3.10. Arrays
An array is a data structure that stores a collection of values of the same type. You access each individual value through an integer index. For example, if a is an array of integers, then a[i] is the ith integer in the array.
Declare an array variable by specifying the array type—which is the element type followed by []—and the array variable name. For example, here is the declaration of an array a of integers:
int[] a;
However, this statement only declares the variable a. It does not yet initialize a with an actual array. Use the new operator to create the array.
int[] a = new int[100];
This statement declares and initializes an array of 100 integers.
The array length need not be a constant: new int[n] creates an array of length n.
Note
You can define an array variable either as
int[] a;
or as
int a[];
Most Java programmers prefer the former style because it neatly separates the type int[] (integer array) from the variable name.
The array elements are numbered from 0 to 99 (and not 1 to 100). Once the array is created, you can fill the elements in an array, for example, by using a loop:
int[] a = new int[100];
for (int i = 0; i < 100; i++)
a[i] = i; // fills the array with numbers 0 to 99
When you create an array of numbers, all elements are initialized with zero. Arrays of boolean are initialized with false. Arrays of objects are initialized with the special value null, which indicates that they do not (yet) hold any objects. This can be surprising for beginners. For example,
String[] names = new String[10];
creates an array of ten strings, all of which are null. If you want the array to hold empty strings, you must supply them:
for (int i = 0; i < 10; i++) names[i] = "";
Caution
If you construct an array with 100 elements and then try to access the element a[100] (or any other index outside the range 0 . . . 99), your program will terminate with an "array index out of bounds" exception.
To find the number of elements of an array, use array .length. For example:
for (int i = 0; i < a.length; i++)
System.out.println(a[i]);
Once you create an array, you cannot change its size (although you can, of course, change an individual array element). If you frequently need to expand the size of an array while your program is running, you should use a different data structure called an array list. (See Chapter 5 for more on array lists.)
3.10.1. The "for each" Loop
Java has a powerful looping construct that allows you to loop through each element in an array (or any other collection of elements) without having to fuss with index values.
The enhanced for loop
for (variable : collection) statement
sets the given variable to each element of the collection and then executes the statement (which, of course, may be a block). The collection expression must be an array or an object of a class that implements the Iterable interface, such as ArrayList. We discuss array lists in Chapter 5 and the Iterable interface in Chapter 2 of Volume II. For example,
for (int element : a) System.out.println(element);
prints each element of the array a on a separate line.
You should read this loop as "for each element in a". The designers of the Java language considered using keywords, such as foreach and in. But this loop was a late addition to the Java language, and in the end nobody wanted to break the old code that already contained
methods or variables with these names (such as System.in).
Of course, you could achieve the same effect with a traditional for loop:
for (int i = 0; i < a.length; i++)
System.out.println(a[i]);
However, the "for each" loop is more concise and less error-prone, as you don‘t have to worry about those pesky start and end index values.
Note
The loop variable of the "for each" loop traverses the elements of the array, not the index values.
The "for each" loop is a pleasant improvement over the traditional loop if you need to process all elements in a collection. However, there are still plenty of opportunities to use the traditional for loop. For example, you may not want to traverse the entire collection, or you may need the index value inside the loop.
Tip
There is an even easier way to print all values of an array, using the toString method of the Arrays class. The call Arrays.toString(a) returns a string containing the array elements, enclosed in brackets and separated by commas, such as "[2, 3, 5, 7, 11, 13]". To print the array, simply call System.out.println(Arrays.toString(a));
3.10.2. Array Initializers and Anonymous Arrays
Java has a shorthand to create an array object and supply initial values at the same time. Here‘s an example of the syntax at work:
int[] smallPrimes = { 2, 3, 5, 7, 11, 13 };
Notice that you do not call new when you use this syntax.
You can even initialize an anonymous array:
new int[] { 17, 19, 23, 29, 31, 37 }
This expression allocates a new array and fills it with the values inside the braces. It counts the number of initial values and sets the array size accordingly. You can use this syntax to reinitialize an array without creating a new variable. For example,
smallPrimes = new int[] { 17, 19, 23, 29, 31, 37 };
is shorthand for
int[] anonymous = { 17, 19, 23, 29, 31, 37 };
smallPrimes = anonymous;
Note
It is legal to have arrays of length 0. Such an array can be useful if you write a method that computes an array result and the result happens to be empty. Construct an array of length 0 as
new elementType[0]
Note that an array of length 0 is not the same as null.
