Variables can be a primitive type according to their declaration, or they may hold a reference to an object. A special object type is an array. When a variable holds a reference to an array, then it can be indexed with the [ and ] characters, along with an integral value consisting of 0 or a positive value ranging to one less than the array's length, to access a certain element of the array. Multi-dimensional arrays are also supported by Java when an array has elements that are also arrays. Arrays are indexed from zero in Java. Under or over indexing is checked at runtime, and the result is an exception.

When a code has an array of a primitive type, the array contains many memory slots, each holding the value of the type. When the array has a reference type, in other words, when it is an array of objects, then the array elements are references to objects, each containing the type. In the case of int for example, each element of the array is 32-bit, which is 4 bytes. If the array is a type of Integer, then the elements are references to objects, pointers, so to say, which is usually 64-bit using 64-bit JVM and 32-bit on 32-bit JVM. In addition to that, there is an Integer object somewhere in memory that contains the 4-byte value and also an object header that may be as much as 24 bytes.

Memory consumption is one issue, but there is something else. When the program works with a large amount of data and the work needs the consecutive elements of the array, then the CPU loads a chunk of memory into the processor cache. It means that the CPU can access elements of the array that are consecutively faster. If the array is of a primitive type, it is fast. If the array is of some class type, then the CPU has to access memory to get the actual value, which may be as much as 50 times slower.