how to use variable inside function

Question

Sorry for asking such a basic question, but I just can't figure it out and I have no idea how to search for it.

I have the following code:

letters = new ArrayList<JButton>();
String[] abc = new String[] {"A", "Á", "B", "C", "D", "E", "É", "F", "G", "H", "I", "Í", "J", "K", "L", "M", "N", "O", "Ó", "Ö", "Ő", "P",
        "Q", "R", "S", "T", "U", "Ú", "Ü", "Ű", "V", "W", "X", "Y", "Z" };
for (Object o: abc)
{
    letters.add(new JButton((String)o));
    int i = letters.size() - 1;
    letters.get(i).setBounds(i%10 * 60 + 40, 350 + ((i / 10) * 50), 55, 45);
    letters.get(i).addActionListener(new ActionListener()
    {
        public void actionPerformed(ActionEvent event)
        {
            System.out.println(o);
        }
    });
    mF.add(letters.get(i));
}

As you can see, I have a for loop and I'd like to use its variable 'o' inside the function. How can I do that? It says:

java: local variable o is accessed from within inner class; needs to be declared final

What exactly does this mean?

make it final, for (final Object o: abc)

Nandkumar Tekale
– Nandkumar Tekale

2013-06-11 09:18:39 +00:00
Commented Jun 11, 2013 at 9:18 — Nandkumar Tekale
– Nandkumar Tekale, Commented Jun 11, 2013 at 9:18
Define one final object and use that object to iterate loop

shreyansh jogi
– shreyansh jogi

2013-06-11 09:19:29 +00:00
Commented Jun 11, 2013 at 9:19 — shreyansh jogi
– shreyansh jogi, Commented Jun 11, 2013 at 9:19

darijan · Accepted Answer · 2013-06-11 09:18:41Z

3

In the for loop, write like this:

for (final Object o: abc)
{
    ...

So the compiler knows that the object o will not change, i.e. you will not assign a new value to it with =.

answered Jun 11, 2013 at 9:18

darijan

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Comments

gma · Accepted Answer · 2013-06-11 09:19:06Z

3

It means that to be accessed in an inner class, the compiler has to be sure that this variable cannot be modified, as the code will be probably be executed asynchronously. change for (Object o: abc) to for (final String o: abc)

answered Jun 11, 2013 at 9:19

gma

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Comments

Sanjaya Liyanage · Accepted Answer · 2013-06-11 09:19:26Z

2

you can not perform System.out.println(o);

as o is not defined as final.

for (final Object o: abc)

answered Jun 11, 2013 at 9:19

Sanjaya Liyanage

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Comments

NimChimpsky · Accepted Answer · 2013-06-11 09:26:45Z

2

letters.get(i).addActionListener(new ActionListener()
{
    public void actionPerformed(ActionEvent event)
    {
        System.out.println(o);
    }
});

Is creating an inner class, simply make the foreach loop use final Object o.

edited Jun 11, 2013 at 9:26

answered Jun 11, 2013 at 9:19

NimChimpsky

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3 Comments

gma Over a year ago

it is not the array instantiation that is creating an inner class, it is the new ActionListener ...

gma Over a year ago

quite sure, yes. new String[] {"A", ....} is just creating an instance, not a Class, new ActionListener(... does as he is overriding actionPerformed method

NimChimpsky Over a year ago

@gma you are aboslutely right, I was thinking of double brace initialization

NickJ · Accepted Answer · 2013-06-11 09:24:58Z

You are using an anonymous inner class, and from within those classes you may access final variables declared outside. Here's an example which will not work because message is not final:

public void myMethod() {
    String message = "You can't see me";

    new SomeInterface() {

        //SomeInterface declares this method:
        public void someMethod() {
            System.out.println(message); //this will not compile
        }
    };
}

But this will work because message is final:

public void myOtherMethod() {
    final String message = "You can see me now!";

    new SomeInterface() {

        //SomeInterface declares this method:
        public void someMethod() {
            System.out.println(message); //this will work!
        }
    };
}

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