HashSet class
The HashSet Class HashSet extends AbstractSet and implements the Set interface.
It creates a collection that uses a hash table for storage. HashSet is a generic class that has this declaration:
class HashSet<E>
E specifies the type of objects that the set will hold.
A demonstration of a hashset with String elements unordered
import java.util.HashSet;
import java.util.Set;
public class Main {
public static void main(String[] args) {
Set<String> ss = new HashSet<String>();
String[] fruits = { "apples", "pears", "grapes", "bananas", "kiwis", "pears", null };
for (String fruit : fruits){
ss.add(fruit);
}
for (String s : ss){
System.out.print(s + " ");
}
}
}
null grapes bananas kiwis pears apples
A custom class not overriding hashCode()
import java.util.HashSet;
import java.util.Set;
public class Main {
public static void main(String[] args) {
Set<Item> sp = new HashSet<Item>();
sp.add(new Item("Q"));
sp.add(new Item("V"));
sp.add(new Item("E"));
sp.add(new Item("M"));
Item p1 = new Item("5");
sp.add(p1);
Item p2 = new Item("b");
sp.add(p2);
System.out.println(p1.equals(p2));
System.out.println(sp);
}
}
class Item {
private String name;
Item(String name) {
this.name = name;
}
@Override
public boolean equals(Object o) {
if (!(o instanceof Item))
return false;
Item p = (Item) o;
return p.name.equals(name);
}
String getName() {
return name;
}
@Override
public String toString() {
return name;
}
}
false
[5, V, E, Q, M, b]
A custom class overriding hashCode()
import java.util.HashSet;
import java.util.Set;
public class Main {
public static void main(String[] args) {
Set<Item> sp = new HashSet<Item>();
sp.add(new Item("A"));
sp.add(new Item("X"));
sp.add(new Item("E"));
sp.add(new Item("B"));
Item p1 = new Item("5");
sp.add(p1);
Item p2 = new Item("5");
sp.add(p2);
System.out.println(p1.equals(p2));
System.out.println(sp);
}
}
class Item {
private String name;
Item(String name) {
this.name = name;
}
@Override
public boolean equals(Object o) {
if (!(o instanceof Item))
return false;
Item p = (Item) o;
return p.name.equals(name);
}
String getName() {
return name;
}
@Override
public int hashCode() {
return name.hashCode();
}
@Override
public String toString() {
return name;
}
}
true
[E, A, 5, B, X]
Constructor
HashSet()- Creates a new, empty set; the backing HashMap instance has default initial capacity (16) and load factor (0.75).
HashSet(Collection<? extends E> c)- Creates a new set containing the elements in the specified collection.
HashSet(int initialCapacity)- Creates a new, empty set; the backing HashMap instance has the specified initial capacity and default load factor (0.75).
HashSet(int initialCapacity, float loadFactor)- Creates a new, empty set; the backing HashMap instance has the specified initial capacity and the specified load factor.
HashSet supports the following methods:
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).
Revised from Open JDK source code