Array processing with loops

Once a program stores its data in an array, it often needs to do something to the whole collection at once. It may need to reset every slot, display the contents in a readable form, put the values in order, or check whether a particular value is present. Writing a loop by hand for each of these tasks is possible, but the Arrays class provides ready-made methods for them. This article follows four of those methods, which handle filling, text conversion, sorting, and searching.

The Arrays class has several versions of each method, with one version for each kind of array element type. The ideas stay the same from version to version, so learning the behavior of one version gives you most of what you need for the others. The examples below use int arrays, but the same reasoning applies to other element types.

Arrays.fill is the method you reach for when every cell needs the same value, or when a stretch of cells needs to be reset. It comes in two forms. The first form takes an array and a value, and fills the entire array with that value. The second form takes an array, a starting index, an ending index, and a value, and fills only the cells in that range.

The whole-array form of fill has no index arguments. Calling Arrays.fill(scores, 3) would set every one of the six cells to 3. Use the whole-array form when the entire collection should be reset, and the range form when only part of it should change.

The range form deserves a careful reading. The starting index is included in the fill, but the ending index is not. The last cell that receives the value is the one just before the ending index. Many learners assume the range is closed on both ends, so this half-open style is worth committing to memory now. It matches the way Java ranges are described throughout the language.

Arrays.toString takes an array and returns a String that describes its contents. The String lists every value, separates the values with commas, and wraps the whole list in square brackets. Each value is converted to text in the same way it would appear if you printed it, so you do not need to write a loop that builds the text yourself.

Arrays.sort rearranges the values in an array so that they end up in increasing order. The change happens in place, which means the same array object is altered rather than a new sorted copy being returned. Like the fill methods, sort has a whole-array form and a range form. The whole-array form sorts every element. The range form sorts only the elements from the starting index up to, but not including, the ending index. The same exclusive-end rule applies here as it did for fill.

Sorting works on arrays of String values and arrays of the primitive types, with one exception: boolean arrays are not supported. Sorting also has a requirement that is easy to overlook. The method must be able to decide which of any two values is smaller. An array whose values cannot be compared in that way cannot be sorted by this method. The sorting algorithms themselves are covered later in the course, in the chapter that discusses algorithms.

Arrays.binarySearch looks for a particular value in an array and reports where it is. It returns an int. If the value is present, the return value is the index of an element that holds it. The method has one firm requirement: the array must already be sorted in increasing order before you call it. The sort step and the search step therefore belong together. Sorting first is what makes the search meaningful. If you search an array that is not sorted, the method's description does not promise a useful answer, so you should not rely on the result.

The source describes the absent-value case as returning -1. Java's documented behavior is more precise: when the key is missing, the method returns -(insertion point) - 1, where the insertion point is the index at which the key would be placed to keep the array sorted. The -1 result from the source is what you get when the insertion point is 0, meaning the key is smaller than every element. For any other missing key, the return value is more negative than -1.

These four methods are often used together. A common pattern is to fill or populate an array, sort it so the values are in order, search it with binarySearch, and print the contents with toString. When you read a program, identify which of these steps each call performs. That habit makes the flow of the program much easier to follow.