JAVA - how do i keep adding value

Question

I have this button and a text field and i wanted to add value on the variable when the button is clicked everything is working apart from I'm unable to add value to string variable

For example if i put value 20 on tempvalue string it should have 20 and i put 30 it should have 50 but what i get is null2050.
I tried += operator which didn't work.

Isn't there any operator that keep adding value on top it or do i have to write new method ?

private String tempvalue;

btnEnter.addActionListener(new ActionListener() {
    public void actionPerformed(ActionEvent e) {
        String getTxt = textField.getText();
        tempvalue += getTxt;

        System.out.println(tempvalue);

    }
});

You show way too little code for us to see what is going wrong. What is "tempValue" ? What is "gtTxt" ? Are they declared, are they initialized ? Do they have a value they refer to ? — Stultuske
– Stultuske, Commented Mar 26, 2015 at 13:13

Loki · Accepted Answer · 2015-03-26 13:35:19Z

4

You get Strings from Textfields.

String input = getTxt;

You have to parse the String to an integer or any other number type.

int value = Integer.parseInt(input);

Then you can do calculations.

You should also always check if the User Input really is a number. Use try/catch to avoid wrong input:

int value = 0;
int firstValue = 5; //example variable
try{
    value = Integer.parseInt(input);
}catch(Exception e1){
    System.out.println("Your input could not be parsed to a number");
}
int result = firstValue + value; //always be sure all your values are numbers and not strings
System.out.println("Result: "+result);

In total:

private int tempvalue = 0;

btnEnter.addActionListener(new ActionListener() {
    public void actionPerformed(ActionEvent e) {
        String getTxt = textField.getText();
        int value = 0;

        try{
            value = Integer.parseInt(getTxt);
        }catch(Exception e1){
            System.out.println("Your input could not be parsed to a number");
        }

        tempvalue += value;

        System.out.println("Result: "+tempvalue);

        }
    });
}

edited Mar 26, 2015 at 13:35

answered Mar 26, 2015 at 13:16

Loki

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5 Comments

WESTKINz Over a year ago

thanks i will try your method. i'm used to visual basic and now i have to learn java which is just different for me so im struggling with java atm T_T.

candied_orange Over a year ago

I don't see where firstValue is being declared

Loki Over a year ago

@CandiedOrange firstValue was just an example to show the try/catch. The applied example for the user is the total example. But I declared it now.

WESTKINz Over a year ago

thanks it works just change "catch(Exception e)" to "catch(Exception e1)" and it worked

Loki Over a year ago

Oh... I oversaw the ActionEvent, sorry

abarisone · Accepted Answer · 2015-03-26 13:17:25Z

1

You're simply concatenating your Strings. In fact you start from null then you add 20 but as a String and then 30 but always as a String. Transform in each step into a number and then you have your result done.

answered Mar 26, 2015 at 13:17

abarisone

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Comments

e2a · Accepted Answer · 2015-03-26 13:44:16Z

1

As commented by @Jesper the code are concatenating strings and not applying computations such as sum, subtraction and son on ...

So, try out to change the code to convert from java.lang.String to java.langInteger

Integer tempValue += new Integer( getText );
System.out.println( tempvalue );

or using a static method from Integer wrap class

Integer tempValue += Integer.parseInt( getText );
System.out.println( tempvalue );

or then using a int Java type (with auto-box automatically)

int tempValue += Integer.parseInt( getText ).intValue();
System.out.println( tempvalue );

Be careful about string to integer conversions. This can rise a NumberFormatException on runtime.

answered Mar 26, 2015 at 13:44

e2a

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