java Hashset去重原理及HashMap key唯一原理

如下,set中添加重复元素是不可以的,如下
php被添加了2次,但是输出的时候只有一次,那么其去重的原理是什么呢?

public class Test {public static void main(String[] args) {HashSet<String> set=new HashSet<>();set.add("hello");set.add("html");set.add("php");set.add("php");System.out.println(set);//[php, html, hello]}
}

查看源码,调用了map的put方法

 public boolean add(E e) {return map.put(e, PRESENT)==null;}

继续溯源

 public V put(K key, V value) {return putVal(hash(key), key, value, false, true);}

继续溯源.可以看出依据hash以及equals判断的.

/*** Implements Map.put and related methods** @param hash hash for key* @param key the key* @param value the value to put* @param onlyIfAbsent if true, don't change existing value* @param evict if false, the table is in creation mode.* @return previous value, or null if none*/final V putVal(int hash, K key, V value, boolean onlyIfAbsent,boolean evict) {Node<K,V>[] tab; Node<K,V> p; int n, i;if ((tab = table) == null || (n = tab.length) == 0)n = (tab = resize()).length;if ((p = tab[i = (n - 1) & hash]) == null)tab[i] = newNode(hash, key, value, null);else {Node<K,V> e; K k;if (p.hash == hash &&((k = p.key) == key || (key != null && key.equals(k))))e = p;else if (p instanceof TreeNode)e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);else {for (int binCount = 0; ; ++binCount) {if ((e = p.next) == null) {p.next = newNode(hash, key, value, null);if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1sttreeifyBin(tab, hash);break;}if (e.hash == hash &&((k = e.key) == key || (key != null && key.equals(k))))break;p = e;}}if (e != null) { // existing mapping for keyV oldValue = e.value;if (!onlyIfAbsent || oldValue == null)e.value = value;afterNodeAccess(e);return oldValue;}}++modCount;if (++size > threshold)resize();afterNodeInsertion(evict);return null;}

如果是自己自定义的类,就不能利用set去重了,需要自己重写继承自Object类的hashCode方法以及equals方法,这样就可以自定义set添加时去重的逻辑了. 为什么用了hashCode方法还要再次重写equals方法呢?因为哈希值相同的未必是同一对象。所以还需要equals方法进一步确认.

如下实例,wangwu只能加进去一个,因为重写了hashCode方法和equals方法,如果不是这样,就会加进去2个wangwu哦.

public class Test {public static void main(String[] args) {HashSet<Student> set = new HashSet<>();set.add(new Student("bob", 23));set.add(new Student("lisi", 21));set.add(new Student("wangwu", 20));set.add(new Student("zhaoliu", 23));set.add(new Student("wangwu", 20));System.out.println(set);// [lisi, wangwu, zhaoliu, bob]}
}
class Student {private String name;private int age;Student(String name, int age) {this.name = name;this.age = age;}@Overridepublic int hashCode() {return name.hashCode()+age*23;}@Overridepublic boolean equals(Object obj) {if(!(obj instanceof Student)){throw new RuntimeException();}Student stu=(Student)obj;return this.name.equals(stu.name)&&this.age==stu.age;}@Overridepublic String toString() {return name;}
}

HashMap key唯一原理

HashMap的key是不能重复的,如果重复,会覆盖.如下. 有两个key为ss,但是输出只有最后一个键值对.

public class Test {public static void main(String[] args) {HashMap<String,String> map=new HashMap<>();map.put("ss","dd");map.put("ss","good");System.out.println(map);//{ss=good}}
}

HashMap保证key唯一的原理和HashSet是一样的.如果是自定义的类,也需要重写Object类的hashCode方法和equals,方法,才可以自定义自己的HashMap的key唯一的原理.
举例如下

public class Test {public static void main(String[] args) {// 保证键不重复的原理和HashSet一样 int hashCode、 boolean equals//key是可以为任意Object的HashMap<Student, String> map = new HashMap<>();map.put(new Student("lisi", 20), "beijing");map.put(new Student("zhangsan", 20), "beijing");map.put(new Student("xiaobai", 20), "beijing");map.put(new Student("liuqian", 27), "beijing");map.put(new Student("xiaobai", 20), "shenzhen");//key看出最后一个xiaobai只进去了一个//如果不定义hashcode和equals方法，结果是{liuqian27=beijing, zhangsan20=beijing, lisi20=beijing, xiaobai20=beijing, xiaobai20=shenzhen}//可以看到xiaobao有两个，因为两个key都是new的，肯定不是同一对象System.out.println(map);//{lisi20=beijing, zhangsan20=beijing, xiaobai20=shenzhen, liuqian27=beijing}//遍历输出map.forEach((t,u)-> System.out.println(t+":"+u));}
}
class Student {String name;int age;public Student(String name, int age) {super();this.name = name;this.age = age;}public int hashCode() {return name.hashCode() + age * 36;}// 姓名年龄相同的认为是相同的键public boolean equals(Object obj) {if (!(obj instanceof Student))throw new ClassCastException();Student stu = (Student) obj;return this.name.equals(stu.name) && this.age == stu.age;}public String toString() {return name + age;}
}

scala中用HashSet去重

在scala中应用HashSet,去重的原理也是一样的.

object Test extends App {private val set = new util.HashSet[Person]()set.add(new Person("wangwu", 23))set.add(new Person("wangwu", 23))//因为重写了hashCode以及equals方法,所以在向set中添加时就会自动判断,重复的内容不会被添加//如果不重写hashCode和equals方法,set中输出就会包含两个wangwu,因为都是new的,属于不同的对象println(set)//[wangwu--23]}class Person(var name: String, var age: Int) {override def toString: String = {s"$name--$age"}override def hashCode(): Int = {return 23 * age}override def equals(obj: Any): Boolean = {val person: Person = obj.asInstanceOf[Person]return person.name == this.name && person.age == this.age}
}

case class与HashSet去重

如果用case class,就可以不用重写,case class默认会重写hashCode和equals方法

object Test extends App {private val set = new util.HashSet[Person]()set.add(Person("wangwu", 23))set.add(Person("wangwu", 23))//case class 会自动写hashCode和equals方法,所以无需自己写,不会添加进重复的内容println(set)//[Person(wangwu,23)]}case class Person(name:String,age:Int)

scala 可变set与去重

scala mutable.Set如果不重写hashCode换equals方法,同样会输出两个,不会去重

object Test extends App {private val set = mutable.Set[Person]()set.add(new Person("wangwu", 23))set.add(new Person("wangwu", 23))//scala mutable.Set如果不重写hashCode换equals方法,同样会输出两个println(set)//Set(wangwu--23, wangwu--23)}class Person(var name: String, var age: Int) {override def toString: String = {s"$name--$age"}//  override def hashCode(): Int = {//    return 23 * age
//  }
//
//  override def equals(obj: Any): Boolean = {//    val person: Person = obj.asInstanceOf[Person]
//    return person.name == this.name && person.age == this.age
//  }
}

如果是case class就不用写了

object Test extends App {private val set = mutable.Set[Person]()set.add(new Person("wangwu", 23))set.add(new Person("wangwu", 23))//即使是自定义类,case class也可以去重println(set)//Set(Person(wangwu,23))
}case class Person(name:String,age:Int)

总结

hashmap key唯一和hashset 去重的原理是一样的
去重的保证是在add 或put元素时调用了重写的HashCode方法以及equals方法.
scala 中set去重原理是java一样
scala case class会自动完成hashCode以及equals方法,用起来会比较方便