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Assume there is a method provided getNextperson() which gives Person objects which have comparable interface implemented, now read from a file records and sort it and give first 1000 records, write code on the paper
maybe use tree data structure
tree data structure ? i hope u meant to say "TreeSet"; but the question doesnt say that getNextPerson() will be unique. So let us not bank on treeSet. So just add them on to a arrayList and do a Collections.sort(arrayList);
The question is actually quite simple since you were given that Comparable interface is already implemented (eg, public int compareTo(Object o) method is overridden in Person object). First step, 'read from a file all records' and store each Person object returned by getNextPerson() in List records = new ArrayList(). Next step, call Collections.sort(records) to sort (by criteria specified in compareTo() method, such as Last and First name, or any other meaningful parameter). Final step, 'give first 1000 records' -- either by iterating over first 1000 (checking size() >= 1000) or by returning the list consisting of only 1000 records (either copy into a new list with size 1000 or by using removeRange method in ArrayList.