removeIf and bulk operations

Key points

• removeIf(pred) deletes every match in a single safe pass and returns whether anything changed.
• removeAll, retainAll and addAll give you difference, intersection and union of two collections.
• replaceAll(fn) rewrites every element in place; on a map it takes both key and value.
• Map views are live, so map.values().removeIf(v -> v == 0) deletes those entries from the map itself.
• Bulk ops call contains on their argument repeatedly. Pass a Set, not a big List, to stay fast.

Example

import java.util.*;

public class Main {
    public static void main(String[] args) {
        List<String> names = new ArrayList<>(List.of("ann", "bob", "cyd", "dee", "eli"));
        names.removeIf(n -> n.startsWith("b") || n.startsWith("d"));
        System.out.println("removeIf          : " + names);

        List<Integer> nums = new ArrayList<>(List.of(1, 2, 3, 4, 5, 6));
        nums.removeAll(List.of(2, 4));
        System.out.println("removeAll         : " + nums);
        nums.retainAll(List.of(1, 3, 9));
        System.out.println("retainAll         : " + nums);
        nums.replaceAll(x -> x * 10);
        System.out.println("replaceAll        : " + nums);

        Map<String, Integer> stock = new LinkedHashMap<>();
        stock.put("nuts", 0);
        stock.put("bolts", 7);
        stock.put("nails", 0);
        stock.values().removeIf(v -> v == 0);
        System.out.println("values().removeIf : " + stock + "   (view writes through)");

        Set<String> a = new LinkedHashSet<>(List.of("x", "y", "z"));
        Set<String> b = Set.of("y", "z", "w");
        Set<String> union = new LinkedHashSet<>(a);
        union.addAll(b);
        Set<String> intersect = new LinkedHashSet<>(a);
        intersect.retainAll(b);
        Set<String> difference = new LinkedHashSet<>(a);
        difference.removeAll(b);
        System.out.println("union             : " + union);
        System.out.println("intersection      : " + intersect);
        System.out.println("difference        : " + difference);
    }
}

If your loop only deletes, filters or rewrites, a bulk method already does it in one line.