불변 값과 with 메서드

public final class IgnoredImmutableReturnBug {
    public static void main(String[] args) {
        Post entry = new Post("immutable", 40);

        entry.addViewCount(20);

        System.out.println("expected=60");
        System.out.println("actual=" + entry.viewCount());
    }

    private record Post(String title, int viewCount) {
        Post addViewCount(int extra) {
            return new Post(title, viewCount + extra);
        }
    }
}

expected=60
actual=40

public final class ImmutableEntryWithMethods {
    public static void main(String[] args) {
        Post original = new Post("immutable", 40, false);
        Post revised = original.withViewCount(60);
        Post published = revised.publish();

        System.out.println("original=" + original);
        System.out.println("revised=" + revised);
        System.out.println("published=" + published);
    }

    private record Post(String title, int viewCount, boolean published) {
        Post {
            if (title == null || title.isBlank()) throw new IllegalArgumentException("title");
            if (viewCount <= 0) throw new IllegalArgumentException("viewCount");
        }

        Post withViewCount(int newViewCount) {
            return new Post(title, newViewCount, published);
        }

        Post publish() {
            return published ? this : new Post(title, viewCount, true);
        }
    }
}

original=Post[title=immutable, viewCount=40, published=false]
revised=Post[title=immutable, viewCount=60, published=false]
published=Post[title=immutable, viewCount=60, published=true]

public final class SharedImmutablePolicy {
    public static void main(String[] args) {
        ViewPolicy policy = new ViewPolicy(25, 5);
        Post first = new Post("object", 40, policy);
        Post second = new Post("string", 50, policy);

        Post revised = first.withViewCount(65);

        System.out.println("first-blocks=" + first.blocks());
        System.out.println("revised-blocks=" + revised.blocks());
        System.out.println("second-blocks=" + second.blocks());
        System.out.println("shared-policy=" + (first.policy() == second.policy()));
    }

    private record ViewPolicy(int pageSize, int reportThreshold) {
        ViewPolicy {
            if (pageSize <= 0 || reportThreshold < 0) throw new IllegalArgumentException("policy");
        }
    }

    private record Post(String title, int viewCount, ViewPolicy policy) {
        Post {
            if (viewCount <= 0) throw new IllegalArgumentException("viewCount");
        }

        Post withViewCount(int value) {
            return new Post(title, value, policy);
        }

        int blocks() {
            return (viewCount + policy.pageSize() - 1) / policy.pageSize();
        }
    }
}

first-blocks=2
revised-blocks=3
second-blocks=2
shared-policy=true

int blocksSafe() {
    return (int) (((long) viewCount + policy.pageSize() - 1) / policy.pageSize());
}

public final class PostHistory {
    public static void main(String[] args) {
        Post[] history = new Post[3];
        int size = 0;
        Post current = new Post("string", 30, false);
        history[size++] = current;

        current = current.withViewCount(45);
        history[size++] = current;
        current = current.publish();
        history[size++] = current;

        for (int index = 0; index < size; index++) {
            System.out.println(index + "=" + history[index]);
        }
    }

    private record Post(String title, int viewCount, boolean published) {
        Post withViewCount(int value) {
            if (value <= 0) throw new IllegalArgumentException("viewCount");
            return new Post(title, value, published);
        }

        Post publish() {
            return new Post(title, viewCount, true);
        }
    }
}

0=Post[title=string, viewCount=30, published=false]
1=Post[title=string, viewCount=45, published=false]
2=Post[title=string, viewCount=45, published=true]