APCS-A Test Review

Array Resources

Arrays are so important that we devoted this entire page to them.

Primitive arrays may be thought of as old-school 'motels:' rooms in a line-up (with rooms numbers starting with zero!) where we can check 'people' into the rooms and find them when necessary.

Declaring/Initializing/Constructing arrays:

Creating from an 'Initializer List':

String stooges[] = {"Moe", "Larry", "Curly"};
char[] vowels = {'a', 'e', 'i', 'o', 'u'};
//[] ('square braces/brackets') may follow either datatype or identifier //Used for small-sized arrays

Declaring/Initializing an array with 'new':

int someOddNumbers[] = new int[3]; //state the size; it is fixed thereafter
someOddNumbers[0] = 1;
someOddNumbers[1] = 3;
someOddNumbers[2] = 5;

Creating from an ArrayList:

ArrayList<Integer> aL = new ArrayList<Integer>(); 
aL.add(10); 
aL.add(20); 
aL.add(30); 
aL.add(40); 

Object[] myIntObjects = aL.toArray(); //must be stored as 'Objects'

int sum = 0; //accumulator
for (Object obj : myIntObjects){ 
    int val = (int)obj; //cast back as ints
    sum += val;
} 
System.out.println("Sum: " + sum);

Creating from a delimited String:

String avengersStr = "Thor|Iron Man|Captain America|Hulk|Dr. Strange|Black Widow|Spider-man|Hawkeye|Captain Marvel|Black Panther|Vision|Ant-Man";
String avengers[] = avengersStr.split("|");

Although arrays are objects, they are not 'birthed' through a class constructor (no 'new'). Therefore, they are considered 'primitive' and have no methods affiliated with them. They do have an important attribute: length:

char[] vowels = {'a', 'e', 'i', 'o', 'u'};
int len = vowels.length;
System.out.println("There are " + len + " vowels in the English language");

Printing arrays:

Using traditional SOPs - Questionable:

char[] vowels = {'a', 'e', 'i', 'o', 'u'};
System.out.println(vowels); //prints 'aeiou' (without '')
int[] intArr = {1, 1, 2, 3, 5, 8};
System.out.println(intArr); //prints memory address: [I@4aa298b7
//DANGER WILL ROBINSON! SOPs behave differently depending on the datatype of the array!

Using loops - Reliable:

char[] vowels = {'a', 'e', 'i', 'o', 'u'};
//traversing the array using 'fixed iteration'
for(int ndx= 0; ndx<vowels.length; ndx++){
    System.out.println(vowels[ndx]);
}

//traversing the array using 'for-each' loop
for(char v : vowels) System.out.println(v);

'Cheating' with Arrays class:

import java.util.Arrays;
public class ArrayDemoDriver{
    public static void main(String[] args){
        System.out.println("=== Array Demo ===");
        
        int primitiveArray[] = {3, 2, 1, 0, 4, 3, 2, 7};
        System.out.println(Arrays.toString(primitiveArray));
        
    }//end main method
}//end ArrayDemoDriver

Helpful array Manipulations:

User Method Call	Code