Java 从列表到集合

给定一个列表（ArrayList或LinkedList），在Java中把它转换成一个字符串的集合（HashSet或TreeSet）。

方法1（简单）

我们简单地创建一个列表。我们遍历给定的集合，并一个一个地将元素添加到列表中。

// Java program to demonstrate conversion of
// list to set using simple traversal
import java.util.*;
  
class Test {
    public static void main(String[] args)
    {
  
        // Creating a list of strings
        List<String> aList = Arrays.asList("Geeks", "for",
                     "GeeksQuiz", "GeeksforGeeks", "GFG");
  
        Set<String> hSet = new HashSet<String>();
        for (String x : aList)
            hSet.add(x);
  
        System.out.println("Created HashSet is");
        for (String x : hSet)
            System.out.println(x);
  
        // We can created TreeSet same way
    }
}

方法2（使用HashSet或TreeSet构造函数）

// Java program to demonstrate conversion of
// list to set using constructor
import java.util.*;
  
class Test {
    public static void main(String[] args)
    {
  
        // Creating a list of strings
        List<String> aList = Arrays.asList("Geeks", "for",
                     "GeeksQuiz", "GeeksforGeeks", "GFG");
  
        // Creating a hash set using constructor
        Set<String> hSet = new HashSet<String>(aList);
  
        System.out.println("Created HashSet is");
        for (String x : hSet)
            System.out.println(x);
  
        System.out.println("Created TreeSet is");
        Set<String> tSet = new TreeSet<String>(aList);
        System.out.println("Created TreeSet is");
        for (String x : tSet)
            System.out.println(x);
    }
}

方法3（使用addAll方法）

// Java program to demonstrate conversion of
// Set to array using addAll() method.
import java.util.*;
  
class Test {
    public static void main(String[] args)
    {
  
        // Creating a list of strings
        List<String> aList = Arrays.asList("Geeks", "for",
                    "GeeksQuiz", "GeeksforGeeks", "GFG");
  
        Set<String> hSet = new HashSet<String>(aList);
        hSet.addAll(aList);
        System.out.println("Created HashSet is");
        for (String x : hSet)
            System.out.println(x);
  
        Set<String> tSet = new TreeSet<String>(aList);
        tSet.addAll(aList);
        System.out.println("Created TreeSet is");
        for (String x : tSet)
            System.out.println(x);
    }
}

方法4（在Java中使用流）
我们在Java中使用流将给定的列表转换为流，然后再将流转换为集合。这只适用于Java 8或之后的版本。

// Java program to demonstrate conversion of
// Set to list using stream
import java.util.*;
import java.util.stream.*;
  
class Test {
    public static void main(String[] args)
    {
  
        // Creating a list of strings
        List<String> aList = Arrays.asList("Geeks", "for",
                    "GeeksQuiz", "GeeksforGeeks", "GFG");
  
        // Converting to set using stream
        Set<String> set = aList.stream().collect(Collectors.toSet());
  
        for (String x : set)
            System.out.println(x);
    }
}