CODEWITHSUNDEEP
javaswingloopsnullpointerexceptionjoptionpane
anthraxX
anthraxX

Reputation: 3

Condition with Swing Options

I get NullPointerException when I press the CANCEL Option in my Swing.

Code:

while(!input.equals(CANCEL_OPTION));

And I get a message that it says incompatible types....

And here's my full code..

do
    {
    input=JOptionPane.showInputDialog("Enter a String.");


    for(int i =input.length()-1;i>=0;i--)
    {           
        result = result + (input.charAt(i));
        disp.append("\n").append(result);
    }       

    JOptionPane.showMessageDialog(null,input+"\n"+disp+"\n"+"\n"+result);
    input="";
    disp.setLength(0);
    result="";
    }
    while(!input.equals(CANCEL_OPTION));

And the error code or something..

Exception in thread "main" java.lang.NullPointerException
at Reverse.main(Reverse.java:15)
Java Result: 1

Upvotes: 1

Views: 81

Answers (4)

jarnthrax
jarnthrax

Reputation: 146

just use try catch statement..

try
    {
    do
    {
        //your codesss......
    }
    while(!input.equals(null));
    }
    catch(Exception ex)
    {
     ////code..
    }

hope it help.

Upvotes: 0

christopher
christopher

Reputation: 27356

Swap your do while for a simple while. The difference is, a do while will run once, before it hits the test condition. Your code is receiving the CANCEL_OPTION value which is null, attempting to use it, then testing if it is null. You need to move your test to the start.

Example 

input=JOptionPane.showInputDialog("Enter a String.");

while(input != null) {
    // Your code here.
}

NOTE: This will give the user the option to cancel should they wish to. If they do, it will skip processing the user input.

Upvotes: 0

René Link
René Link

Reputation: 51403

As the javadoc of JOptionPane.showInputDialog() states

Returns user's input, or null meaning the user canceled the input

if you want the user to enter something.

do {
    input = JOptionPane.showInputDialog("Enter a String.");
} while (input == null); // user can't cancel

for (int i = input.length() - 1; i >= 0; i--) {
    result = result + (input.charAt(i));
    disp.append("\n").append(result);
}

....

Upvotes: 5

Salah
Salah

Reputation: 8657

When you press the cancel button the JOptionPane returns nothing, so your input is equal null. and you code will already enter the do scoop, and when your code reach the line :

for(int i =input.length()-1;i>=0;i--)

the input is already null.

i prefer to add after the that line the follwing:

if(input == null) {
            break;
        }

Upvotes: 0

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