Two-dimensional arrays in Java - Cheat Sheet & Quick Revision

overview

Recall & Review

beginner

What is a two-dimensional array in Java?

A two-dimensional array in Java is like a table with rows and columns. It stores data in a grid format, where each element is accessed by two indexes: one for the row and one for the column.

beginner

How do you declare a two-dimensional array of integers with 3 rows and 4 columns in Java?

You declare it like this:
int[][] array = new int[3][4];
This creates a 3x4 grid of integers.

beginner

How do you access the element in the second row and third column of a 2D array named 'matrix'?

You use matrix[1][2]. Remember, counting starts at 0, so the second row is index 1 and the third column is index 2.

intermediate

How can you find the number of rows and columns in a two-dimensional array named 'data'?

The number of rows is data.length. The number of columns in the first row is data[0].length. This assumes all rows have the same number of columns.

beginner

What happens if you try to access an index outside the bounds of a two-dimensional array in Java?

Java throws an ArrayIndexOutOfBoundsException. This means you tried to use a row or column number that does not exist in the array.

How do you declare a 2D array of doubles with 5 rows and 2 columns in Java?

A double arr[][] = new double[2][5];

B double[][] arr = new double[5][2];

C double arr = new double[5,2];

D double arr[5][2];

What is the index of the first element in a two-dimensional array?

A 1,0

B 1,1

C 0,0

D 0,1

If you have int[][] grid = new int[4][3];, what is grid.length?

A 4

B 3

C 7

D 12

How do you access the element in the last row and last column of a 2D array named 'table'?

A table[table.length-1][table[0].length-1]

B table[table.length][table[0].length]

C table[0][0]

D table[table.length+1][table[0].length+1]

What exception is thrown if you try to access an invalid index in a 2D array?

A NullPointerException

B ClassCastException

C IllegalArgumentException

D ArrayIndexOutOfBoundsException

Explain how to declare, initialize, and access elements in a two-dimensional array in Java.

Describe how to find the size (number of rows and columns) of a two-dimensional array and why it matters.