Extract List of object from another List of objects

Question

I have a List<User> users, where classUser has one property username. I also have anotherList<User> activeUsers. Let users = activeUsers + inactiveUsers. Now I want to extract inactiveUsers from users based on username property. I solved this problem using two for-loops. I think this is not efficient way. so if anyone know how it can be done efficiently please let me know.

e.g. I have activeUsers[1,3] and users[1,2,3,4] and want to build inactiveUsers[2,4].

Answer 1

In Java, you can use the Collection interface's removeAll method.

// 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:

First List: [user1, user2]
Second List: [user1, user2, user3, user4]
Result: [user3, user4]