1.

int counter = 0;
for (int j = 0; j < text.length(); j++)
	if (text.charAt(j) == ch)
		counter++;
return counter;

2.

while (message.length() < 10) 
	message = JOptionPane(.....);

OR

String message;
do {
	message = JOptionPane(.....);
} while (message.length() < 10);

3.

boolean valid;
do {
	try {
		System.out.println("How many messages?");
		numMsgs = scan.nextInt();
		if (numMsgs < 1) {
			System.out.println("ERROR");
			valid = false;
		}
		else
			valid = true;
	}
	catch (InputMismatchException e) {
		System.out.println("ERROR");
		valid = false;
	}
	scan.nextLine();	// clear input buffer
} while (!valid);

4.

int[] countArray = new int[26];
for (int i = 0; i < text.length; i++) {
	char ithchar = text.charAt(i);
	if (Character.isLetter(ithchar))
		countArray[ithChar-'A']++;
}
for (int i = 0; i < countArray.length; i++) {
	System.out.println((char)('A'+i) + " " + countArray[i]);