Core Java (23) Set (set Interface)

I have previously written a blog post about hash table, which can be used to implement hashset and hashmap ).

Hashset

The hashset <t> class has four constructor types:

Constructor and description
HashSet() Constructs a new, empty set; the backingHashmapInstance 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 backingHashmapInstance has the specified initial capacity and default load factor (0.75 ).
HashSet(int initialCapacity, float loadFactor) Constructs a new, empty set; the backingHashmapInstance has the specified initial capacity and the specified load factor.

All of its methods:

Modifier and type	Method and description
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 thisHashsetInstance: The elements themselves are not cloned.
boolean	contains(Object o) ReturnsTrueIf this set contains the specified element.
boolean	isEmpty() ReturnsTrueIf 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 ).

Treeset

A treeset is an ordered collection ). This class is sorted in a certain order, and the red and black trees are used.

Constructor:

TreeSet() Constructs a new, empty tree set, sorted according to the natural ordering of its elements.

TreeSet(Collection<? extends E> c) Constructs a new tree set containing the elements in the specified collection, sorted according toNatural OrderingOf its elements.

TreeSet(Comparator<? super E> comparator) Constructs a new, empty tree set, sorted according to the specified comparator.

TreeSet(SortedSet<E> s) Constructs a new tree set containing the same elements and using the same ordering as the specified sorted set.

There are two sorting methods:

1. Implement the comparable Interface

By default, treeset assumes that the inserted elements implement the comparable interface. This interface defines a method:

Public interface comparable <t> {

Int comparto (t other );

}

For example, for the string class, if A is equal to B, A. comparto (B) returns 0. If A> B, a positive value is returned.

However, this method has the disadvantage that only one sorting method can be used for a class. For example, if the employee class wants to sort by ID or salary, is it swollen? This requires the second method.

2. Implement the comparator Interface

Notice the constructorTreeset(Comparator <?
Super E> comparator), you only need to pass a comparator object to the treeset.

For example:

Set <employee> otherstaff = new treeset <employee> (new comparator <employee> () {public int compare (Employee A, employee B) {return (INT) (. getsalary ()-B. getsalary (); // sort by salsay size }});

Test procedure

Test Programs for hashset and treeset:

Hashcode and equals methods are rewritten here.

Package COM. xujin; import Java. util. comparator; import Java. util. hashset; import Java. util. iterator; import Java. util. set; import Java. util. treeset; public class implements listtest {public static void main (string... arg) {employee Jim = new employee ("Jim", 8000); employee Bob = new employee ("Bob", 9000); Manager Jin = new manager ("gin ", 10000,400 0 ); *************************** * *******/set <employee> Staff = new hashset <employee> (); staff. add (Jim); staff. add (Bob); staff. add (Jin); staff. add (Bob); // The hashcode is the same and the system is not allowed to be added. out. println (staff. size (); // 3 iterator <employee> iter = staff. iterator (); While (ITER. hasnext () {employee e = ITER. next (); system. out. println (E );} *************************** * *******/set <employee> otherstaff = new treeset <employee> (new comparator <employee> () {public int compare (Employee A, employee B) {return (INT) (. getsalary ()-B. getsalary (); // sort by salsay size}); otherstaff. add (Bob); otherstaff. add (Jim); system. out. println (otherstaff); // [ID: 1 Name: Jim salary: 8000.0, ID: 2 Name: Bob salary: 9000.0]} class employee {public employee (string name) {This. name = Name; id = nextid; nextid ++;} Public String tostring () {return "ID:" + ID + "name:" + name + "salary: "+ salary;} public int hashcode () {return ID;} public Boolean equals (employee e) {If (ID-e. GETID () = 0) return true; return false;} public employee (string name, double salary) {This (name); // call another constructor this. salary = salary;} // defines the accessors method public final string getname () {return name;} public double getsalary () {return salary;} public final int GETID () {return ID;} // defines the modifier method public final void setname (string name) {This. name = Name;} public final void setsalary (double salary) {This. salary = salary;} public final void raisesalary (double percent) {This. salary * = (1 + percent);} // defines the variable private string name = ""; // instance domain initialization private double salary; private int ID; private Static int nextid = 1;} class manager extends employee {Public Manager (string name, double salary, double bonus) {super (name, salary ); // super is used in the constructor. You can call the constructor setbonus (bonus);} Public String tostring () {return Super. tostring () + ", bonus:" + bonus;} public double getbonus () {return bonus;} // rewrite the getsalary method public double getsalary () {double basesalary = super. getsalary (); // call the getsalary method of the superclass return basesalary + bonus;} public void setbonus (double bonus) {This. bonus = bonus;} private double bonus ;}

Usage Policy

Hashset and treeset

The hashset has a higher speed and does not need to be sorted.

If you do not need to sort data, you do not need to use treeset.