Java Mapping ArrayList到HashMap

Question

我给页面获取了一个ArrayList <Document>,其中每个文档都有一个名为type的属性.

我不知道唯一类型或文档的数量.

我想将这个ArrayList排序为HashMap <type,document []>但是我在解决它时遇到了一些麻烦.

有些伪代码会喜欢

for (int i = 0; i < documents.size(); i++) 
{
   if there is an array for documents[i].type
   add to this array
   else create a new array for this type
   add document[i].type and the array of documents with matching type to the hashmap
}

我知道这是错误的做法,显然不会奏效.我对任何建议持开放态度.

谢谢

Answer 1

// create the map to store stuff, note I'm using a List instead of an array
// in my opinion it's a bit cleaner
Map<String, List<Document>> map = new HashMap<String, List<Document>>();

// now iterate through each document
for(Document d : documents){

    // check to see if this type is already known
    List<Document> list = map.get(d.type);

    if(list == null){
        // list is null when it wasn't found in the map
        // this is a new type, create a new list
        list = new ArrayList<Document>();

        // store the list in the map
        map.put(d.type, list);
    }

    // finally, whether we got a hit or a miss, we want
    // to add this document to the list for this type
    list.add(d);
}