1. Creating An Array¶

An array is a sequence of values; the values in the array are called elements. You can make an array of ints, doubles, Strings, or any other type, but all the values in an array must have the same type. The type of values stored in the array is called the array’s element type.

To create an array, you have to declare a variable with an array type and then create the array itself. Array types look like other Java types, except they are followed by square brackets ([]). No, there is no special “array” keyword in Java; you just add square brackets onto the end of any type to turn it into an array type. For example, the following lines declare that counts is an “integer array” and values is a “double array”.

int[] counts = new int[4];
double[] values = new double[size];

To create the array itself, you have to use the new operator, which you have used to create all other objects. The new operator allocates memory for the array and automatically initializes all of its elements to zero. The first declaration initializes the variable counts to refer to a new array of four integers. The second initalizes values to refer to an array of doubles, but the number of elements depends on the value of size (at the time the array is created).

You can use any integer expression for the size of an array, as long as the value is non-negative. If you try to create an array with -4 elements, for example, you will get a NegativeArraySizeException. An array with zero elements is allowed, and there are special uses for such arrays. They are often called “empty arrays”.

Importantly, once we establish the size of our array, we can’t change it. If we suddenly decide we need an array of a different size, we’ll need to create a new array object instead.

Like objects, arrays are instantiated with the new operator and they have fields (for example, length). Like variables for objects, variables that refer to arrays are considered reference variables. When arrays are used as parameters, a reference to the array is passed rather than a copy of the entire array. The primary difference between arrays and full-fledged objects is that arrays aren’t defined in terms of an Array class. Thus, arrays don’t fit into Java’s Object hierarchy. They don’t inherit any properties from Object and they cannot be subclassed.

Most importantly, that means that arrays themselves, while they are objects, have no useful methods. Instead, you need to use library classes to access methods that work with arrays.

When creating an array of Strings or any type of object (like Jeroo or Pixel), each of the items in our array will be initialized to null. If we created an int (or double) array, each item in the array will be initialized to 0, and an array of booleans will have each item initialized to false when the array is created.

There is a second way we can create an array if we know what we want to put in the array when we create it.

For example, lets say we wanted to create an array to store the numbers: 97, 43, and 2.

int[] counts = {97, 43, 2};

This will make an array of size 3 that contains these three values. The sequence of values in curly braces serves to specify the initial values we want to place in each slot of the array we are creating.

2. Accessing Items in Arrays¶

However, arrays access the values a little bit differently, making use of the square brackets again.

When referencing elements in an array, we refer to the position of that particular element within the array. For example, if the array is named values, then the elements are named values[0], values[1], values[2], … values[n - 1], where n gives the number of elements in the array. This naming also reflects the fact that the array’s data are contained in storage locations that are next to each other.

Note that this is the same concept as referring to positions in a List or character positions within a String. The item in the first “slot” of our array is at index 0 and the last item in the array is going to be one less than the size of the array. For example, the first item in an array of size 5 is at index 0, and the last is at index 4. Trying to access the index 5 would cause an ArrayIndexOutOfBoundsException at runtime.

The syntax for referring to elements of an array uses square brackets to provide the desired position (index):

arrayname[subscript]

Here, arrayname is the name of the array (any valid identifier will do) and subscript is the position of the element within the array.

A subscript is an integer quantity contained in square brackets that is used to identify an array element by its position or index value. A subscript must be either an integer value or an integer expression. For example, all the of the following lines of code are valid ways to access a value in an array values

values[4]
values[x]
values[x + y]

These examples show that when an expression, such as x + y, is used as a subscript, it is evaluated to a specific integer value before the reference is made.

It is a syntax error to use a non-integer type as an array subscript. Each of the following expressions would be invalid:

// will not work!
arr[5.0]
arr["5"]

3. Setting Items in an Array¶

When working in Lists , if we wanted to change a value, we could use the set method. For example:

ArrayList <String> arr = new ArrayList<String>();
arr.add("Brazil");
arr.add("France");
arr.add("Ethiopia");

arr.set(0, "Columbia"); // changes the item at index 0 from Brazil to Columbia

After this code is executed, the List would contain the strings: Columbia, France, and Ethiopia, in that order.

When working with an array, things are simpler. We don’t need to call a method (because arrays have no useful methods). Instead, referring to positions within an array uses a subscript in square brackets. So when we want to change a value in the array, we can treat arrayname[<any subscript>] just like a variable.

String[] countries = new String[3];
countries[0] = "Brazil";
countries[1] = "France";
countries[2] = "Ethiopia";

countries[0] = "Columbia"; // changes the item at index 0 from Brazil to Columbia

4. Arrays Compared to Lists (or ArrayList)¶

5. Putting It All Together¶

To put this all together, consider a brand new array object:

int[] counts = new int[4];

Remember that when you create an array with the new operator, the elements are initialized to zero, as shown in this memory diagram of the counts array so far.

