Use the size() method of the HashMap class in Java to obtain the number of key-value pairs in the map

Use the size() method of the HashMap class in Java to obtain the number of key-value pairs in the map

In the Java programming language, HashMap is a commonly used data structure used to store key-value pairs. It provides fast find, insert, and delete operations, and can quickly locate values ​​based on keys. When using HashMap, we often need to know how many key-value pairs are currently stored in the HashMap. In this case, we can use the size() method of HashMap to obtain the number of key-value pairs in the map.

The size() method of HashMap returns an int type value, indicating the number of key-value pairs in the HashMap. The following is a simple sample code:

import java.util.HashMap;

public class HashMapExample {
    public static void main(String[] args) {
        // 创建一个HashMap对象
        HashMap<String, Integer> hashMap = new HashMap<>();

        // 向HashMap中插入一些键值对
        hashMap.put("apple", 3);
        hashMap.put("banana", 5);
        hashMap.put("orange", 2);

        // 使用size()方法获取HashMap中键值对的数量
        int size = hashMap.size();
        System.out.println("HashMap中键值对的数量为： " + size);
    }
}

In the sample code, we first create a HashMap object and use the put() method to insert three key-value pairs into the HashMap. Then, we use the size() method to get the number of key-value pairs in the HashMap and print the result.

Using the size() method of HashMap can easily obtain the number of key-value pairs stored in the HashMap. In actual programming, we often need to make some judgments or perform some operations based on the number of key-value pairs in the HashMap, such as expanding the HashMap when it reaches a certain capacity, or determining whether the HashMap is empty, etc.

It should be noted that the time complexity of HashMap's size() method is O(1), so no matter how many key-value pairs are stored in HashMap, the operation of obtaining the number of key-value pairs is very fast. of. However, if you need to traverse all key-value pairs in the HashMap, the time complexity will become O(n), where n is the number of key-value pairs stored in the HashMap.

To sum up, using the size() method of HashMap can easily obtain the number of key-value pairs stored in the HashMap. It is a very practical method provided by the HashMap class. In actual programming, we can judge the status of HashMap based on its return value and perform corresponding operations. Whether it is to determine whether the capacity has reached the upper limit or whether the HashMap is empty, it can be achieved through the size() method.

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