CODEWITHSUNDEEP
javastring
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Reputation: 147

Checking String length returns NullPointerException?

For some reason, I am getting a NullPointerException upon checking a String length in an 'if' statement. I am probably doing it wrong, but I wouldn't really know. What the code I am trying to write basically just changes a button's label, but only if the String 'label1' is 0 characters long (or not set), so that it can only be changed once.

import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;

         public class Class1 {

public static String label1;


public static String one = ("Hello");
public static String two = ("Goodbye");

public static void main(String args[]) {

    JFrame frame = new JFrame();
    JPanel pane = new JPanel();
    JButton button = new JButton();


    button.addActionListener(new ActionListener() {

        @Override
        public void actionPerformed(ActionEvent e) {
            try {
            if(label1.length() == 0) {

                label1 = one;
                JButton button = (JButton) e.getSource();
                button.setText(label1);
            }
            if(label1.length() < 0) {

                label1 = two;
                JButton button = (JButton) e.getSource();
                button.setText(label1);
            }
        } catch(Exception ex) {
                System.out.println("ERROR");
                ex.printStackTrace();
            }

        }       
    });

    frame.setSize(350, 350);
    frame.setVisible(true);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    frame.add(pane);
    pane.add(button);


     }

 }

Upvotes: 2

Views: 12369

Answers (5)

Ken
Ken

Reputation: 31161

Always check for null. For example:

if(label != null && label.length() > 0) {...}

Out of interest, when is (label1.length() < 0) ever true?

Upvotes: 0

zero0
zero0

Reputation: 877

In your example, the String object "label1" is not initialized and NULL. This means, if you try to call a method on a non-existing object, the NullPointerException must happen.

To check if there is an Object assigned, try if (label1 != null).

To assign a value, do label1 = "my String" - this initializes the String object.

Upvotes: 0

Daahrien
Daahrien

Reputation: 10320

label1 is null because you didn't initialize it.

label1 = "";

should fix your problem (it will be non-null size 0)

Upvotes: 0

Prasad Kharkar
Prasad Kharkar

Reputation: 13556

Assign some string value to label

public static String label1 = new String("someString");

We get NullPointerException when calling some method on an object reference which has not been assigned any object yet and the reference which is erferring to null

Upvotes: 0

Suresh Atta
Suresh Atta

Reputation: 121998

public static String label1;

to use the line if(label1.length() == 0) { you have to intialize lable1 before

You are not intialised lable! anywhere any using it

Try to change that line 

public static String label="";

or add extra null condition

if(label1 != null && label1.length() == 0) {

Upvotes: 8

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