The arrow indicates that the value of counts is a reference to the array. You should think of the array and the variable that refers to it as two different things. As you’ll soon see, we can assign a different variable to refer to the same array, and we can change the value of counts` to refer to a different array.

The boldface numbers inside the boxes are the elements of the array. The lighter numbers outside the boxes are the indexes used to identify each location in the array. As with strings, the index of the first element is 0, not 1. For this reason, we sometimes refer to the first element as the “zeroth” element.

The [] operator selects elements from an array:

System.out.println("The zeroth element is " + counts[0]);

You can use the [] operator anywhere in an expression:

counts[0] = 7;
counts[1] = counts[0] * 2;
counts[2]++;
counts[3] -= 60;

After executing these lines, the contents of the array will be changed to:

You can use any expression as an index, as long as it has type int. One of the most common ways to index an array is with a loop variable. For example:

int i = 0;
while (i < 4)
{
    System.out.println(counts[i]);
    i++;
}

This while loop counts up from 0 to 4. When i is 4, the condition fails and the loop terminates. So the body of the loop is executed only when i is 0, 1, 2, or 3. In this context, the variable name i is short for “index”.

Each time through the loop, we use i as an index into the array, displaying the ith element. This type of array processing is usually written as a for loop:

for (int i = 0; i < 4; i++)
{
    System.out.println(counts[i]);
}

For the counts array, the only legal indexes are 0, 1, 2, and 3. If the index is negative or greater than 3, the result is an ArrayIndexOutOfBoundsException.

6. Check Your Understanding: Arrays¶

7. Syntax Practice 10a¶

8. Iterating Over Arrays¶

Lets say we wanted to iterate over all values in an array of integers to print them all out.

We could do this with a counter-controlled loop or a for-each loop.

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

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

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

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

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

9. Check Your Understanding: Iterating with Arrays¶

10. Syntax Practice 10b¶

11. Initializing Array Contents¶

As mentioned above, when a new array object is created, it is automatically initialized. Normally, values are initialized to zero, false, or null, depending on what is appropriate for the array’s element type.

However, what if you do not want an array of all zeroes or nulls? As you’ve seen, you can explicitly list out your own initial values:

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

This is great for small arrays or situations where it is convenient to write out all the values. However, what if you have an array with 100 slots? Or 1,000 slots?

For example, what if you want to initialize all of the pixels in a large image to be blue?

If you have an array you want to initialize to something other than zeroes or nulls, just use a numeric for loop to iterate over all slots in the array, and use an assignment statement to set values as needed. For example, to initialize an array of 100 integers to the values 0-99:

int[] values = new int[100];

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

12. Printing Arrays¶

You can use println() to display an array, but it probably doesn’t do what you would like. For example, say you print an array like this:

int[] values = {1, 2, 3, 4};
System.out.println(values);

The output is something like this:

[I@bf3f7e0

The bracket indicates that the value is an array, I stands for “integer”, and the rest represents the address of the array in memory.

If we want to display the elements of the array, we can do it ourselves:

public void printArray(int[] values)
{
    System.out.print("[" + values[0]);
    for (int i = 1; i < values.length; i++)
    {
        System.out.print(", " + values[i]);
    }
    System.out.println("]");
}

Given the previous array, the output of printArray() is as follows:

[1, 2, 3, 4]

Fortunately, the Java library already includes a class, java.util.Arrays, that provides methods for working with arrays. One of them, toString(), returns a string representation of an array. Remember that arrays do not provide useful methods themselves, but this utility class does provide them for you. After importing Arrays, we can invoke toString() like this:

System.out.println(Arrays.toString(values));

And the output is shown here:

[1, 2, 3, 4]

Notice that Arrays.toString() uses square brackets around the elements of the array, and produces output that looks the same as the toString() method for List objects.

13. Copying Array Variables¶

Array variables contain references to arrays. When you make an assignment to an array variable, it simply copies the reference. But it doesn’t copy the array itself. For example:

double[] a = new double[3];
double[] b = a;

These statements create an array of three doubles and make two different variables refer to it.

Any changes made through either variable will be seen by the other. For example, if we set a[0] = 17.0;, and then display b[0], the result is 17.0. Because a and b are different names for the same thing, they are sometimes called aliases.

If you actually want to copy the array, not just the reference, you have to create a new array and copy the elements from one to the other, like this:

double[] b = new double[3];
for (int i = 0; i < 3; i++)
{
    b[i] = a[i];
}

java.util.Arrays provides a method named copyOf() that performs this task for you. So you can replace the previous code with one line:

double[] b = Arrays.copyOf(a, 3);

The second parameter is the number of elements you want to copy, so copyOf() can also be used to copy part of an array. After invoking Arrays.copyOf() in this way, the two variables would end up in the following state:

The examples so far work only if the array has three elements. It is better to generalize the code to work with arrays of any size. We can do that by replacing the magic number, 3, with a.length:

double[] b = new double[a.length];
for (int i = 0; i < a.length; i++)
{
    b[i] = a[i];
}

The last time the loop gets executed, i is a.length - 1, which is the index of the last element. When i is equal to a.length, the condition fails and the body is not executed–which is a good thing, because trying to access a[a.length] would throw an exception.

