Question

Part III. Common String Errors

The following two example programs demonstrate common errors that often occur when programming with Strings.

public class StringErrors1 {     public static void main(String [] args) {       String greeting = "hello world";       greeting.toUpperCase();       System.out.println(greeting);     } }

15. Type above code in Dr. Java or Eclipse. Compile and run the program a few times.
    1. What does this program do?

2. How might you make the program more user-friendly?

16. Now run the program two more times. The first time, type “a b c” and then the <enter> key when you run the program. The second time, just type the <enter> key.
    1. What runtime error message do you see?

2. What line of code caused the error message?

3. How did you know which line of code caused the error message?

Answer 1

What does this program do?

The program creates a String ‘hello world’ and store in a variable, convert to upper case (but not store the resultant String) and display the String. As a result, the program prints ‘hello world’ (as in original text). But storing the result in a variable (or the same variable itself), the program displays ‘HELLO WORLD’.

How might you make the program more user-friendly?

By asking user to enter a text and convert to upper case and display the resultant text, preferably in a GUI.

Updated program:

import javax.swing.JOptionPane;

public class StringErrors1 {

      public static void main(String[] args) {

            // prompting and reading a text from user

            String greeting = JOptionPane.showInputDialog("Enter some text: ");

            // converting to upper case and storing the resultant text in greeting

            greeting = greeting.toUpperCase();

            // displaying the converted text

            JOptionPane.showMessageDialog(null, greeting);

      }

}

If we enter “a b c”, the program will display “A B C”, if we do leave the input empty and press enter, no text will be displayed as the input is an empty String. (Same thing happens if we use nextLine() method in Scanner class or readLine() method in BufferedReader class), but if we press cancel button on JOptionPane window or closed the window, an Exception will be thrown.

The thrown exception will be NullPointerException.

The line ‘greeting=greeting.toUpperCase();’ is the one throwing the exception.

This is because when we click cancel or close the JoptionPane window, it returns null value, so that null value will be stored in greeting. If the greeting variable is null, we cannot call toUpperCase() method in it. If we do, it will cause NullPointerException.