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
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;
}
}
Common Mistakes
- 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 primitiveint
datatype to perform operations on it.
- This is important because Java by default stores data in arrayLists using wrapper classes liek
- Failure to use
num % 10
ornum /= 10
.- These lines are meant to target the last digit to either add to the arrayList or to remove from the integer