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📜  Java Hashtable类

📅  最后修改于: 2020-10-12 10:09:06             🧑  作者: Mango

Java Hashtable类

Java Hashtable类实现了一个哈希表，该哈希表将键映射到值。它继承了Dictionary类并实现Map接口。

要记住的要点

  • 哈希表是列表的数组。每个列表称为存储桶。桶的位置通过调用hashcode()方法来标识。哈希表包含基于键的值。
  • Java Hashtable类包含唯一的元素。
  • Java Hashtable类不允许使用null键或值。
  • Java Hashtable类已同步。
  • Hashtable类的初始默认容量为11，而loadFactor为0.75。

哈希表类声明

我们来看一下java.util.Hashtable类的声明。

public class Hashtable extends Dictionary implements Map, Cloneable, Serializable

哈希表类参数

让我们看一下java.util.Hashtable类的参数。

  • K ：这是此映射维护的键的类型。
  • V ：这是映射值的类型。

Java Hashtable类的构造方法

Constructor Description
Hashtable() It creates an empty hashtable having the initial default capacity and load factor.
Hashtable(int capacity) It accepts an integer parameter and creates a hash table that contains a specified initial capacity.
Hashtable(int capacity, float loadFactor) It is used to create a hash table having the specified initial capacity and loadFactor.
Hashtable(Map t) It creates a new hash table with the same mappings as the given Map.

Java Hashtable类的方法

Method Description
void clear() It is used to reset the hash table.
Object clone() It returns a shallow copy of the Hashtable.
V compute(K key, BiFunction remappingFunction) It is used to compute a mapping for the specified key and its current mapped value (or null if there is no current mapping).
V computeIfAbsent(K key, Function mappingFunction) It is used to compute its value using the given mapping function, if the specified key is not already associated with a value (or is mapped to null), and enters it into this map unless null.
V computeIfPresent(K key, BiFunction remappingFunction) It is used to compute a new mapping given the key and its current mapped value if the value for the specified key is present and non-null.
Enumeration elements() It returns an enumeration of the values in the hash table.
Set> entrySet() It returns a set view of the mappings contained in the map.
boolean equals(Object o) It is used to compare the specified Object with the Map.
void forEach(BiConsumer action) It performs the given action for each entry in the map until all entries have been processed or the action throws an exception.
V getOrDefault(Object key, V defaultValue) It returns the value to which the specified key is mapped, or defaultValue if the map contains no mapping for the key.
int hashCode() It returns the hash code value for the Map
Enumeration keys() It returns an enumeration of the keys in the hashtable.
Set keySet() It returns a Set view of the keys contained in the map.
V merge(K key, V value, BiFunction remappingFunction) If the specified key is not already associated with a value or is associated with null, associates it with the given non-null value.
V put(K key, V value) It inserts the specified value with the specified key in the hash table.
void putAll(Map t)) It is used to copy all the key-value pair from map to hashtable.
V putIfAbsent(K key, V value) If the specified key is not already associated with a value (or is mapped to null) associates it with the given value and returns null, else returns the current value.
boolean remove(Object key, Object value) It removes the specified values with the associated specified keys from the hashtable.
V replace(K key, V value) It replaces the specified value for a specified key.
boolean replace(K key, V oldValue, V newValue) It replaces the old value with the new value for a specified key.
void replaceAll(BiFunction function) It replaces each entry’s value with the result of invoking the given function on that entry until all entries have been processed or the function throws an exception.
String toString() It returns a string representation of the Hashtable object.
Collection values() It returns a collection view of the values contained in the map.
boolean contains(Object value) This method returns true if some value equal to the value exists within the hash table, else return false.
boolean containsValue(Object value) This method returns true if some value equal to the value exists within the hash table, else return false.
boolean containsKey(Object key) This method return true if some key equal to the key exists within the hash table, else return false.
boolean isEmpty() This method returns true if the hash table is empty; returns false if it contains at least one key.
protected void rehash() It is used to increase the size of the hash table and rehashes all of its keys.
V get(Object key) This method returns the object that contains the value associated with the key.
V remove(Object key) It is used to remove the key and its value. This method returns the value associated with the key.
int size() This method returns the number of entries in the hash table.

