Java集合与数组的互转

心已赠人 2022-05-23 01:48 264阅读 0赞

在实际工作中，我们经常需要Collection类型与数组的互相转换，Java API也提供了相应的方法帮我们完成操作。

>  *  集合转为数组  
>     java.util.Collection.toArray(T\[\] a)
>  *  数组转为集合  
>     （不推荐，不支持集合的add()、remove()、clear()方法）java.util.Arrays\#asList(T… a)  
>     （推荐，新建List）List list = new ArrayList<>(Arrays.asList(“a”, “b”, “c”))

### 集合转数组 ###

*  源码

@Test
    public void testList2Array() { 
    	// 构建一个集合
    	List<String> list = new ArrayList<>();
    	list.add("Calligraphy is the art of beautiful handwriting.");
    	list.add("He really takes good care of his sister.What a thoughtful boy!");
    	list.add("Working hard in the gymnasium keeps us fit.");
    
    	// 转为数组
    	String[] strings = list.toArray(new String[list.size()]);
    
    	// 遍历(JDK8)
    	Arrays.stream(strings).forEach(System.out::println);
    }

*  运行结果

Calligraphy is the art of beautiful handwriting.
    He really takes good care of his sister.What a thoughtful boy!
    Working hard in the gymnasium keeps us fit.

### 数组转集合 ###

*  源码

//数组转化为集合
     @Test
     public void array2List() { 
         String[]arr = new String[]{ "Calligraphy is the art of beautiful handwriting."
        		 ,"He really takes good care of his sister.What a thoughtful boy!"
        		 ,"Working hard in the gymnasium keeps us fit."};
         
         // 转为集合
         List<String> list = Arrays.asList(arr);
         // 遍历
         list.forEach(n -> System.out.println(n));  
     }

*  运行结果

Calligraphy is the art of beautiful handwriting.
    He really takes good care of his sister.What a thoughtful boy!
    Working hard in the gymnasium keeps us fit.

数组转集合使用的是Arrays.asList(T…a)方法。这里特别需要注意的是，使用这种方式转来的list的类型是Arrays的一个内部类【即java.util.Arrays.ArrayList】，拥有的方法数量有限，不具备add 、remove等的常用操作。

如下操作：

//数组转化为集合，并尝试添加
     @Test
     public void array2ListAttemptAdd() { 
         String[]arr = new String[]{ "Calligraphy is the art of beautiful handwriting."
        		 ,"He really takes good care of his sister.What a thoughtful boy!"
        		 ,"Working hard in the gymnasium keeps us fit."};
         
         // 转为集合
         List<String> list = Arrays.asList(arr);
         
         try { 
    		//会报java.lang.UnsupportedOperationException
    		 list.add("When it gets cold, I sleep with a thick quilt to stay warm.");
    	} catch (Exception e) { 
    		e.printStackTrace();
    	}
         
         // 遍历
         list.forEach(n -> System.out.println(n));  
     }

若要经转化后有增加删除等操作，可转为ArrayList或其他拥有完整操作的List类

*  源码

//1. 数组转化为集合
     //2. 并将该结果集合转化为java.util.ArrayList【严格规范的做法】
        //3. 然后尝试添加
     @Test
     public void array2ArrayListAndAdd() { 
         String[]arr = new String[]{ "Calligraphy is the art of beautiful handwriting."
        		 ,"He really takes good care of his sister.What a thoughtful boy!"
        		 ,"Working hard in the gymnasium keeps us fit."};
         
         // 转为集合
         // 也可以是java.util.LinkedList、java.util.Vector<E>
         List<String> list = new java.util.ArrayList<>(Arrays.asList(arr));
         
         try { 
    		//正常添加
    		 list.add("When it gets cold, I sleep with a thick quilt to stay warm.");
    	} catch (Exception e) { 
    		e.printStackTrace();
    	}
         
         // 遍历
         list.forEach(n -> System.out.println(n));  
     }

*  运行结果

Calligraphy is the art of beautiful handwriting.
    He really takes good care of his sister.What a thoughtful boy!
    Working hard in the gymnasium keeps us fit.
    When it gets cold, I sleep with a thick quilt to stay warm.

--------------------

参考文档：

1.  https://blog.csdn.net/wthfeng/article/details/61416464