Interpretation of Java documentation: Detailed explanation of the usage of the put() method of the HashMap class

HashMap is a commonly used data structure in Java. It implements the Map interface and provides a storage method based on key-value pairs. When using HashMap, the put() method is one of the commonly used operations. This article will introduce in detail the usage of the put() method of the HashMap class.

The put() method of the HashMap class can store the specified key-value pair into the Map. If the key already exists, the original value will be overwritten. The syntax of the put() method is as follows:

V put(K key, V value);

Among them, K represents the type of key, and V represents the type of value. In the put() method, the corresponding hash bucket is found through the key and the key-value pair is stored in the bucket. Next, we explain the usage of the put() method in detail through specific code examples.

1. Create HashMap object

Before calling the put() method, we need to create a HashMap object first. In the following code, a HashMap object map is created, the key type is Integer, and the value type is String.

HashMap<Integer, String> map = new HashMap<Integer, String>();

2. Add key-value pairs

Use the put() method to add key-value pairs to a HashMap. The following code shows how to add a key-value pair with key 1 and value "Java".

map.put(1, "Java");

In the above code, the key is 1 and the value is "Java", that is, the key 1 and the value "Java" are stored in the HashMap.

3. Overwrite the original value

If the added key already exists in the HashMap, the put() method will overwrite the original value. The code below shows how to overwrite the original value.

map.put(1, "C++");

In the above code, the key 1 already exists in the HashMap, and the value "C" overwrites the original value "Java" through the put() method.

4. Return value

The put() method will return the overwritten original value. If the original value does not exist, it will return null. The following code shows the use of the return value of the put() method.

String oldValue = map.put(1, "Python");
System.out.println(oldValue);

In the above code, we overwrite the value "C" of key 1 to "Python" and assign the original value "C" to the variable oldValue. After running the program, the console will output the original value "C".

5. It is recommended to use generics

After Java 5, the generic mechanism was introduced, which can specify the type during the creation process of HashMap, and when calling the put() method , avoiding type conversion operations on key values. Therefore, it is recommended to use the generic mechanism when using HashMap. The following code shows how to use generics to create a HashMap object.

HashMap<String, Integer> scoreMap = new HashMap<String, Integer>();
scoreMap.put("Tom", 90);
scoreMap.put("Jerry", 80);

6. Summary

Through the above code examples, we have a detailed understanding of the use of the put() method of the HashMap class. When using the put() method, we need to pay attention to the uniqueness of the key. If the key already exists, the put() method will overwrite the original value. At the same time, it is recommended to use generics to avoid type conversion operations.

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