3.10.3. Array Copying
You can copy one array variable into another, but then both variables refer to the same array:
int[] luckyNumbers = smallPrimes; luckyNumbers[5] = 12; // now smallPrimes[5] is also 12
Figure 3.14 shows the result. If you actually want to copy all values of one array into a new array, you use the copyOf method in the Arrays class:
int[] copiedLuckyNumbers = Arrays.copyOf(luckyNumbers, luckyNumbers.length);
The second parameter is the length of the new array. A common use of this method is to increase the size of an array:
luckyNumbers = Arrays.copyOf(luckyNumbers, 2 * luckyNumbers.length);
The additional elements are filled with 0 if the array contains numbers, false if the array contains boolean values. Conversely, if the length is less than the length of the original array, only the initial values are copied.
C++ Note
A Java array is quite different from a C++ array on the stack. It is, however, essentially the same as a pointer to an array allocated on the heap. That is,
int[] a = new int[100]; // Java
is not the same as
int a[100]; // C++
but rather
int* a = new int[100]; // C++
In Java, the [] operator is predefined to perform bounds checking. Furthermore, there is no pointer arithmetic—you can‘t increment a to point to the next element in the array.
3.10.4. Command-Line Parameters
You have already seen one example of a Java array repeated quite a few times. Every Java program has a main method with a String[] args parameter. This parameter indicates that the main method receives an array of strings—namely, the arguments specified on the command line.
For example, consider this program:
public class Message
{
public static void main(String[] args)
{
if (args[0].equals("-h"))
System.out.print("Hello,");
else if (args[0].equals("-g"))
System.out.print("Goodbye,");
// print the other command-line arguments
for (int i = 1; i < args.length; i++)
System.out.print(" " + args[i]);
System.out.println("!");
}
}
If the program is called as
java Message -g cruel world
then the args array has the following contents:
args[0]: "-g" args[1]: "cruel" args[2]: "world"
The program prints the message
Goodbye, cruel world!
C++ Note
In the main method of a Java program, the name of the program is not stored in the args array. For example, when you start up a program as
java Message -h world
from the command line, then args[0] will be "-h" and not "Message" or "java".
3.10.5. Array Sorting
To sort an array of numbers, you can use one of the sort methods in the Arrays class:
int[] a = new int[10000]; . . . Arrays.sort(a)
This method uses a tuned version of the QuickSort algorithm that is claimed to be very efficient on most data sets. The Arrays class provides several other convenience methods for arrays that are included in the API notes at the end of this section.
The program in Listing 3.7 puts arrays to work. This program draws a random combination of numbers for a lottery game. For example, if you play a "choose 6 numbers from 49" lottery, then the program might print this:
Bet the following combination. It‘ll make you rich!
478
19
30
44
To select such a random set of numbers, we first fill an array numbers with the values 1, 2, . . ., n:
int[] numbers = new int[n];
for (int i = 0; i < numbers.length; i++)
numbers[i] = i + 1;
Listing 3.7. LotteryDrawing/LotteryDrawing.java
A second array holds the numbers to be drawn:
int[] result = new int[k];
Now we draw k numbers. The Math.random method returns a random floating-point number that is between 0 (inclusive) and 1 (exclusive). By multiplying the result with n, we obtain a random number between 0 and n–1.
int r = (int) (Math.random() * n);
We set the ith result to be the number at that index. Initially, that is just r + 1, but as you‘ll see presently, the contents of the numbers array are changed after each draw.
result[i] = numbers[r];
Now we must be sure never to draw that number again—all lottery numbers must be distinct. Therefore, we overwrite numbers[r] with the last number in the array and reduce n by 1.
numbers[r] = numbers[n - 1];
n--;
The point is that in each draw we pick an index, not the actual value. The index points into an array that contains the values that have not yet been drawn.
After drawing k lottery numbers, we sort the result array for a more pleasing output:
Arrays.sort(result);
for (int r : result)
System.out.println(r);
3.10.6. Multidimensional Arrays
Multidimensional arrays use more than one index to access array elements. They are used for tables and other more complex arrangements. You can safely skip this section until you have a need for this storage mechanism.
