从 LinkedHashSet 中获取第一个元素的Java程序

LinkedHashSet 是集合抽象数据类型 (ADT) 的实现。它从 HashSet 类扩展而来，后者又实现了 Set 接口。 LinkedHashSet 和 HashSet 的区别在于维护元素排序的属性。 LinkedList 只是一个包含元素序列的容器。根据定义，Set 不应包含重复元素。此属性不需要对 LinkedList ADT 保持良好。当元素应该被复制并且必须保持顺序时，应该使用 LinkedHashSet。

方法：

基本上有三种标准方法可以从 LinkedHashSet 获取元素而不将其更改为不同的数据结构。

1. 将其转换为数组或列表
2. 使用迭代器
3. 使用流

实现：从 LinkedHashSet 中获取第一个元素

方法一：通过将其转换为数组或列表

例子

Java

// Java Program to Get the First Element from LinkedHashSet
// by converting it to an array or List
// Array is demonstrated below so do with List
 
// Importing generic java packages
import java.io.*;
import java.lang.*;
import java.util.*;
 
// Class
public class GFG {
    // Main driver method
    public static void main(String[] args)
        throws java.lang.Exception
    {
        // Creating a LinkedHashMap object
        // Declaring object of Integer type
        LinkedHashSet hashSet
            = new LinkedHashSet();
 
        // Adding elements to LinkedHashMap
        hashSet.add(2);
        hashSet.add(1);
        hashSet.add(4);
        hashSet.add(6);
        hashSet.add(8);
 
        // Condition check using isEmpty() method which
        // holds
 
        // True till there is a single element in an object
        // is remaining False, when there is no object left
        // or if initially there was no element added
        if (!hashSet.isEmpty()) {
            // Converting the above Map to an array
            Integer arr[] = new Integer[hashSet.size()];
            arr = hashSet.toArray(arr);
 
            // Accessing the first element by passing 0
            // as an argument which by default
            // accesses and prints out first element
            System.out.println("First element: " + arr[0]);
        }
    }
}

输出

First element: 2

方法 2：使用迭代器

例子

Java

// Java Program to Get the First Element from LinkedHashSet
// Using Iterators
 
// Importing generic java packages
import java.util.*;
import java.lang.*;
import java.io.*;
 
// Class
public class GFG {
    // Main driver method
    public static void main(String[] args)
        throws java.lang.Exception
    {
        // Creating a LinkedHashMap
        LinkedHashSet hashSet
            = new LinkedHashSet();
 
        // Adding elements to LinkedHashMap
        hashSet.add(1);
        hashSet.add(2);
        hashSet.add(3);
        hashSet.add(4);
        hashSet.add(5);
 
        // Iterator over LinkedHashMap
        Iterator iter = hashSet.iterator();
 
        if (iter != null && iter.hasNext()) {
            // Display the first element of Map using next()
            // ethod
            System.out.println(
                "First element in LinkedHashSet: "
                + iter.next());
        }
    }
}

输出

First element in LinkedHashSet: 1

方法 3：使用流

例子：

Java

// Java Program to Get the First Element from LinkedHashSet
// Using Streams
 
// Importing generic java packages
import java.util.*;
import java.lang.*;
import java.io.*;
 
// Class
class GFG {
   
    // Main driver method
    public static void main(String[] args) throws java.lang.Exception
    {
        // Creating a LinkedHashMap
        LinkedHashSet hashSet
            = new LinkedHashSet();
 
        // Adding elemens to LinkedHashMap
        hashSet.add(1);
        hashSet.add(2);
        hashSet.add(3);
        hashSet.add(4);
        hashSet.add(5);
 
        // Checking whether Map is empty or not
        if (hashSet.size() == 0)
 
            // Display message
            System.out.println("The Set is Empty!");
        else {
            // Using stream() through findFirst() method
            // over the elements of LinkedHashMap
            int first = hashSet.stream().findFirst().get();
 
            // Printing the first element of LinkedHashMap
            System.out.println(
                "First element in LinkedHashSet: " + first);
        }
    }
}

输出

First element in LinkedHashSet: 1