Use java's HashMap.get() function to get elements in a HashMap

Use Java's HashMap.get() function to obtain elements in HashMap

HashMap is one of the commonly used data structures in Java. It is used to store the mapping relationship of key-value pairs. In the actual development process, it is often necessary to obtain specific elements from HashMap. The HashMap class provides the get() method, allowing us to obtain the corresponding value through the key. This article will introduce in detail how to use the get() function of HashMap and provide corresponding code examples.

1. The basic concept of HashMap

HashMap is a data structure based on hash tables, which implements the Map interface. It allows null as a key, and there is no fixed order of key-value pairs. In HashMap, each key corresponds to a value, and the corresponding value can be uniquely determined by the key. The working principle of HashMap is to pass the key to the hash function and calculate the hash value corresponding to the key. Then, the storage location is calculated from the hash value and the key-value pair is stored at that location. When we need to obtain the value corresponding to a certain key, we only need to calculate the hash value corresponding to the key through the same hash function and obtain the stored value at the corresponding position.

2. Use HashMap's get() function to obtain elements

HashMap's get() function receives a key as a parameter and returns the value corresponding to the key. If the key does not exist in the HashMap, null is returned.

The following is a sample code for using the HashMap.get() function to obtain elements:

import java.util.HashMap;

public class HashMapExample {
    public static void main(String[] args) {
        HashMap<String, Integer> map = new HashMap<>();
        
        // 添加键值对
        map.put("apple", 1);
        map.put("banana", 2);
        map.put("orange", 3);
        
        // 获取元素
        int value1 = map.get("apple");
        int value2 = map.get("banana");
        int value3 = map.get("orange");
        
        // 输出结果
        System.out.println("apple对应的值为：" + value1);
        System.out.println("banana对应的值为：" + value2);
        System.out.println("orange对应的值为：" + value3);
    }
}

The above code creates a HashMap object and adds three key-value pairs. Then use the get() function to obtain the values ​​corresponding to "apple", "banana" and "orange" respectively, and print out the results.

3. Notes

When using the get() function of HashMap, you need to pay attention to the following points:

• Make sure that the key exists: Before calling the get() function, you should first determine whether the key exists. You can use the containsKey() function to determine whether a key exists in a HashMap.
• Null pointer exception: If you try to get a key that does not exist, the get() function will return null. Therefore, when using the get() function, you should first perform a non-null judgment to avoid null pointer exceptions.
• Performance considerations: In HashMap, the performance of finding elements is good, with a time complexity of about O(1). Therefore, HashMap is an efficient data structure, especially suitable for quickly finding the value corresponding to a certain key.

The above is the relevant content of using Java's HashMap.get() function to obtain the elements in HashMap. Through the get() function, we can easily obtain the corresponding value according to the key. In actual development, hash tables are a very common and practical data structure. They are very convenient for processing key-value pairs and can greatly improve coding efficiency.

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