9.1.1. 2D Arrays

Arrays in Java can store many items of the same type. You can even store items in two-dimensional (2D) arrays which are arrays that have both rows and columns. A row has horizontal elements. A column has vertical elements. In the picture below there are 3 rows of lockers and 6 columns.

Figure 1: Lockers in rows and columns

Two dimensional arrays are especially useful when the data is naturally organized in rows and columns like in a spreadsheet, bingo, battleship, theater seats, classroom seats, or a picture. In battleship, letters map to the rows (A is the first row, B is the second row, and so on) and the column indices start with 1.

9.1.2. Array Storage

Many programming languages actually store two-dimensional array data in a one-dimensional array. The typical way to do this is to store all the data for the first row followed by all the data for the second row and so on. This is called row-major order. Some languages store all the data for the first column followed by all the data for the second column and so on. This called column-major order.

Figure 1: A 2D array stored in row-major order or column-major order as a 1D array.

9.1.3. How Java Stores 2D Arrays

Java actually stores two-dimensional arrays as arrays of arrays. Each element of the outer array has a reference to each inner array. The picture below shows a 2D array that has 3 rows and 7 columns. Notice that the array indices start at 0 and end at the length - 1.

Figure 3: Java arrays of arrays

On the exam assume that any 2 dimensional (2D) array is in row-major order. The outer array can be thought of as the rows and the inner arrays the columns. On the exam all inner arrays will have the same length even though it is possible in Java to have inner arrays of different lengths (also called ragged arrays).

Check your understanding

Try to answer the following questions. Click on the value or values to select them. Click again to unselect a value.

9-1-1: Click on all the values in the row at index 2Rows are horizontal and columns are vertical and both start with index 0.

8	-2	3	-1
4	5	0	-7
2	-3	-4	-5

9-1-2: Click on all the values in the column at index 1Rows are horizontal and columns are vertical and both start with index 0.

8	-2	3	-1
4	5	0	-7
2	-3	-4	-5

9-1-3: Click on the value at row index 2 and column index 1Rows are horizontal and columns are vertical and both start with index 0.

8	-2	3	-1
4	5	0	-7
2	-3	-4	-5

9-1-4: Click on the value at row index 0 and column index 2Rows are horizontal and columns are vertical and both start with index 0.

8	-2	3	-1
4	5	0	-7
2	-3	-4	-5

9-1-5: Click on the value at row index 1 and column index 1Rows are horizontal and columns are vertical and both start with index 0.

8	-2	3	-1
4	5	0	-7
2	-3	-4	-5

9.1.4. Declaring 2D Arrays

To declare a 2D array, specify the type of elements that will be stored in the array, then ([][]) to show that it is a 2D array of that type, then at least one space, and then a name for the array. Note that the declarations below just name the variable and say what type of array it will reference. The declarations do not create the array. Arrays are objects in Java, so any variable that declares an array holds a reference to an object. If the array hasn’t been created yet and you try to print the value of the variable, it will print null (meaning it doesn’t reference any object yet). Try the the following in DrJava’s interaction pane.

int[][] ticketInfo;
String[][] seatingChart;

Check your understanding

To create an array use the new keyword, followed by a space, then the type, and then the number of rows in square brackets followed by the number of columns in square brackets, like this new int[numRows][numCols].

The code below creates a 2D array with 2 rows and 3 columns named ticketInfo and a 2D array with 3 rows and 2 columns named seatingChart. The number of elements in a 2D array is the number of rows times the number of columns.

ticketInfo = new int [2][3];
seatingChart = new String [3][2];

Check your understanding

Coding Exercise

What will the following code print out? Can you change ticketInfo to be an array of 5 rows and 10 columns? Can you declare another array called studentNames that has 10 rows and 5 columns? The length property of arrays will be explained in the next lesson.

public class TicketInfo { public static void main(String[] args) { // declare arrays int[][] ticketInfo = new int[2][3]; System.out.println(ticketInfo.length + " rows"); System.out.println(ticketInfo[0].length + " columns"); } } ==== import static org.junit.Assert.*; import org.junit.*;; import java.io.*; public class RunestoneTests extends CodeTestHelper { public RunestoneTests() { super("TicketInfo"); } @Test public void test1() { String output = getMethodOutput("main"); String expected = "5 rows\n10 columns"; boolean passed = output.contains(expected);passed = getResults(expected, output, "Changed ticketInfo to 5 rows and 10 columns", passed); assertTrue(passed); } @Test public void test2() { String expected = "studentNames = new String[10][5]"; boolean passed = checkCodeContains(expected); assertTrue(passed); } }

This lesson is continued on the next page.

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