Course:
Learn Java Programming
In this lesson, you will learn.
This structure is particularly useful when you need to store tabular data, like a grid or a spreadsheet, where you have rows and columns.
The syntax for declaring a 2-D array in Java is similar to that of a one-dimensional array, but with an additional set of square brackets.
dataType[][] arrayName;
Here, dataType can be any valid Java data type, and arrayName is the variable name for the array.
arrayName = new dataType[rows][columns];
This creates a 2-D array with the specified number of rows and columns.
int twoD[][] = new int[3][3];
The below figure shows the memory representations of the 2-D Array
Static initialization is when the size and elements of the array are known beforehand.
You can initialize a 2-D array inline while declaring it:
int[][] matrix = {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}};
Dynamic initialization is when the elements of the array are assigned values after declaration.
int[][] arr = new int[3][3]; // 3x3 array
arr[0][0] = 1;
arr[0][1] = 2;
arr[0][2] = 3;
// and so on for the rest of the elements...
Below is the conceptual view of elements of 2-D arrays.
Since a 2D array is an array of arrays, you can also initialize it by directly creating arrays for each of its elements.
int[][] arr = new int[3][];
arr[0] = new int[]{1, 2, 3};
arr[1] = new int[]{4, 5, 6};
arr[2] = new int[]{7, 8, 9};
package ch6;
public class Basic2DArray {
public static void main(String[] args) {
int[][] matrix = {
{1, 2, 3},
{4, 5, 6},
{7, 8, 9}
};
for (int i = 0; i < matrix.length; i++) {
for (int j = 0; j < matrix[i].length; j++) {
System.out.print(matrix[i][j] + " ");
}
System.out.println();
}
}
}
1 2 3
4 5 6
7 8 9
package ch6;
public class Sum2DArray {
public static void main(String[] args) {
int[][] numbers = {
{1, 2, 3},
{4, 5, 6},
{7, 8, 9}
};
int sum = 0;
for (int[] row : numbers) {
for (int num : row) {
sum += num;
}
}
System.out.println("Sum of all elements: " + sum);
}
}
Sum of all elements: 45
Note: Memory allocation for a multidimensional array requires specifying the first dimension, allowing for the separate allocation of remaining dimensions, as demonstrated in the code.
Example
int twoD[][] = new int[4][];
twoD[0] = new int[1];
twoD[1] = new int[2];
twoD[2] = new int[3];
twoD[3] = new int[4];
The below figure shows the conceptual view of a 2-D Jagged Array.
package twodarray;
public class JaggedArray {
public static void main(String[] args) {
int[][] jaggedArray = new int[3][];
jaggedArray[0] = new int[2];
jaggedArray[1] = new int[3];
jaggedArray[2] = new int[1];
// Initializing values
int num = 1;
for (int i = 0; i < jaggedArray.length; i++) {
for (int j = 0; j < jaggedArray[i].length; j++) {
jaggedArray[i][j] = num++;
}
}
// Printing the jagged array
for (int[] row : jaggedArray) {
for (int value : row) {
System.out.print(value + " ");
}
System.out.println();
}
}
}
Output
1 2
3 4 5
6
package twodarray;
public class WeeklyHours {
public static void main(String[] args) {
// Creating a jagged array for hours
// worked on different days of the week
double[][] weekHours = {
{8.5}, // Monday
{8.0, 3.5}, // Tuesday (two shifts)
{8.0}, // Wednesday
{8.0, 4.0}, // Thursday (two shifts)
{6.5}, // Friday
{}, // Saturday (no work)
{} // Sunday (no work)
};
// Displaying hours
String[] days = {"Monday", "Tuesday", "Wednesday",
"Thursday", "Friday", "Saturday", "Sunday"};
for (int i = 0; i < weekHours.length; i++) {
System.out.print(days[i] + ": ");
for (double hours : weekHours[i]) {
System.out.print(hours + " ");
}
System.out.println();
}
}
}
Output
Monday: 8.5
Tuesday: 8.0 3.5
Wednesday: 8.0
Thursday: 8.0 4.0
Friday: 6.5
Saturday:
Sunday:
This example demonstrates a jagged array representing grades of students in different subjects, where each subject might have a different number of assessments.
package twodarray;
public class StudentGrades {
public static void main(String[] args) {
// Grades for three subjects, each having
// a different number of assessments
int[][] grades = {
{87, 92}, // Grades in Subject 1 (e.g., Math)
{78, 85, 90}, // Grades in Subject 2 (e.g., Science)
{85, 80} // Grades in Subject 3 (e.g., History)
};
// Subject names
String[] subjects = {"Math", "Science", "History"};
// Displaying grades for each subject
for (int i = 0; i < grades.length; i++) {
System.out.print(subjects[i] + " Grades: ");
for (int grade : grades[i]) {
System.out.print(grade + " ");
}
System.out.println();
}
}
}
Output
Math Grades: 87 92
Science Grades: 78 85 90
History Grades: 85 80
Objective: Write a Java program that takes two 2D arrays (matrices) of the same dimensions as input and outputs their sum. Assume the size of both matrices is 3×3 for simplicity.
Sample Input:
Matrix 1:
1 2 3
4 5 6
7 8 9
Matrix 2:
9 8 7
6 5 4
3 2 1
Sample Output:
10 10 10
10 10 10
10 10 10
Objective: Create a Java program that calculates the sum of the elements on the diagonal of a square matrix. For an extra challenge, calculate both the primary and secondary diagonals.
Sample Input:
1 2 3
4 5 6
7 8 9
Sample Output:
Primary Diagonal Sum: 15
Secondary Diagonal Sum: 15
Objective: Write a Java method that checks if a given matrix is symmetric. A matrix is symmetric if it is equal to its transpose.
Sample Input:
1 2 3
2 4 5
3 5 6
Sample Output:
True
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