Chapter 7.1.1 – Arrays | Introduction to Programming Using Java
7.1.1 Arrays
Like a record, an array is a sequence of items. However, where items in a record are referred to by name, the items in an array are numbered, and individual items are referred to by their position number. Furthermore, all the items in an array must be of the same type.
The definition of an array is: a numbered sequence of items, which are all of the same type. The number of items in an array is called the length of the array. The position number of an item in an array is called the index of that item. The type of the individual items in an array is called the base type of the array.
The base type of an array can be any Java type, that is, one of the primitive types, or a class name, or an interface name. If the base type of an array is int, it is referred to as an “array of ints.” An array with base type String is referred to as an “array of Strings.” However, an array is not, properly speaking, a list of integers or strings or other values. It is better thought of as a list of variables of type int, or of type String, or of some other type.
As always, there is some potential for confusion between the two uses of a variable: as a name for a memory location and as a name for the value stored in that memory location. Each position in an array acts as a variable. Each position can hold a value of a specified type (the base type of the array). The value can be changed at any time. Values are stored in an array. The array is the container, not the values.
The items in an array—really, the individual variables that make up the array—are more often referred to as the elements of the array. In Java, the elements in an array are always numbered starting from zero. That is, the index of the first element in the array is zero. If the length of the array is N, then the index of the last element in the array is N-1. Once an array has been created, its length cannot be changed.
Java arrays are objects. This has several consequences. Arrays are created using a form of the new operator. No variable can ever hold an array; a variable can only refer to an array. Any variable that can refer to an array can also hold the value null, meaning that it doesn’t at the moment refer to anything. Like any object, an array belongs to a class, which like all classes is a subclass of the class Object. The elements of the array are, essentially, instance variables in the array object, except that they are referred to by number rather than by name.
Nevertheless, even though arrays are objects, there are differences between arrays and other kinds of objects, and there are a number of special language features in Java for creating and using arrays.
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