HashSet
1.it's very important not to set the initial capacity too high (or the load factor too low) if iteration performance is important.
2.is not synchronized
settlement: Set s = Collections.synchronizedSet(new HashSet(...));
3.the fail-fast behavior of iterators should be used only to detect bugs
4.permits the null element
Constructor Summary
HashSet()
Constructs a new, empty set; the backing HashMap instance has default initial capacity (16) and load factor (0.75).
HashSet(Collection<? extends E> c)
Constructs a new set containing the elements in the specified collection.
HashSet(int initialCapacity)
Constructs a new, empty set; the backing HashMap instance has the specified initial capacity and default load factor (0.75).
HashSet(int initialCapacity, float loadFactor)
Constructs a new, empty set; the backing HashMap instance has the specified initial capacity and the specified load factor.
| Method Summary | |
|---|---|
boolean |
add(E e) Adds the specified element to this set if it is not already present. |
void |
clear() Removes all of the elements from this set. |
Object |
clone() Returns a shallow copy of this HashSet instance: the elements themselves are not cloned. |
boolean |
contains(Object o) Returns true if this set contains the specified element. |
boolean |
isEmpty() Returns true if this set contains no elements. |
Iterator<E> |
iterator() Returns an iterator over the elements in this set. |
boolean |
remove(Object o) Removes the specified element from this set if it is present. |
int |
size() Returns the number of elements in this set (its cardinality). |
Sorts the Hashset two different situations:
if sort type is HashSet, it will use default Comparator as nature sort ,sample 1
else sort type is HashSet<T> ,you must implements Comparator as sort way ,sample 2
else you can use Collections.sort(); sample 3
sample 1
HashSet hashSet=new HashSet();
hashSet.add("2");
hashSet.add("a");
hashSet.add("i");
hashSet.add("1");
TreeSet treeSetOne=new TreeSet(hashSet);
Iterator iteratorOne=treeSetOne.iterator();
while(iteratorOne.hasNext()){
System.out.println(iteratorOne.next());
}
sample 2
class Person {
String name;
Integer age;
Person(String name,Integer age){
this.name=name;
this.age=age;
}
}
class MyComparator implements Comparator{
public int compare(Object o1, Object o2) {
// TODO Auto-generated method stub
Person person1=(Person)o1;
Person person2=(Person)o2;
if(person1.age>person2.age){
return 1;
}else if(person1.age<person2.age){
return -1;
}else if(person1.age==person2.age){
if(person1.name.compareTo(person2.name)>0){
return 1;
}
}
return 0;
}
}
public class test {
public static void main(String[] args) {
HashSet<Person> set=new HashSet<Person>();
Person person=new Person("张三",14);
set.add(person);
person=new Person("张三",14);
set.add(person);
person=new Person("李四",43);
set.add(person);
person=new Person("利益",11);
set.add(person);
person=new Person("里尔",24);
set.add(person);
person=new Person("红旗",75);
set.add(person);
person=new Person("试试",24);
set.add(person);
TreeSet<Person> treeSet=new TreeSet<Person>(new MyComparator());
treeSet.addAll(set);
Iterator<Person> i1=treeSet.iterator();
while(i1.hasNext()){
Person t1=i1.next();
System.out.println(t1.name+": "+t1.age);
}
}
}
sample 3