Java Program example to demonstrate String charAt method

Program

import java.util.Scanner;
public class StringCharAt
{
	public static void main(String args[])
	{
		String str = "Welcome to oodlescoop";
		int pos;
		Scanner sc = new Scanner(System.in);
		System.out.println("Enter an integer number:");
		pos = sc.nextInt();
		System.out.println("Character at pos " + pos + " is: " + str.charAt(pos));
	}
}

Purpose

This Java program demonstrates how to use the charAt method of the String class to extract a character at a specific position in a string.

Code Walkthrough

1. Import the Scanner Class

import java.util.Scanner;

• Allows the program to take input from the user.

2. Class Definition

public class StringCharAt

• Defines the class StringCharAt.

3. Main Method

public static void main(String args[])

• The entry point of the program where execution begins.

4. Declare and Initialize Variables

String str = "Welcome to oodlescoop";
int pos;

• str: Stores the string "Welcome to oodlescoop".
• pos: Will store the position entered by the user to fetch the character from the string.

5. Take Input from the User

Scanner sc = new Scanner(System.in);
System.out.println("Enter an integer number:");
pos = sc.nextInt();

• Step 1: Create a Scanner object to take input.
• Step 2: Prompt the user to enter an integer, which will represent the index of the character in the string str.
• Step 3: Store the entered integer in the variable pos.

6. Fetch and Display the Character

System.out.println("Character at pos " + pos + " is: " + str.charAt(pos));

• Step 1: Call str.charAt(pos) to get the character at the specified position (pos).
  • The charAt method returns the character at the zero-based index pos in the string.
• Step 2: Display the result to the console.

Important Notes

1. Indexing in Java Strings:

   • Java strings are zero-indexed, meaning:
     • The first character is at index 0.
     • The second character is at index 1, and so on.

2. Potential Exception:

   • If the user enters a position (pos) that is negative or greater than or equal to the string’s length, the program will throw an exception:
     • java.lang.StringIndexOutOfBoundsException.

Example Execution

Input 1

Enter an integer number:
0

Execution Steps

1. The user enters pos = 0.
2. str.charAt(0) retrieves the character at index 0, which is 'W'.

Output 1

Character at pos 0 is: W

Input 2

Enter an integer number:
7

Execution Steps

1. The user enters pos = 7.
2. str.charAt(7) retrieves the character at index 7, which is 't'.

Output 2

Character at pos 7 is: t

Key Methods and Concepts

1. String.charAt(int index):

   • Returns the character at the specified index of the string.
   • Throws an IndexOutOfBoundsException if the index is out of bounds.

2. Input Handling:

   • The program assumes valid input. To handle invalid inputs (like out-of-bounds indexes), you could add error handling.

Enhanced Version with Validation

To make the program robust, you can add validation:

if (pos < 0 || pos >= str.length()) {
    System.out.println("Invalid position. Please enter a number between 0 and " + (str.length() - 1));
} else {
    System.out.println("Character at pos " + pos + " is: " + str.charAt(pos));
}

Output

Enter an integer number:
11
Character at pos 11 is: o