Question

I have a list of Objects, each of which has a boolean “selected” property. I want these in a set if the “selected” property is true. Other than a for loop through the objects adding each to the set, is there another way to achieve this?

Answer 1

For something this simple just write a loop and be done with it.

If this is a pattern you’re going to implement over and over:

(untested so there may be a typo or two)

public interface Selector<T> {
    public boolean select(T t); }

public <T> Set<T> grep(Collection<T> coll, Selector<T> sel) {
    Set<T> result = new Set<T>;
    for (T item : coll)
        if (sel.select(T)) 
            result.add(T);
    return result;
}

And the invocation

List<SomeClasss> input = ...;
Set<SomeClass>   output = grep(
    input, 
    new Selector<SomeClass> { 
        public boolean select(SomeClass item) {
            return some-boolean-function-of-item; // decide which items to keep here
        }
    }
);

Implementations of Selector know how to examine objects and decide if they should be kept.