我可以根据特定条件对每个循环进行迭代吗？

Question

Suppose there are two lists of two classes say Class A and Class B respectively listOfClassesA and listOfClassesB .

Now I need to iterate these lists in a single loop on specific condition that suppose boolean variable test is true than iterate list A otherwise B, So I want something like below

Boolean test = true;
For( test ? A a:B b   :   test ? listOfClassesA : listOfClassesB){
     String id = test?a.getId():b.getId();
}

I have this requirement because in almost both cases almost all code are same so I dont want to repeat loop two times, but I didn't find any way to do this. Is there any way to achieve this?

Answer 1

No, Java does not let you write a loop like that. What you can do is defining a generic method ^* , and put a call to that method in a body of your loop. This way you would need to repeat only the loop header, and not the code inside the loop body.

if (test) {
    for (A a : listOfA) loopBody(a);
} else {
    for (B b : listOfB) loopBody(b);
}
...
private static <T> void loopBody(T item) {
    ...
}

---

^* If the two classes are related, a method that takes their common superclass would work as well.

Answer 2

You can implement a new class (can be anonymous) implementing the Iterable interface that will return your iterator responsible for iterating over listOfClassesA or listOfClassesB . However, in the definition of your iterator you can can only use the class A and B common super type.

That is what I had on mind:

interface I { }
class A implements I { }
class B implements I { }

class MyIterator implements Iterable<I> {
    private List<I> list;

    public MyIterator(boolean test, List<A> listOfClassesA, List<B> listOfClassesB) {
        super();
        this.list = new ArrayList<I>(test ? listOfClassesA : listOfClassesB);
    }

    @Override
    public Iterator<I> iterator() {
        return list.iterator();
    }   
}

public class Test {
    public static void main(String[] args) throws Exception {
        List<A> listOfClassesA = new ArrayList<A>();
        List<B> listOfClassesB = new ArrayList<B>();

        for(I elm : new MyIterator(true, listOfClassesA, listOfClassesB)) {
            // elm is of I type 
        }
    }
}