Java教程-Java Hashtable类

Java Hashtable类

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

要记住的要点

• Hashtable是一个数组的列表。每个列表称为一个桶(bucket)。通过调用hashcode()方法来确定桶的位置。Hashtable根据键来存储值。
• Java Hashtable类包含唯一的元素。
• Java Hashtable类不允许空键或空值。
• Java Hashtable类是同步的。
• Hashtable类的初始默认容量为11，负载因子为0.75。

Hashtable类声明

让我们看一下java.util.Hashtable类的声明。

public class Hashtable<K, V> extends Dictionary<K, V> implements Map<K, V>, Cloneable, Serializable

Hashtable类参数

让我们看一下java.util.Hashtable类的参数。

• K：此映射维护的键的类型。
• V：此映射将键映射到的值的类型。

Java Hashtable示例

import java.util.*;

class HashtableExample {
  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());
    }
  }
}

输出：

103 Rahul
102 Ravi
101 Vijay
100 Amit

Java Hashtable示例：remove()

import java.util.*;

public class HashtableExample {
  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);

    // 移除键为102的值
    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 HashtableExample {
  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");

    // 在getOrDefault方法中，我们指定了if和else语句作为参数
    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 HashtableExample {
  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);

    // 插入，因为指定的键值对是唯一的
    map.putIfAbsent(104, "Gaurav");
    System.out.println("Updated Map: " + map);

    // 返回当前值，因为指定的键值对已经存在
    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：Book

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) {
    // 创建一个存储Book对象的映射
    Map<Integer, Book> map = new Hashtable<Integer, Book>();

    // 创建Book对象
    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);

    // 将Book对象添加到映射中
    map.put(1, b1);
    map.put(2, b2);
    map.put(3, b3);

    // 遍历映射
    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