Checking whether intersection of two lists is valid, returning intersection itself not necessary

Question

I have two lists ( List1 and List2) containing objects of type Column. Column has two private variables - Name and Type. The intersection of List1 and List2 is done by Column's name. For example:

List1 = { Column (name = "A", type = "STRING"), Column (name = "B", type = "INT") }
List2 = { Column (name = "A", type = "STRING"), Column (name = "D", type = "INT") }

then intersection of List1 and List2 = { Column (name = "A", type = "STRING") }

How can I efficiently write code in Java 8 without using a double for loop (O(n^2)) to check whether an intersection is valid (if it is not valid, then I would like to know the Column names along with conflicting types, if it is valid, just returning True is good enough for me - I don't need the intersection itself). A valid intersection is defined as follows: the columns in the intersection done by comparing the field name, must have the same type. For example, the following is invalid:

List1 = { Column (name = "A", type = "STRING"), Column (name = "B", type = "INT") }
List2 = { Column (name = "A", type = "INT"), Column (name = "D", type = "INT") }

then an invalid intersection of List1 and List2 = { Column (name = "A", type = "STRING") } since the types do not match. Another way of thinking about it is Given a list of columns in a List1 and a list of columns in List2, I would like to check whether columns which are in both List1 and List2 have the same type.

First attempt:

 for (final Column newColumn : newMySQLTableMetaData.getColumns()) {
            for (final Column originalColumn : originalMySQLTableMetaData.getColumns()) {
                if (newColumn.getName().equals(originalColumn.getName())) {
                    if (!newColumn.getType().equals(ColumnTypeConverter.toLogicalColumnType(originalColumn.getType()))) {
                        throw new UploadException("The column types have a mismatch. Original column" +
                                " named " + originalColumn.getName() + " a type of " + originalColumn.getType().toString() + " " +
                                "while new column with the same name has a type of " + newColumn.getType().toString());
                    }
                }
            }
        }

can't you use sets? [This](http://stackoverflow.com/questions/4349369/list-intersection-in-java)could help you. — Olimpiu POP, Sep 12 '14 at 14:55
wouldn't set intersection only give me a correct intersection without listing the columns which had the same name but not the same type? — Tad, Sep 12 '14 at 14:56

maaartinus · Accepted Answer · 2014-09-12T16:12:24.417

Put all elements from list1 into a HashMap index by name. Go throw the second list and look if there's a matching column from the first one (O(1) per element). If so, check for conflict.

private List<String> conflictList = new ArrayList<String>();

private final Map<String, Column> map = new HashMap<>();
for (Column c1: list1) map.put(c1.name, c1);
for (Column c2: list2) {
    Column c1 = map.put(c2.name, c2);
    if (c1==null) continue;
    if (c1.type.equals(c2.type)) continue;
    conflictList.add(c1); // or add the name only or both elements or whatever
}

If conflictList is empty, then you've won. If you don't care about the details, return early instead of processing the remaining elements.

Checking whether intersection of two lists is valid, returning intersection itself not necessary

1 Answers1