java 根据对象属性将java集合拆分为子集合
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split a java collection into sub collections based on a object property
提问by james
I have a List of MyObjects ... MyObject{ int id,String name}. Now I want to split the the list into sublists that have identical "id" values, can any one suggest an efficient approach for doing this.
我有一个 MyObjects 列表... MyObject{ int id,String name}。现在我想将列表拆分为具有相同“id”值的子列表，任何人都可以提出一种有效的方法来做到这一点。
回答by Mark Elliot
// create the thing to store the sub lists
Map<Integer, List<MyObject>> subs = new HashMap<Integer, List<MyObject>>();
// iterate through your objects
for(MyObject o : list){
// fetch the list for this object's id
List<MyObject> temp = subs.get(o.getId());
if(temp == null){
// if the list is null we haven't seen an
// object with this id before, so create
// a new list
temp = new ArrayList<MyObject>();
// and add it to the map
subs.put(o.getId(), temp);
}
// whether we got the list from the map
// or made a new one we need to add our
// object.
temp.add(o);
}
回答by Kevin Bourrillion
回答by Damien O'Reilly
If you are using JDK 1.8, you can use an elegant solution like:
如果您使用的是 JDK 1.8，则可以使用优雅的解决方案，例如：
Map<Integer, List<MyObject>> myObjectsPerId =
myObjects.stream().collect(Collectors.groupingBy(MyObject::getId));
回答by Ian Henry
Loop through the elements, check their idvalues, and place them in a Hashtablewith idas the key. That's O(N), which is as efficient as you're going to get.
循环遍历元素，检查它们的id值，并将它们放在Hashtablewith 中id作为键。那是 O(N)，它与您将获得的效率一样高。
回答by Piotr
Using JDK 1.8:
使用 JDK 1.8：
List<MyObject> objects= new ArrayList();
Map<Integer, List<MyObject>> obejctMap = new HashMap();
objects.stream().map(MyObject::getId).distinct().forEach(id -> obejctMap .put(id,
objects.stream().filter(object -> id.equals(object.getId())).collect(Collectors.toList())));
回答by Emil
ArrayList<MyObject> list=new ArrayList<MyObject>();
//fill Objects..
HashMap<Integer,ArrayList<MyObject>> hash=new HashMap<Integer,ArrayList<MyObject>>();
for(MyObject elem:list)//iterate the list
{
ArrayList<MyObject> tmp=null; //temporary variable
if((tmp=hash.get(elem.getId()))==null) // check if id already present in map
{
tmp=new ArrayList<MyObject>();
hash.put(elem.getId(),tmp); //if not put a new array list
}
names.add(elem); //if present add the name to arraylist
}
