[arrayListName] = new ArrayList<Integer>();

// add to the list
[arrayListName].add([position], [value])
// ---OR---
[arrayListName].add([value])

// get from the list at a position
[arrayListName].get([position]);

// modify a list at a specific position
[arrayListName].set([position], [value]);

// get the size of an array
[arrayListName].size();

// not pertaining to arrays but still good to know
// the .intValue() method is used to convert an object of a wrapper class for primitive types (such as Integer, Double, Float, etc.) into its corresponding primitive int value.
Integer integerValue = new Integer(10); // Creating an Integer object with value 10
int intValue = integerValue.intValue(); // Extracting the int value from the Integer object
// This is needed because java arrayLists store wrapper objects for 'int' primitive values

public class Digits
{
    private ArrayList<Integer> digitList;

    // Part (a)
    public Digits(int num)
    {
        digitList = new ArrayList<Integer>();
        if (num == 0)
        {
            digitList.add(new Integer(0));
        }
        while (num > 0)
        {
            digitList.add(0, new Integer(num % 10));
            num /= 10;
        }
    }

    // Part (b)
    public boolean isStrictlyIncreasing()
    {
        for (int i = 0; i < digitList.size()-1; i++)
        {
            if (digitList.get(i).intValue() >= digitList.get(i+1).intValue())
            {
                return false;
            }
        }
        return true;
    } 

    public void printDigits(){
        for (int num : this.digitList){
            System.out.println(num);
        }
        return;
    }

    public static void main(String[] args){
        Digits digits = new Digits(125);
        digits.printDigits();
        System.out.println(digits.isStrictlyIncreasing());
    }
} 

Digits.main(null);

1
2
5
true

Common Mistakes

  1. Failure to use the intValue() method.
    • This is important because Java by default stores data in arrayLists using wrapper classes liek Interger. In this case the wrapper class needs to be converted into a primitive int datatype to perform operations on it.
  2. Failure to use num % 10 or num /= 10.
    • These lines are meant to target the last digit to either add to the arrayList or to remove from the integer