Java哈希表示例

import java.util.*;
class Hashtable1{
 public static void main(String args[]){
  Hashtable hm=new Hashtable();

  hm.put(100,"Amit");
  hm.put(102,"Ravi");
  hm.put(101,"Vijay");
  hm.put(103,"Rahul");

  for(Map.Entry m:hm.entrySet()){
   System.out.println(m.getKey()+" "+m.getValue());
  }
 }
}

输出：

103 Rahul
102 Ravi
101 Vijay
100 Amit

Java Hashtable示例：remove()

import java.util.*;
public class Hashtable2 {
   public static void main(String args[]) {
  Hashtable map=new Hashtable();    
 map.put(100,"Amit");  
 map.put(102,"Ravi"); 
 map.put(101,"Vijay");  
 map.put(103,"Rahul");  
 System.out.println("Before remove: "+ map);  
   // Remove value for key 102
   map.remove(102);
   System.out.println("After remove: "+ map);
   }    
}

输出：

Before remove: {103=Rahul, 102=Ravi, 101=Vijay, 100=Amit}
After remove: {103=Rahul, 101=Vijay, 100=Amit}

Java Hashtable示例：getOrDefault()

import java.util.*;
class Hashtable3{
 public static void main(String args[]){
 Hashtable map=new Hashtable();    
 map.put(100,"Amit");  
 map.put(102,"Ravi"); 
 map.put(101,"Vijay");  
 map.put(103,"Rahul");  
 //Here, we specify the if and else statement as arguments of the method
 System.out.println(map.getOrDefault(101, "Not Found"));
 System.out.println(map.getOrDefault(105, "Not Found"));
 }
}

输出：

Vijay
Not Found

Java Hashtable示例：putIfAbsent()

import java.util.*;
class Hashtable4{
 public static void main(String args[]){
 Hashtable map=new Hashtable();    
 map.put(100,"Amit");  
 map.put(102,"Ravi"); 
 map.put(101,"Vijay");  
 map.put(103,"Rahul");  
 System.out.println("Initial Map: "+map);
 //Inserts, as the specified pair is unique
 map.putIfAbsent(104,"Gaurav");
 System.out.println("Updated Map: "+map);
 //Returns the current value, as the specified pair already exist
 map.putIfAbsent(101,"Vijay");
 System.out.println("Updated Map: "+map);
 }
}

输出：

Initial Map: {103=Rahul, 102=Ravi, 101=Vijay, 100=Amit}
Updated Map: {104=Gaurav, 103=Rahul, 102=Ravi, 101=Vijay, 100=Amit}
Updated Map: {104=Gaurav, 103=Rahul, 102=Ravi, 101=Vijay, 100=Amit}

Java Hashtable示例：书

import java.util.*;  
class Book {  
int id;  
String name,author,publisher;  
int quantity;  
public Book(int id, String name, String author, String publisher, int quantity) {  
    this.id = id;  
    this.name = name;  
    this.author = author;  
    this.publisher = publisher;  
    this.quantity = quantity;  
}  
}  
public class HashtableExample {  
public static void main(String[] args) {  
    //Creating map of Books  
    Map map=new Hashtable();  
    //Creating Books  
    Book b1=new Book(101,"Let us C","Yashwant Kanetkar","BPB",8);  
    Book b2=new Book(102,"Data Communications & Networking","Forouzan","Mc Graw Hill",4);  
    Book b3=new Book(103,"Operating System","Galvin","Wiley",6);  
    //Adding Books to map 
    map.put(1,b1);
    map.put(2,b2);
    map.put(3,b3);    
    //Traversing map
    for(Map.Entry entry:map.entrySet()){  
    int key=entry.getKey();
    Book b=entry.getValue();
        System.out.println(key+" Details:");
        System.out.println(b.id+" "+b.name+" "+b.author+" "+b.publisher+" "+b.quantity); 
    }  
}  
}

输出：

3 Details:
103 Operating System Galvin Wiley 6
2 Details:
102 Data Communications & Networking Forouzan Mc Graw Hill 4
1 Details:
101 Let us C Yashwant Kanetkar BPB 8