Java Collection comparison ---- collection (continued), java ---- Collection

Java Collection comparison ---- collection (continued), java ---- Collection

Set Interface

The Set interface is rarely mentioned in class, so there are not many understandings of it. The elements in the class that implement the set interface are not repeated. It depends on map. In fact, the saved value is the saved key. In map, the key will not be repeated, so there will be no repeated elements in set. Its implementation class has three: hashSet, TreeSet, LinkedHashSet.

1 public static void main (String [] args) {2 // Set <Integer> set = new HashSet <Integer> (); 3 // Set <Integer> set = new TreeSet <Integer> (); 4 Set <Integer> set = new LinkedHashSet <Integer> (); 5 set. add (null); 6 Random rd = new Random (); 7 System. out. println ("add order:"); 8 for (int I = 0; I <20; I ++) {9 int x = rd. nextInt (30); 10 set. add (x); 11 System. out. print (x + ","); 12} 13 System. out. println (); 14 15 System. out. println ("traversal order:"); 16 Iterator iterator = set. iterator (); 17 while (iterator. hasNext () {18 System. out. print (iterator. next () + ","); 19} 20}

 

 

HashSet: It is unordered and can save null. The sequence and traversal result are as follows:

LinkedHashSet: It maintains the order of adding elements and can save null. The order of adding and traversing results are as follows:

TreeSet: It is an ordered set. elements are arranged in order, but null cannot be saved. The order of adding and traversing results are as follows:

1 public class Model implements Comparable {2 String str; 3 int num; 4 @ Override 5 public int compareTo (Object arg0) {6 Model tmp = (Model) arg0; 7 int cmp = this. num-tmp. num; 8 if (cmp = 0) {9 return str. compareTo (tmp. str); 10} 11 return cmp; 12} 13} 14 15 public static void main (String [] args) {16 TreeSet <Model> ts = new TreeSet <Model> (); 17 Model m2 = new Model (); 18 m2.str = "121"; 19 m2.num = 1; 20 ts. add (m2); 21 22 Model m3 = new Model (); 23 m3.str = "121"; 24 m3.num = 2; 25 ts. add (m3); 26 27 Model m1 = new Model (); 28 m1.str = "321"; 29 m1.num = 1; 30 ts. add (m1); 31 32 Iterator <Model> it = ts. iterator (); 33 while (it. hasNext () {34 Model tmp = it. next (); 35 System. out. println (tmp. str + ":" + tmp. num); 36} 37}