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类不允许空键或空值。
• 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<K, V> extends Dictionary<K, V> implements Map<K, V>, Cloneable, Serializable

哈希表类参数

让我们看一下java.util.Hashtable类的参数。

• K：这是此地图维护的键的类型。
• V：这是映射值的类型。

Java Hashtable类的构造方法

Java Hashtable类的方法

Java哈希表示例

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

  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());
  }
 }
}

立即测试

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<Integer, String> map=new Hashtable<Integer, String>();  	  
	 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<Integer, String> map=new Hashtable<Integer, String>();  	  
	 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<Integer, String> map=new Hashtable<Integer, String>();  	  
	 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<Integer, Book> map=new Hashtable<Integer, Book>();  
    //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<Integer, Book> 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