1.3.4.1. Counter-Controlled Loops Over Arrays¶

To create a numeric for loop over an array, we need to know how many slots the array has. Fortunately, every array knows its own length, and we can access it using its length field.

int[] values = new int[ ... ];

for (int i = 0; i < values.length; i++)
{
    values[i] = 2 * i;
}

Notice here that we use values.length to access the array’s length. Unlike most other objects where you would use a method, arrays have no useful methods and provide their length in a special read-only field called length. Don’t get this confused with the length() method on strings or the size() method on lists–it is just a field, so you never include parentheses after the field name. Remember that since the size of an array can’t be changed once it has been created, you cannot assign a value to the length field of the array–it is read-only.

To summarize this numeric for loop:

• The first subscript we want to use is at postion 0, so we create a new variable called i and initialize it to 0.

• Our last subscript position is one less than the length of the array. This means we should stop when our counter is no longer less than the length of the array. Thus, our loop condition is i < values.length.

• We want to go through every index in the array so we write i++ for the update step to increment i by one each time the loop repeats.

When working with both Lists and arrays, it’s very easy to mix up when to use the size() method and when to use length. Equally tricky is that when accessing the length of a String variable, we’d use the method length().

String[] words = new String[3];
int x = words.length;   // x is set to 3

String str = "Hello";
int y = str.length();   // y is set to 5

List<String> moreWords = new ArrayList<>();
int z = moreWords.size();  // z is set to 0

Be sure to keep careful track of what type of data you’re working with so you can access its length correctly.

1.3.4.2. For-Each Loops Over Arrays¶

For-each loops over arrays work exactly the same as with lists or other structures:

String[] coffees = {"Espresso", "Mocha", "Decaf", "Americano"};

for (String coffee : coffees)
{
    System.out.println(coffee);
}

However, there are two critical differences compared to a numeric for loop:

1. You do not have access to the current position or subscript value inside your loop, so you cannot use it in any computations inside the loop.

2. You can only access each value stored in the array, but cannot change the values stored in the array. The loop variable (for example, coffee in this loop) is a local variable inside the loop. While you can assign a new value to the variable, that will not affect the array itself, or the contents of the array.

For-each loops have many advantages, since they are short to write and near bullet-proof in terms of making mistakes with management of the index/position or condition, making it nearly impossible to write infinite loops. However, these advantages do come with limitations. Fortunately, Arrays naturally support either style of for loop, so use the style that best fits your needs.