Java while-loop create text fields from string characters

Question

I'm currently working on text fields, and I'm using NetBeans with Java. The assignment requires a user to enter a sentence. The program reads the sentence, and displays a frame with the sentence that the user entered, but with each character in its own text field. Below is my code along with some comments that point out where I'm getting errors.

package lab.pkg2.pkg2;

import java.awt.FlowLayout;
import java.util.Scanner;
import javax.swing.JFrame;
import javax.swing.JTextField;

public class Lab22 {

    public static void main(String[] args) {

        // initialize string 
        String str1;

        Scanner sc = new Scanner(System.in); // scanner reads string
        System.out.print("Enter a sentence");
        str1 = sc.next();
        System.out.println(str1);

        // Create frame 
        JFrame frame = new JFrame("Characters in Text field :");
        frame.setLayout(new FlowLayout());

        // Create a text field with a single character
        While(str1 == length) { // error
            JTextField tf = new JTextField(4);
            System.out.print(str1.length());

            String ch = str1;
            tf.setText(String.valueOf(ch));

            str1++; // error
        }

        String ch = str1;
        frame.add(tf); // error

        // make another text field with a single character
        // Set up the frame and displays it
        frame.pack();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);

    }

}

Answer 1

While(str1 == length) Explanation:

There are multiple reasons why the line While(str1 == length) results in errors.

1. While is not the correct syntax, while with a lowercase w is the correct syntax.

2. str1 is a String, String is an Object, when checking for equality of objects through the operator ==, the references (locations in memory) are checked, not the contents of the objects. To check for equality between objects use the .equals() method.
   Note: Using the == operator between objects will not result in a compilation error. But if used to check for object equality then it will result in a logical error, which is usually way worse than a compilation error.

3. You never declared a variable length, I'm assuming you meant to write the loop's condition to resemble while(str1.length() != 0). If you did mean to write while(str1 == length), assuming length is an int, it does not make since to compare an object (String) to a primitive (int) and it's not allowed.

str1++ Explanation:

The reason str1++; results in error is similar to the second point above. str1 is a String which is an Object. Most primitives (int, float, double, ...) have +, -, *, /, ++, --, ** ... operators, but most objects don't, with the exception of a few like Iterators, and the objects that do are set in stone. Languages like C++ allow you to overload operators but Java does not. Thus, str1++ does not work, you have to use the methods supplied by the String class to alter the contents of str1.

frame.add(tf) Explanation:

The reason frame.add(tf) results in error is because tf is no longer in scope. tf was declared inside the while loop, anything declared inside curly braces ({ }) cannot be referenced outside of the curly braces. If you need to alter a variable within curly braces and then use it outside of curly braces, then declare it before the curly braces.

Future Logical Error:

The assignment requires a user to enter a sentence.

If the words in the sentence are separated by spaces then you're going to wonder why only the first word in the sentence is being processed. The reason lies in the difference between the next and nextLine methods of the Scanner class. next reads until a specified delimiter is encountered (space by default), nextLine reads until the operating system's new-line character is encountered. Thus you're going to want to use:

str1 = sc.nextLine();

Here's a good StackOverflow question with answers, in case you run into any trouble while using the Scanner class' next methods: Skipping nextLine() after using next(), nextInt() or other nextFoo() methods

Side Note:

You can use a while loop to achieve the desired results, but a for loop is much easier to implement in this situation. If you use a for loop then you won't have to truncate str1 on every iteration. A for loop example:

// create text fields with a single character
for(int i = 0; i < str1.length(); i++) {
    JTextField tf = new JTextField(4);
    char ch = str1.charAt(i);
    
    // set the newly created text fields text to ch
    tf.setText(ch + "");
    
    // add the text field to frame while it's still in scope
    frame.add(tf);
}

However, if you have to use a while loop then something similar to the following will work:

// create text fields with a single character
while(str1.length() != 0) {
    JTextField tf = new JTextField(4);
    char ch = str1.charAt(0);
    
    // chop off first (zeroeth) character from str1
    // unless it's the last character
    str1 = (str1.length() > 1) ? str1.substring(1) : "";
    
    // set the newly created text fields text to ch
    tf.setText(ch + "");
    
    // add the text field to frame while it's still in scope
    frame.add(tf);
}

Working main Method Code:

public static void main(String[] args) {
    // initialize string 
    String str1;

    Scanner sc = new Scanner(System.in); // scanner reads string
    System.out.print("Enter a sentence");
    str1 = sc.next();
    
    System.out.println(str1);

    // create and set up frame 
    JFrame frame = new JFrame("Characters in Text field :");
    frame.setLayout(new FlowLayout());
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    
    // create text fields with a single character
    for(int i = 0; i < str1.length(); i++) {
        JTextField tf = new JTextField(4);
        char ch = str1.charAt(i);
        
        // output the character for debugging?
        System.out.println(ch);
        
        // set the newly created text fields text to ch
        tf.setText(ch + "");
        
        // add the text field to frame while it's still in scope
        frame.add(tf);
    }
    
    // let frame's layout manager do it's thing
    frame.pack();
    // show the frame
    frame.setVisible(true);
}