[英]Extract List of object from another List of objects
I have a List<User> users
, where class User
has one property username
. 我有一个
List<User> users
,其中class User
有一个属性username
。 I also have another List<User> activeUsers
. 我还有另一个
List<User> activeUsers
。 Let users = activeUsers + inactiveUsers
. 让
users = activeUsers + inactiveUsers
。 Now I want to extract inactiveUsers
from users
based on username
property. 现在我想根据
username
属性从users
提取inactiveUsers
。 I solved this problem using two for-loops. 我使用两个for循环解决了这个问题。 I think this is not efficient way.
我认为这不是有效的方法。 so if anyone know how it can be done efficiently please let me know.
所以如果有人知道如何有效地完成,请告诉我。
eg I have activeUsers[1,3] and users[1,2,3,4] and want to build inactiveUsers[2,4]. 例如,我有activeUsers [1,3]和用户[1,2,3,4],并希望构建inactiveUsers [2,4]。
In Java, you can use the Collection
interface's removeAll
method. 在Java中,您可以使用
Collection
接口的removeAll
方法。
// Create a couple ArrayList objects and populate them
// with some delicious fruits.
Collection firstList = new ArrayList() {{
add("user1");
add("user2");
}};
Collection secondList = new ArrayList() {{
add("user1");
add("user1");
add("user3");
add("user4");
}};
// Show the "before" lists
System.out.println("First List: " + firstList);
System.out.println("Second List: " + secondList);
// Remove all elements in firstList from secondList
secondList.removeAll(firstList);
// Show the "after" list
System.out.println("Result: " + secondList);
The above code will produce the following output: 上面的代码将产生以下输出:
First List: [user1, user2]
Second List: [user1, user2, user3, user4]
Result: [user3, user4]
Well, based on the restriction of modifying the User
class and nature of your question there is no way of doing it lesser than o(n^2)
with two lists (ie two loops). 好吧,基于修改
User
类的限制和问题的性质,没有办法比使用两个列表(即两个循环)小于o(n^2)
)。 Of course if you could have two lists of each type your problem is done. 当然,如果你可以有两个每种类型的列表,你的问题就完成了。
But since you cannot than the logic is : (assuming your structures are lists only) 但是因为你不能逻辑:(假设你的结构只是列表)
Iterate users list (o(n)) :
- Search the active users list for the current user (o(n))
No matter how you look at it you get o(n^2)
无论你如何看待它,你都得到
o(n^2)
If you could modify User
to have activation property you can easily reduce the problem to o(n)
by a single search 如果您可以修改
User
以具有激活属性,则可以通过单次搜索轻松将问题减少为o(n)
You could store users in a Set based on if they're active or not, I would prefer a property on User but I guess you don't want that in this assignment of yours. 您可以将用户存储在一个Set中,如果他们是否处于活动状态,我会更喜欢User上的一个属性,但我想你不希望在你的这个分配中。 Using this code your problem is solved in O(n) time.
使用此代码,您的问题将在O(n)时间内得到解决。 Assuming you haven't messed up equals/hashCode in User.
假设你没有弄乱用户中的equals / hashCode。
package com.stackoverflow.q15999468;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashSet;
import java.util.List;
import java.util.Set;
public class Users
{
public static void main(String[] args)
{
List<User> users = Arrays.asList(u1,u2,u3,u4,u5);
List<User> activeUsers = Arrays.asList(u3,u5);
Set<User> activeUsersSet = new HashSet<User>(activeUsers);
List<User> inactiveUsers = new ArrayList<User>();
for(User user : users)
{
if(!activeUsersSet.contains(user))
{
inactiveUsers.add(user);
}
}
System.out.println("Inactive users: " + inactiveUsers);
}
}
This should work 这应该工作
List<User> inactiveUsers = new ArrayList<User>(users);
inactiveUsers.removeAll(activeUsers) ;
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