Trying to compare two attributes from two lists. I usually compare it iterating two lists and comparing each element (which I think is not optimal). Like:

list1.forEach(x -> {
    list2.forEach(y -> {
        if (x.getId().compareTo(y.getId()) == 0) 
            x.setMyAttribute(y.getNameAttribute());
    });
});

Is there a better way to compare a specific attribute from two lists? Still, I don't understand how to work with HashMap, but I want to know if is better use HashMap for this comparing and how to use it.

I guess I can create a HashMap only with id and name (attributes I need).

CodePudding user response：

What you've shared is basically a brute-force solution, which checks every element in the list2 for each element in the list1.

And you were thinking in the right direction. To avoid performing redundant iterations, you can index the contents of list2 by generating a HashMap associating a particular element with its id.

I'll assume that the natural order of id is consistent with it equals/hashCode implementation, i.e. (x.compareTo(y)==0) == (x.equals(y)) because it's a recommended practice (and if you're using as id standard JDK classes like Long, String, UUID it would be the case).

That's how it might be implemented:

List<Foo> list1 = // initializing list1
List<Foo> list2 = // initializing list1
        
Map<ID, Foo> list2FoosById = list2.stream()
    .collect(Collectors.toMap(
        Foo::getId,
        Function.identity(),
        (left, right) -> right // remove merge function if IDs are expected to be unique
    ));

for (Foo x : list1) {
    Foo y = list2FoosById.get(x.getId());                  // retrieving Foo from list2 with the corresponding ID
    if (y != null) x.setMyAttribute(y.getNameAttribute()); // reassign the attribute if the Foo having identical ID exists
}