Set＜String＞

HashSet

It has several characteristics , First, it doesn't have duplicate elements , Secondly, it is disordered , In addition, it can contain empty elements .

Set<String> str = new HashSet<String>();
 str.add("first");// Additive elements 
 str.add("second");
 str.remove("first");// Remove elements 
 // Traverse , By iterator 
 Iterator<String> it = str.iterator();

TreeSet

First of all, it can not write the same data , Second, you cannot write null data , The last data written is ordered

Set<String> str = new TreeSet<String>();
 str.add("first");// Additive elements 
 str.add("second");
 str.add("third");
 // Traverse , By iterator 
 Iterator<String> it = str.iterator();