CODEWITHSUNDEEP
javacollections
Ken de Guzman
Ken de Guzman

Reputation: 2810

Does addAll() return false?

While looking at the source code of CollectionUtils addAll method. I notice it uses |= symbol

public static <T> boolean addAll(Collection<? super T> c, T... elements) {
 boolean result = false;
 for (T element : elements)
    result |= c.add(element);

 return result;
}

From my understanding |= is a bitwise or operator and just a shorthand of result = result|c.add(element), so for example : 

System.out.println(false|true); //true
System.out.println(false|false); //false
System.out.println(true|false); //true
System.out.println(true|true); //true

This means that if any item successfully added, it will return true. Now I've been wondering is there will be an instance, it will return false? If not why it has a return?

Upvotes: 8

Views: 5139

Answers (4)

Gayathri
Gayathri

Reputation: 1

Operation : all of the elements in the specified collection appends to the end of this list. if the collection changed as a result of the call then it returns true. if the collection not changed as a result of the call then it returns false.

Upvotes: 0

Zaheer Ahmed
Zaheer Ahmed

Reputation: 28528

|= is bitwise OR

| (Bitwise OR) sets a bit to 1 if one or both of the corresponding bits in its operands are 1, and to 0 if both of the corresponding bits are 0. In other words, | returns one in all cases except where the corresponding bits of both operands are zero. The resulting bit pattern is the "set" (1 or true) bits of any of the two operands. This property is used to "set" or "turn on" a "flag" (bit set to one) in your flags or options variable regardless of whether that flag was set previously or not. Multiple flag bits can be set if a combo MASK is defined.

// To set or turn on a flag bit(s)
flags = flags | MASK;
// or, more succintly
flags |= MASK;

So your code is equivalent to:

boolean result = false;
for (T element : elements){
   result = result | c.add(element);
}
return result;

Initially result will be false and as one of elements successfully get added to collection will be set to true i.e c.add(element);. So it will return true if one of elements get added.

Upvotes: 3

Suresh Atta
Suresh Atta

Reputation: 121998

From docs of addAll()

returns:

true if the collection changed as a result of the call.

If the collections not at all modified, then false.

Consider the below program.(follow the result in comments)

public static void main(String[] args) {
        List<String> l1= new ArrayList<String>();
        l1.add("test");
        List<String> l2= new ArrayList<String>();
        System.out.println(l2.addAll(l1));//true
        System.out.println(l2.addAll(l1));//true
        
        Set<String> s1= new HashSet<String>();
        s1.add("test");
        Set<String> s2= new HashSet<String>();
        System.out.println(s2.addAll(s1));//true
        System.out.println(s2.addAll(s1));//false
        }

Upvotes: 2

Eran
Eran

Reputation: 393781

If all the elements to be added were already in the Collection (prior to the call to addAll), and the Collection doesn't allow duplicates, it will return false, since all the individual add method calls would return false. This is true for Collections such as Set.

For other Collections, add always returns true, and therefore addAll would return true, unless you call it with an empty list of elements to be added.

Upvotes: 8

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