Creating and Filling an Array of Objects

Imagine a teacher who keeps track of a class of students, where each student is represented by a Student object. Storing those objects in an array lets a program hold the whole class under one name, such as classlist. That solves the first problem, but it raises a harder one: once a particular student is sitting inside that array, how do you reach them? This article follows a single array, 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.

An array of objects has a numbered row of slots, and each slot is reached by an index written in square brackets. Before the array is filled, the slots do not yet hold students. Once every slot has been filled with a Student object, the expression classlist[i] stands for the Student that lives at position i. The brackets select the slot, and 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]. The index counts from zero in Java, so the box labeled 3 is the fourth slot of the array. Everyday speech may call this student number 3, but the index used in code is the position counted from zero. This five-slot array is an illustration chosen for this article, not a fixed size from the source.

The reason this holds is that the array is only a way of naming the object. Once the index has selected a slot, the object in that slot behaves exactly as it always does. The source makes this point directly: you can do anything with classlist[i] that you could do with any other variable of type Student. That includes reading properties, calling methods, and passing the object to other code. So classlist[i] is not a special kind of value that needs its own rules. It is a Student, reached by a different route.

A property belongs to the object, not to the array. To reach a property through the array, first select the object with the index, then attach the property name with a dot. The pattern is always the array name, then the index in brackets, then a dot and the property name. The source gives the form classlist[i].name for the name of the ith Student, and the specific case classlist[3].name for the name of the Student stored at index 3.

Methods are reached the same way as properties. You select the object with the index, then use the dot to name the method and add parentheses for the call. The source gives classlist[i].getAverage() as the way to compute the average for student number i. Because the index picks out an object, the method call runs on that object and uses that object's own data.

The same index-then-dot step works for both forms because the index has already done its job by the time the dot appears. It has chosen one Student. The dot then asks that Student for something, whether a property value or a method result. Calling getAverage() on classlist[2] and calling it on classlist[4] therefore usually give different results, since each Student carries different data.