用 Java 将集合转换为列表的程序

原文:https://www . geesforgeks . org/program-to-convert-set-to-list-in-Java/

Java Set 是 java.util 包的一部分，扩展了 java.util.Collection 接口。它不允许使用重复元素，最多只能容纳一个空元素。

列表是收藏的子界面。它是对象的有序集合，其中可以存储重复的值。因为列表保留了插入顺序，所以它允许元素的位置访问和插入。列表接口由数组列表、链表、向量和堆栈类实现。

下面是用 Java 将集合转换为列表的方法。

1. Brute Force or Naive method: This method includes creating an empty list and add every element of the set to it.

   算法:

   1. 获取要转换的集合。
   2. 创建一个空列表
   3. 将集合中的每个元素推入列表
   4. 返回形成的列表

   程序:

   ```java // Java Program to convert // Set to List in Java 8

   import java.util.; import java.util.stream.;

   class GFG {

   // Generic function to convert set to list     public static List convertSetToList(Set set)     {         // create an empty list         List list = new ArrayList<>();

   // push each element in the set into the list         for (T t : set)             list.add(t);

   // return the list         return list;     }

   public static void main(String args[])     {

   // Create a Set using HashSet         Set hash_Set = new HashSet();

   // Add elements to set         hash_Set.add("Geeks");         hash_Set.add("For");         hash_Set.add("Geeks");         hash_Set.add("Example");         hash_Set.add("Set");

   // Print the Set         System.out.println("Set: " + hash_Set);

   // construct a new List from Set         List list = convertSetToList(hash_Set);

   // Print the List         System.out.println("List: " + list);     } } ```

   Output:

   ```java Set: [Set, Example, Geeks, For] List: [Set, Example, Geeks, For]

   ```

2. With help of Constructor: Collections in Java have a constructor in which the direct set can be passed to create a List from it.

   算法:

   1. 获取要转换的集合。
   2. 通过将 set as 参数传递给构造函数来创建列表。
   3. 返回形成的列表

   程序:

   ```java // Java Program to convert // Set to List in Java 8

   import java.util.; import java.util.stream.;

   class GFG {

   // Generic function to convert set to list     public static List convertSetToList(Set set)     {         // create a list from Set         List list = new ArrayList<>(set);

   // return the list         return list;     }

   public static void main(String args[])     {

   // Create a Set using HashSet         Set hash_Set = new HashSet();

   // Add elements to set         hash_Set.add("Geeks");         hash_Set.add("For");         hash_Set.add("Geeks");         hash_Set.add("Example");         hash_Set.add("Set");

   // Print the Set         System.out.println("Set: " + hash_Set);

   // construct a new List from Set         List list = convertSetToList(hash_Set);

   // Print the List         System.out.println("List: " + list);     } } ```

   Output:

   ```java Set: [Set, Example, Geeks, For] List: [Set, Example, Geeks, For]

   ```

3. Using Java 8 Stream: In Java 8, Stream can be used convert a set to a list by converting the set to a sequential Stream using Set.stream() and using a Collector to collect the input elements into a new List.

   算法:

   1. 获取要转换的 HashMap。
   2. 从集合中创建流
   3. 将集合转换为列表并收集
   4. 返回收集的列表

   程序:

   ```java // Java Program to convert // HashMap to TreeMap in Java 8

   import java.util.; import java.util.stream.;

   class GFG {

   // Generic function to convert set to list     public static List convertSetToList(Set set)     {         // create a list from Set         return set

   // Create stream from the Set             .stream()

   // Convert the set to list and collect it             .collect(Collectors.toList());     }

   public static void main(String args[])     {

   // Create a Set using HashSet         Set hash_Set = new HashSet();

   // Add elements to set         hash_Set.add("Geeks");         hash_Set.add("For");         hash_Set.add("Geeks");         hash_Set.add("Example");         hash_Set.add("Set");

   // Print the Set         System.out.println("Set: " + hash_Set);

   // construct a new List from Set         List list = convertSetToList(hash_Set);

   // Print the List         System.out.println("List: " + list);     } } ```

   Output:

   ```java Set: [Set, Example, Geeks, For] List: [Set, Example, Geeks, For]

   ```

4. Using Guava Library List.newArrayList(set): Lists.newArrayList(set) creates a mutable ArrayList instance containing the elements of the specified set.

   算法:

   1. 获取要转换的集合。
   2. 使用 Lists.newArrayList()创建一个新列表，方法是将 set 作为参数传递给 Guava 库的这个函数
   3. 返回形成的列表

   程序:

   ```java // Java Program to convert // HashMap to TreeMap in Java 8

   import java.util.; import java.util.stream.;

   class GFG {

   // Generic function to convert set to list     public static List convertSetToList(Set set)     {         // create a list from Set         return Lists.newArrayList(set);     }

   public static void main(String args[])     {

   // Create a Set using HashSet         Set hash_Set = new HashSet();

   // Add elements to set         hash_Set.add("Geeks");         hash_Set.add("For");         hash_Set.add("Geeks");         hash_Set.add("Example");         hash_Set.add("Set");

   // Print the Set         System.out.println("Set: " + hash_Set);

   // construct a new List from Set         List list = convertSetToList(hash_Set);

   // Print the List         System.out.println("List: " + list);     } } ```

   输出:

   ```java Set: [Set, Example, Geeks, For] List: [Set, Example, Geeks, For]

   ```

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