doc: update docs/java.md (#321)

docs/java.md

@@ -860,6 +860,7 @@ try (InputStream inputStream = new ByteArrayInputStream(inputContent);
 ```
 
 ### Collections 工具类
+
 ```java
 // 计算出现频率
 List<Integer> list = new ArrayList<>();
@@ -873,6 +874,7 @@ int frequency = Collections.frequency(list, 2); // frequency = 2
 ```
 
 ### Stream 流
+
 ```java
 // 统计词频
 List<Integer> list = new ArrayList<>();