Arrays of Objects

Suppose a teacher keeps track of a class of students, and each student is represented by a Student object. Storing those objects in an array lets a program hold the whole class under one name. The harder question is how to reach one particular student once they are sitting inside that array. This article follows a single array, called classlist, and shows exactly what happens when you ask it for one of its students, what you can ask that student to do, and where learners most often go wrong.

Many learners who first meet arrays of objects assume that one statement does all the work. They expect that writing an array declaration and a creation statement will produce a set of ready-to-use objects. This is the most common way this topic goes wrong, so the sections that follow are built to test that belief directly.

An array's base type can be a class, not only a primitive type such as int or a type such as String. When the base type is Student, every element of the array is a variable whose type is Student. A variable of that type is a place that can hold a reference to a Student object. It is not the object itself.

Creating the array allocates the slots and sets each slot to null. A null element means the slot exists but refers to nothing. The array has three places ready for students, but no student has been made yet. In the source's example, new Student[30] allocates thirty elements, each starting as null. This separation between the container and the objects it is meant to hold is the central idea of the whole topic.

To create the actual objects, the program must visit each index and assign a new object to it. A for loop does this. The loop variable i takes the values 0 through 2 in this small array, and on each pass the statement creates one Student and stores it in the slot at that index. The loop is the part that turns three nulls into three objects.

Trace the loop by hand before running it. On the first pass, the slot at index 0 receives a new Student. On the second pass, index 1 receives one. On the third pass, index 2 receives one, and then the condition fails because i has reached the array length. Every slot is now filled, and every element is a real object rather than null.

Once every slot has been filled with a Student object, the expression classlist[i] stands for the Student that lives at position i. From that point on, you can treat classlist[i] the same way you would treat any variable declared as type Student. The brackets select the slot; the slot leads you to the object.

Read the diagram from left to right. Each box is one slot of the array, labeled with its index, and each slot holds a Student object. The highlighted box is classlist[3]. Java indices count from zero, so the box labeled 3 is the fourth slot. The source describes this same slot as student number 3, which is the everyday way of speaking, while the index used in code is the position counted from zero.

Properties and methods are reached in the same way. A property belongs to the object, not to the array, so the pattern is always the array name, then the index in brackets, then a dot and the name. The source gives classlist[i].name for the name of the ith Student. For methods, you select the object with the index, use the dot to name the method, and add parentheses for the call. The source gives classlist[i].getAverage() to compute the average for student number i.

The method call deserves a closer look. Because the index picks out an object, getAverage() runs on that object and uses its own data. Calling it on classlist[2] and on classlist[4] therefore usually gives different results, because each Student carries different data. The array is only a way of naming the object. Once the index has selected it, the object behaves as it always does, and you can pass it to other code just as you would any Student variable.

The same reasoning applies to the declaration-only case from the opening. Java cannot read a property from an object that was never created. In a filled array, classlist[i] refers to an object. In an array that was only declared and allocated, classlist[i] refers to null, and the attempt to use it is the failure point.

Almost all of these errors come from forgetting that the array creation statement stops at null. Each one is easier to spot once you know which line should have created the object.