Python:

string = "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"

# Count the occurrences of a specific character
count_a = string.count('a')
print("Number of 'a' occurrences:", count_a)

# Check if the string starts with a specific substring
starts_with_a = string.startswith('aa')
print("Starts with 'aa':", starts_with_a)

# Replace a specific substring with another
new_string = string.replace('a', 'b')
print("New string:", new_string)

JavaScript:

var string = "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa";

// Count the occurrences of a specific character
var count_a = string.split('a').length - 1;
console.log("Number of 'a' occurrences:", count_a);

// Check if the string starts with a specific substring
var starts_with_a = string.startsWith('aa');
console.log("Starts with 'aa':", starts_with_a);

// Replace a specific substring with another
var new_string = string.replace(/a/g, 'b');
console.log("New string:", new_string);

Java:

public class Main {
   public static void main(String[] args) {
       String string = "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa";

       // Count the occurrences of a specific character
       int count_a = string.length() - string.replace("a", "").length();
       System.out.println("Number of 'a' occurrences: " + count_a);

       // Check if the string starts with a specific substring
       boolean starts_with_a = string.startsWith("aa");
       System.out.println("Starts with 'aa': " + starts_with_a);

       // Replace a specific substring with another
       String new_string = string.replace("a", "b");
       System.out.println("New string: " + new_string);
   }
}