Interpretation of Java documentation: Detailed explanation of the usage of the size() method of the HashSet class

Interpretation of Java documentation: Detailed explanation of the usage of the size() method of the HashSet class, specific code examples are required

In Java programming, we often use collection classes to manage data . HashSet is one of the Java collection classes. It implements the Set interface and can be used to store unique collection elements. The elements in HashSet are not ordered, and its internal implementation uses HashMap to store elements. In HashSet, we can use the size() method to get the number of elements in the set. This article will explain in detail the size() method in HashSet and how to use it.

Definition of size() method

HashSet inherits from the collection interface Set, so it has various methods defined in Set. Among them, the size() method is a unique method of HashSet, and its definition is as follows:

public int size() 

This method has no parameters and returns an integer indicating the number of elements in the HashSet.

Usage of size() method

In Java programming, we often need to get the number of elements in a collection. The size() method is provided in HashSet to obtain the number of elements in the set. The size() method returns an integer representing the number of elements in the collection.

A simple sample code is given below:

import java.util.HashSet;
 
public class HashSetDemo {
    public static void main(String[] args) {
        HashSet<String> hashSet = new HashSet<String>();
        hashSet.add("apple");
        hashSet.add("orange");
        hashSet.add("banana");
        System.out.println("HashSet中元素的个数为: " + hashSet.size());
    }
}

In the above code, we created a HashSet object and added three string elements to it. Then use the size() method to get the number of elements in the HashSet, and output the result to the console.

After running the program, the output result is:

HashSet中元素的个数为: 3

It can be seen from the output result that the number of elements in the HashSet is 3.

Time complexity of the size() method

When using the size() method, you need to pay attention to its time complexity. The time complexity of the size() method of HashSet is O(1). That is to say, no matter how many elements there are in the HashSet, the time to obtain the number of elements is constant. This is because HashSet uses HashMap internally to store elements, and HashMap maintains a variable to store the number of elements. When the size() method is called, only the value of the variable needs to be returned. Therefore, no matter how many elements there are in the HashSet, the time to obtain the number of elements is constant.

Summary

This article explains the size() method in the HashSet class in detail. The size() method is used to obtain the number of elements in the collection. Its time complexity is O(1), so it can be used with confidence in actual programming. When using the size() method, please note that this method returns the number of elements in the collection, not the amount of space available in the collection.

The above is the interpretation and usage instructions of the size() method in the HashSet class in this article. I hope it can be helpful to everyone.

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