Suppose you want to make a table of numbers that shows how much an investment of $10,000 will grow under different interest rate scenarios in which interest is paid annually and reinvested (Table 3.8).
You can store this information in a two-dimensional array (matrix), which we call balances.
Declaring a two-dimensional array in Java is simple enough. For example:
double[][] balances;
As always, you cannot use the array until you initialize it with a call to new. In this case, you can do the initialization as follows:
balances = new double[NYEARS][NRATES];
In other cases, if you know the array elements, you can use a shorthand notation for initializing a multidimensional array without a call to new. For example:
int[][] magicSquare =
{
{16, 3, 2, 13},
{5, 10, 11, 8},
{9, 6, 7, 12},
{4, 15, 14, 1}
};
Once the array is initialized, you can access individual elements by supplying two pairs of brackets—for example, balances[i][j].
The example program stores a one-dimensional array interest of interest rates and a twodimensional array balances of account balances, one for each year and interest rate. We initialize the first row of the array with the initial balance:
for (int j = 0; j < balances[0].length; j++)
balances[0][j] = 10000;
Then we compute the other rows, as follows:
for (int i = 1; i < balances.length; i++)
{
for (int j = 0; j < balances[i].length; j++)
{
double oldBalance = balances[i - 1][j];
double interest = . . .;
balances[i][j] = oldBalance + interest;
}
}
Listing 3.8 shows the full program.
Note
A "for each" loop does not automatically loop through all elements in a twodimensional array. Instead, it loops through the rows, which are themselves onedimensional arrays. To visit all elements of a two-dimensional array a, nest two loops, like this:
for (double[] row : a)
for (double value : row)
do something with value
Tip
To print out a quick-and-dirty list of the elements of a two-dimensional array, call
System.out.println(Arrays.deepToString(a));
The output is formatted like this:
[[16, 3, 2, 13], [5, 10, 11, 8], [9, 6, 7, 12], [4, 15, 14, 1]]
3.10.7. Ragged Arrays
So far, what you have seen is not too different from other programming languages. But there is actually something subtle going on behind the scenes that you can sometimes turn to your advantage: Java has no multidimensional arrays at all, only one-dimensional arrays.
Multidimensional arrays are faked as "arrays of arrays."
For example, the balances array in the preceding example is actually an array that contains ten elements, each of which is an array of six floating-point numbers (Figure 3.15).
The expression balances[i] refers to the ith subarray—that is, the ith row of the table. It is itself an array, and balances[i][j] refers to the jth element of that array.
Since rows of arrays are individually accessible, you can actually swap them!
double[] temp = balances[i];
balances[i] = balances[i + 1];
balances[i + 1] = temp;
It is also easy to make "ragged" arrays—that is, arrays in which different rows have different lengths. Here is the standard example. Let us make an array in which the element at row i and column j equals the number of possible outcomes of a "choose j numbers from i
numbers" lottery.
1
1 1
1 2 1
1 3 3 1
1 4 6 4 1
1 5 10 10 5 1
1 6 15 20 15 6 1
As j can never be larger than i, the matrix is triangular. The ith row has i + 1 elements. (We allow choosing 0 elements; there is one way to make such a choice.) To build this ragged array, first allocate the array holding the rows.
int[][] odds = new int[NMAX + 1][];
Next, allocate the rows.
for (int n = 0; n <= NMAX; n++)
odds[n] = new int[n + 1];
Now that the array is allocated, we can access the elements in the normal way, provided we
do not overstep the bounds.
for (int n = 0; n < odds.length; n++)
for (int k = 0; k < odds[n].length; k++)
{
// compute lotteryOdds
. . .
odds[n][k] = lotteryOdds;
}
Listing 3.9 gives the complete program.
C++ Note
In C++, the Java declaration
double[][] balances = new double[10][6]; // Java
is not the same as
double balances[10][6]; // C++
or even
double (*balances)[6] = new double[10][6]; // C++
Instead, an array of ten pointers is allocated:
double** balances = new double*[10]; // C++
Then, each element in the pointer array is filled with an array of six numbers:
for (i = 0; i < 10; i++)
balances[i] = new double[6];
Mercifully, this loop is automatic when you ask for a new double[10][6]. When you want ragged arrays, you allocate the row arrays separately.
You have now seen the fundamental programming structures of the Java language. The next chapter covers object-oriented programming in Java.