Arrays – Introduction to Programming

Arrays are containers that can hold multiple values of the same data type.

Declaring Arrays

We can declare a reference to an array as follows. These statements don’t, however, allocate memory for the array.

String args[];
String[] args;

int heights[];
int[] heights;

Allocating Arrays

In order to allocate space you need to use the new keyword and specify either the size or a list of initial elements.

String[] arr1 = new String[3];  // elements initialized to null
int[] arr2 = new int[12];       // elements initialized to 0

String s1 = "one";
String s2 = "two";
String[] arr3 = {s1, s2, "three"};
int[] arr4 = {10, 20, 17};

arr3 = new String[] {"four", "five"};  // array declared earlier
arr4 = new int[] {31, 47};             // array declared earlier

Accessing Elements of an Array

We can get the value stored at a particular index in the array using brackets and the index.

String first = arr3[0];
int last = arr4[1];

Arrays are implemented as objects on the heap and have a length property that can be used to get the size of the array. Note that length is not a method, but rather a property. We can access properties (a.k.a. fields) using the dot operator as we did with methods, however we omit the parenthesis.

System.out.println("Size: " + arr4.length);   // prints 2

We can access all of the elements in an array using a for-loop to iterate over all of the possible array indices. In the example below, we initialize all of the elements in the array to 1.

for (int i = 0; i < arr4.length; i++) {
    arr4[i] = 1;
}

For-each Loops

For-each loops allow us to iterate through all of the elements in the array without having to use indices. For-loops can not be used to modify an array( add element, remove element, change element), but are useful when you need to process each element.

for (int element : arr4) {
    System.out.printf("%d ", element);
}

Arrays are Reference Types

Stored on the heap
We can pass references to arrays into functions
Arrays passed into a function can be modified within the function
We can return reference to arrays.
Memory is reserved so long as it is referenced by some variable.

Passing and Returning Arrays

public static void main(String[] args) {
    int[] ages = initArray(10);
    System.out.println("Total ages: " + sum(ages));
}

static int[] initArray(int size) {
    int[] arr = new int[size];

    for (int i = 0; i < arr.length; i++) { 
        arr[i] = i; 
    }

    return arr;
}

static int sum(int[] arr) {
    if (arr == null) {
        return 0; 
    }
    int sum = 0;
    for (int elm : arr) {
        sum += elm;
    }
    return sum;
}

Notice that we check to see if arr is null in the sum function. Before ever calling a method or accessing a field on a reference type variable, make sure it is not null (or use a try-catch statement).