How to compare equality of lists of arrays with modern Java?
The for
loop at least can be streamified, leading to:
return (list1.size()==list2.size() &&
IntStream.range(0, list1.size())
.allMatch(i -> Arrays.equals(list1.get(i), list2.get(i)));
1) Solution based on Java 8 streams:
List<List<String>> first = list1.stream().map(Arrays::asList).collect(toList());
List<List<String>> second = list2.stream().map(Arrays::asList).collect(toList());
return first.equals(second);
2) Much simpler solution (works in Java 5+):
return Arrays.deepEquals(list1.toArray(), list2.toArray());
3) Regarding your new requirement (to check the contained String arrays length first), you could write a generic helper method that does equality check for transformed lists:
<T, U> boolean equal(List<T> list1, List<T> list2, Function<T, U> mapper) {
List<U> first = list1.stream().map(mapper).collect(toList());
List<U> second = list2.stream().map(mapper).collect(toList());
return first.equals(second);
}
Then the solution could be:
return equal(list1, list2, s -> s.length)
&& equal(list1, list2, Arrays::asList);
using zip
(which originates from lambda b93) function from https://stackoverflow.com/a/23529010/755183, code could look like:
boolean match = a.size() == b.size() &&
zip(a.stream(), b.stream(), Arrays::deepEquals).
allMatch(equal -> equal)
update
in order to check size of arrays first and then content this could be a solution to consider
final boolean match = a.size() == b.size()
&& zip(a.stream(), b.stream(), (as, bs) -> as.length == bs.length).
allMatch(equal -> equal)
&& zip(a.stream(), b.stream(), Arrays::deepEquals).
allMatch(equal -> equal);
You could use a stream if the lists are random access lists (so that a call to get
is fast - generally constant time) leading to:
//checks for null and size before
boolean same = IntStream.range(0, list1.size()).allMatch(i -> Arrays.equals(list1.get(i), list2.get(i)));
However, you might give as parameters some implementations that are not (such as LinkedLists). In this case, the best way is to use the iterator explicitly. Something like:
boolean compare(List<String[]> list1, List<String[]> list2) {
//checks for null and size
Iterator<String[]> iteList1 = list1.iterator();
Iterator<String[]> iteList2 = list2.iterator();
while(iteList1.hasNext()) {
if(!Arrays.equals(iteList1.next(), iteList2.next())) {
return false;
}
}
return true;
}
You could stream over one list and compare to each element of the other by using an iterator:
Iterator<String[]> it = list1.iterator();
boolean match = list1.size() == list2.size() &&
list2.stream().allMatch(a -> Arrays.equals(a, it.next()));
Using an iterator instead of the get(index)
method on the first list is better because it doesn't matter whether the list is RandomAccess
or not.
Note: this only works with a sequential stream. Using a parallel stream will lead to wrong results.
EDIT: As per the question last edit, which indicates it would be better to check the length of every pair of arrays in advance, I think it could be achieved with a slight modification to my previous code:
Iterator<String[]> itLength = list1.iterator();
Iterator<String[]> itContents = list1.iterator();
boolean match =
list1.size() == list2.size()
&&
list2.stream()
.allMatch(a -> {
String[] s = itLength.next();
return s == null ? a == null :
a == null ? s == null :
a.length == s.length;
})
&&
list2.stream()
.allMatch(a -> Arrays.equals(a, itContents.next()));
Here I'm using two iterators and am streaming list2
twice, but I see no other way to check all lengths before checking the contents of the first pair of arrays. Check for lengths is null-safe, while check for contents is delegated to the Arrays.equals(array1, array2)
method.