Array Basics and Syntax

Suppose a program must process the names of one thousand people. The first idea many learners reach for is one variable per name, with one thousand variables declared and one thousand print statements to display them. That approach quickly becomes unworkable. An array is a structure designed for exactly this situation: it keeps the whole list under a single name, and each individual item is reached through its position in that list.

Because every item has a position number, any single item can be referred to directly by that number. Java imposes two rules on this structure. First, every item in the array must be of the same type. Second, the numbering always begins at zero. The same-type rule belongs to the Java setting rather than to arrays in general, since some other programming languages do not require it.

Three terms describe any array, and keeping them apart is the key to reading array descriptions correctly. The length is the number of items the array contains. The base type is the type shared by the individual items. The index is the position number of one particular item.

Each element in an array has an index, which is its numerical position in the sequence. If the array is named A, then element number i is written A[i]. Numbering starts at zero, so the first element is at position zero, the second at position one, and so on. These position numbers are offsets into the sequence, with the first element sitting at offset zero. They are not labels that happen to be counted from one.

The diagram below shows a five-element array with its values. The elements are A[0] through A[4]. Read from left to right, the first value is at index 0 and the last value is at index 4. A learner who reads the first position as index 1 has made the most common indexing error.

The valid indexes therefore run from 0 through A.length minus 1. In the five-element array, A.length is 5, so the highest valid index is 4. There is no slot at index 5 because the highest offset is always one less than the count.

The length of an array A is written A.length. It is a property that defines the size of the array, meaning the number of elements it holds. Because length is the count that fixes the valid index range, it matters for every index you write.

Length is determined when the array is created and cannot be changed afterward. Whatever size you choose at creation is the size the array keeps for its entire life. So the number of slots available to you is decided up front, and every index you write must fall inside the range that this fixed length defines.

The word type appears in two places when you work with arrays, and separating them clears up much of the confusion. The first is the type of each element. Every element in an array has the same type, called the base type of the array. The second is the type of the array itself, considered as a whole. The array's type is built from the base type rather than being a separate, unrelated choice. This is written btype[], where btype stands for the base type.

Read the type name from left to right. In int[], the part before the brackets, int, tells you what each element holds, and the brackets mark that the whole thing is an array of those elements. The same pattern applies to any base type: an array of Strings has base type String and array type String[].

The source describes an array as a collection of variables rather than simply a list of integers or strings. Each position in an array acts as a variable that can hold a value of the base type. The individual variables that make up an array are called its elements. An array of ints can be understood as a list of variables of type int, and an array of Strings as a list of variables of type String.

It helps to separate two things. An element is a storage position, identified by its index. The value currently stored in that position is separate from the position itself. The value of an element can be changed at any time, just as the value of an ordinary variable can be changed. Changing the value does not change the position, its index, or the array's length.

In the diagram, the slot labeled A[2] stays the same position before and after the assignment. Only the value inside it changes, from 30 to 75. The index remains 2, and the length remains 5.