Of course, we can replace the loop altogether by using Arrays.copyOf() and a.length for the second argument. The following line produces the same result shown above:

double[] b = Arrays.copyOf(a, a.length);

The Arrays class provides many other useful methods like Arrays.compare(), Arrays.equals(), Arrays.fill(), and Arrays.sort. Take a moment to read the documentation by searching the web for java.util.Arrays.

14. Naming Array Variables¶

Picking names for array variables can be tricky. For example, consider this array.

String[] words = {"loop", "array", "method", "class"};

Here, we have chosen a plural name, which is the most common pattern in naming arrays. Use a plural name when you most often refer to the whole array as an object, and want to give it a name that represents the entire collection of values as a whole. This aids readability when writing loops, passing the array as a parameter to other methods, and so on, since the plural name serves as a reminder that the array is a whole group of values combined into one object.

However, there are other situations where you may instead primarily focus on individual slots within the array, rather than on the array itself. If code primarily involves working with just a single array slot, then a singular name can be more useful. For example, if we have an array of words, but we most often think about referring to “word 1” in the array, or “word 3” in the array, or “word 0” in the array, then naming the array word instead of words would allow us to use subscript expressions like word[1] or word[3] or word[0]. Since this pattern exactly matches the natural phrasing we would use when talking about a single value within an array, sometimes it is a better choice.

In practice, you will make a judgement call about which is a better fit for your situation. When in doubt, use a plural name, since that is more common and avoids misleading impressions even if there are some uses of the name where a singular name would be a better fit.

15. Writing Test Assertions Involving Arrays¶

When testing with array values, sometimes basic assertions are convenient, but other times it can be complicated. You can use assertThat(...).isEqualTo(...) to check that an array has an expected value, as you would expect, but there are many other options.

// We expect result is {"Espresso", "Mocha", "Decaf", "Americano"}
String[] result = someObject.MethodThatReturnsArray();

// If you want to check the size
assertThat(result.length).isEqualTo(4);

// Alternate way of checking the size
assertThat(result).hasSize(4);

// If you want to check a specific index
assertThat(result[1]).isEqualTo("Mocha");

// If you want to check the entire array
String[] coffees = {"Espresso", "Mocha", "Decaf", "Americano"};
assertThat(result).isEqualTo(words);

However, sometimes an array may be long, or it may not be convenient to write out all the values, or you may want to avoid declaring/creating/initializing a separate array to compare against. Fortunately, there are some additional methods you can use when making assertions about arrays.

The first method is containsExactly(), which allows you to directly list out the values you expect to find in the array:

// If you want to check the entire array
assertThat(result).containsExactly(
    "Espresso", "Mocha", "Decaf", "Americano");

When you make an assertion using containsExactly(), you list all of the array’s contents in the order you expect, and that’s it. containsOnly() succeeds if the array contains exactly the values you specify, in the order you list them, and nothing else. It is a useful shorthand for having to create a separate array object to compare against. Even better, it works for list objects too!

However, sometimes if you have a really long array, you don’t want to list all the values. Instead, you can use:

// If you want to check the entire array
assertThat(result).contains("Americano", "Mocha");

The contains() method allows you to list out as many values as necessary. However, contains() will succeed if the array has the specified values anywhere in the array in any order, mixed in among its slots. The array may be longer, and may have other values besides the ones you list, but it must include the values you specify. This can be useful when there are a few values in the array that matter, but the array is too long to list out all the values. Again, this works for lists too.

There are also several other methods you can use on arrays or lists when constructing tests, where you can specify as many or few of the values as you need:

// If you want to check values are not present
assertThat(result).doesNotContain("table", "chair");

// To check the first few elements, without worrying about others
assertThat(result).startsWith("Espresso", "Mocha");

// To check the last few elements, without worrying about others
assertThat(result).endsWith("Mocha", "Decaf", "Americano");

// To check these occur only once in array, are not duplicated
assertThat(result).containsOnlyOnce("Mocha", "Americano");

16. Applying Arrays in a Problem¶

This video discusses a “Web-Analyzer” project and discusses how arrays could be used in this type of problem. You don’t need to worry too much about what this project does or how it works, or about completing the exercise given in the video. It is just a more detailed example used to explain arrays and how they might be used in a problem.

17. Syntax Practice 10c¶

18. Check Your Understanding¶

19. Programming Practice 10a¶

20. Programming Practice 10b¶