How to output an error message to the user and repeat to allow them to enter the word again if the user enters more than one word

Please Help!
If the user enters more than one word, output an error message to the user and repeat to allow them to enter the word again.

This is the code I have:

public void computeLetters(){
	for(int i = 0; i < term.length()-1; i++){
		if(term.charAt(i)==' ' && term.charAt(i+1)!=' ')
		numberWords = 1;
	}

	for(int numberWords = 1; numberWords > 0;){
		JOptionPane.showMessageDialog(null,"Please try again");
		JOptionPane.showInputDialog(null,"Enter a word");
		numberWords = 0;
	}

	for(int numberWords = 0; numberWords < 1;){
		for(int j = 0; j < term.length()-1; j++){
						numberLetters++;
		}
	}
}

A loop should work...

public void computeLetters()
{
    while (true) {
        // Get user input
        String term = JOptionPane.showInputDialog("Enter a word");
        if (null == term) return;
        // Check if more than 1 word (has a space)
        String [] words = term.split(" ");
		// Single word?
		if (words.length == 1) {
			// Get letter count
			int numberLetters = words[0].length();
            // Do something else here???
            // Done
            return;
        }
        // Show error message
        JOptionPane.showMessageDialog(null, "Please try again");
        // And try again...
    }
}

Edit based on comment using only if statements, loops or chars & strings (and ints).

public void computeLetters()
{
    int numberLetters = 0;
	while (numberLetters == 0) {
		// Get user input
		String term = JOptionPane.showInputDialog("Enter a word");
		if (null != term) {
			// Check if more than 1 word (has a space)
			for (int i = 0; i < term.length(); i++) {
				if (' ' == term.charAt(i)) {
					numberLetters = 0;
					break;
				}
				numberLetters += 1;
			}
		}
		if (numberLetters == 0) {
			JOptionPane.showMessageDialog(null, "Please try again");
		}
	}
	
	// Do something with numberLetters here....
}

If this method is trying to get a single word from the user, validate it is one word, then count the number of letters in that word. This is how I would do it.

public int computeLetters()
 {
   
   String myWord = JOptionPane.showInputDialog("enter a single word.");
   while(myWord.contains(" "))
   {
    JOptionPane.showMessageDialog("Cannot be more than one word.");
    myWord = JOptionPane.showInputDialog("enter a single word.");
   }
  return myWord.toCharArray().length;
}

public void computeLetters(){
    numberWords = term.trim().split("\\s+").length;


    if(numberWords > 0 ){
        JOptionPane.showMessageDialog(null,"Please try again");
        term = JOptionPane.showInputDialog(null,"Enter a word");
        computeLetters();
    }

}

Number of words check, changed to a single line of code.

Next, the for loop in your code can be replaced by a simple 'if' as shown.

Then the next loop would have been an infinite loop. Just calling the function again after getting value from the user will do!