Make String from String[] and int

Question

String callsign;
String airlines[] = {"DLH","BER","TUI","EZY","ACA","AAL","FDX","SKW","ABY","SWR"};
public void assignCallsign()
{
    Random r = new Random();
    int airline = r.nextInt(10);
    int number = r.nextInt(900) + 100;
    callsign = airlines[airline] + number;
}

The String Array airlines[] contains 3 letters designating an airline. The random integer airline is used to choose one of those airlines. The random integer number should designate the last 3 characters of an airplanes callsign.

I'm trying to get an output like "BER219", "AAL814" and so on, but upon executing the class, the String callsign is still null :/

I probably meant running, the german word translates into executing though :') [As in assignCallsign();] — Tom
– Tom, Commented Mar 10, 2015 at 15:38
It isn't possible to tell what the problem is without a runnable example. — David Conrad
– David Conrad, Commented Mar 10, 2015 at 15:53

peppermcknight · Accepted Answer · 2015-03-10 15:56:57Z

2

Java passes variables by value. If you are testing the value of callsign variable outside this function then it will be null because you have set it to null outside of the assignCallsign method.

To remedy this, you can either:

return the callsign value from the function and set a variable with it.

public String assignCallSign() {
return airlines[airline] + number;
}
String callsign = assignCallSign()

make callsign a member variable of the class, and your code will function as you expect:
```
private String callsign;
```

edited Mar 10, 2015 at 15:56

answered Mar 10, 2015 at 15:39

peppermcknight

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1 Comment

meister_reineke Over a year ago

just to state the obvious: your second code sniplet returns a String and is therefore not public VOID assignCallSign()

Adam · Accepted Answer · 2015-03-10 15:40:25Z

It is difficult to see the issue without seeing the class that is using it.

Speculating this is part of some "Flight" object. This demonstrates the callsign being set and displayed correctly.

public static void main(String[] args) {
    Flight flight = new Flight();
    flight.assignCallsign();
    System.out.println(flight);
}

private static class Flight {
    private static final String AIRLINES[] = { "DLH", "BER", "TUI", "EZY", "ACA", "AAL", "FDX", "SKW", "ABY", "SWR" };
    private String callsign;
    public void assignCallsign() {
        Random r = new Random();
        int airline = r.nextInt(10);
        int number = r.nextInt(900) + 100;
        callsign = AIRLINES[airline] + number;
    }
    @Override
    public String toString() {
        return "Flight [callsign=" + callsign + "]";
    }
}

Output

Flight [callsign=SKW534]

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