Improve my if-statement to prevent double checks

Question

I write the following version of an if-statement very often. And every time, I hate the way that I make it, but I don't know how to improve it.

For example, I have two ArrayList's. If one of them is not empty, then I convert it to a StringBuilder and send it per mail. The problem is, first I check if one of them is not empty to know if I should send a mail, and then I check it again to know which of them is not empty. Example code:

if(!array1.isEmpty() || !array2.isEmpty()){
   // I need to distinguish the StringBuilder
   StringBuilder array1string = new StringBuilder();
   StringBuilder array2string = new StringBuilder();
   // so one of them is not empty. but which? now I must double check it
   if(!array1.isEmpty()){
      for(String string : array1){
         array1string.append(string);
      }
   }
   if(!array2.isEmpty()){
      for(String string : array2){
         array2string.append(string);
      }
   }
   mail.sendMail("Array1: " + array1string == null?"-":array1string.toString() 
      + ", Array2: " + array2string == null?"-":array2string.toString());
}

So is there a way to prevent this double check?

Answer 1

Your code can be shortened to:

if(!array1.isEmpty() || !array2.isEmpty()) {
    mail.sendMail("Array1: " + getContent(array1) + ", Array2: " + getContent(array2));
}

When you have:

String getContent(ArrayList<String> list) {
    if(!list.isEmpty()) {
        StringBuilder sb = new StringBuilder();
        for(String string : list) {
            sb.append(string);
        }
        return sb.toString();
    }
    return "-";
}

Answer 2

Adam's answer is good, but I just couldn't resist modifying his method a bit:

<E> String getArrayContent(Collection<E> list) {
    if(list.isEmpty())
        return "-";

    StringBuilder sb = new StringBuilder();
    for(E value : list) {
        sb.append(value);
    }
    return sb.toString();
}

This method has the following modifications compared to Adam's:

• It uses Collection because you stated that you were using ArrayList in the question. (And this method works with any type of collection, not only List or ArrayList). Collection is the superinterface which declares both isEmpty() and extends Iterable
• It uses generics so it can be used for any kind of collection, not only Collection<String>
• I prefer to return early to simplify readability and not have to indent code more than necessary

If you also want a method for regular arrays, that method could call the method for collections by converting the array to a list:

<E> String getArrayContent(E[] array) {
    return getArrayContent(Arrays.asList(array));
}