Aim for FiveComputer Science A

Unit 4 · Topic 4.3

4.3 Array Creation and Access

An array stores a fixed number of values of the same type under one name. This topic covers creating arrays, their default values, initializer lists, getting and changing elements by index, and the exception you get for a bad index.

Key terms

  • array
  • element
  • index
  • length
  • initializer list
  • ArrayIndexOutOfBoundsException

What an array is

An array holds several values, called elements, all of the same type. The type can be a primitive, like int[], or a reference type, like String[]. The number of elements is set when the array is created and can never change. You get it from the length attribute: scores.length, with no parentheses (unlike a string's length() method).

An array is an object, so an array variable holds a reference. Assigning one array variable to another, like int[] b = a;, makes them aliases of the same array. Changing b[0] changes a[0] too.

The same thing happens when you pass an array to a method. The parameter gets a copy of the reference (3.6), so a method that changes arr[i] changes the caller's array. That's how a method can fill in or rearrange an array without returning anything.

Creating arrays

There are two ways to create an array.

  • With new and a size: int[] scores = new int[4]; makes 4 elements, each set to the default value for the type: 0 for int, 0.0 for double, false for boolean and null for any reference type.
  • With an initializer list: double[] prices = {2.5, 4.0, 1.25}; makes an array of length 3 holding exactly those values.

Indexes

Each element has an index, starting at 0. Use square brackets to read or change an element: scores[2] is the third element, and scores[2] = 85; changes it.

The valid indexes run from 0 to length - 1. The last element is always arr[arr.length - 1]. Using any other index, such as -1 or arr.length, compiles fine but throws an ArrayIndexOutOfBoundsException when it runs.

int[] nums = {4, 8, 15}; System.out.println(nums[nums.length - 1]); System.out.println(nums[nums.length]);

This prints 15 and then crashes on the third line, because index 3 doesn't exist in an array of length 3. The compiler can't catch this, since the index is only known when the code runs.

Arrays of objects

In an array of objects, like String[], each element holds a reference. A new array of objects starts full of null, not empty objects. Calling a method on one of those null elements, like names[0].length(), throws a NullPointerException. Fill in the elements before you use them.

Worked examples

Try each one yourself first, then open the solution.

  1. Example 1

    Creating and accessing

    What does this code print?int[] scores = new int[4]; scores[0] = 90; scores[2] = scores[0] - 5; double[] prices = {2.5, 4.0, 1.25}; String[] names = new String[2]; System.out.println(scores[1] + " " + scores[2] + " " + scores.length); System.out.println(prices[prices.length - 1] + " " + names[0]);

    Show the solution
    1. Step 1: scores is made with new int[4], so it starts as 0, 0, 0, 0.
    2. Step 2: scores[0] = 90 changes the first element. scores[2] becomes 90 - 5, which is 85. scores[1] is still 0, and scores.length is 4.
    3. Step 3: prices has length 3, so prices[prices.length - 1] is prices[2], 1.25.
    4. Step 4: names is a new String array, so every element is null.

    Answer: Two lines: 0 85 4, then 1.25 null.

  2. Example 2

    Two names, one array

    What does this code print?int[] a = {1, 2, 3}; int[] b = a; b[0] = 99; System.out.println(a[0] + " " + b[0]);

    Show the solution
    1. Step 1: int[] b = a; copies the reference, not the array. a and b refer to the same array.
    2. Step 2: b[0] = 99 changes that one shared array.
    3. Step 3: So a[0] and b[0] are both 99.

    Answer: It prints 99 99.

Common mistakes

  • Using arr.length as an index. The last valid index is arr.length - 1.
  • Writing arr.length() with parentheses. For arrays, length is an attribute; for strings, length() is a method.
  • Thinking int[] b = a; copies the array. It copies the reference, so both names share one array.
  • Calling methods on elements of a new object array before filling them in. They start as null.

On the exam

  • Tracing questions on arrays often hinge on default values or on an index of length. Write out the array with its indexes before you trace.

Connected topics

Videos

  • AP Computer Science A - Topic 4.3 - Part 1: Array Creation and Access

    Tim Gallagher Computer ScienceWatch on YouTube (opens in a new tab)

  • AP CSA Unit 4 Array Basics (2025-2026)

    PickcodeWatch on YouTube (opens in a new tab)

  • AP CSA Data Collections – One Dimensional Arrays

    Goldie's Math EmporiumWatch on YouTube (opens in a new tab)

  • AP Computer Science A - Topic 4.3 - Part 2: Array Creation and Access

    Tim Gallagher Computer ScienceWatch on YouTube (opens in a new tab)

  • Learn Java arrays in 9 minutes! 🍎

    Bro CodeWatch on YouTube (opens in a new tab)

  • Arrays (Java Tutorial)

    Bill BarnumWatch on YouTube (opens in a new tab)

Check yourself

4 questions on 4.3 Array Creation and Access. Pick an answer to see if you got it, and why.

Question 1 of 4

Consider the following code segment.double[] d = new double[4]; d[1] = 2.5; System.out.println(d[0] + d[1] + " " + d.length);What is printed as a result of executing the code segment?

Question 2 of 4

Consider the following code segment.int[] a = {3, 1, 4}; System.out.println(a[a.length]);What is printed as a result of executing the code segment?

Question 3 of 4

Which of the following statements creates an array that can hold 5 String references, all initially null?

Question 4 of 4

Consider the following code segment.int[] a = {1, 2, 3}; int[] b = a; b[0] = 9; System.out.println(a[0] + " " + b[0]);What is printed as a result of executing the code segment?

0 of 4 answered