House.java

public class House {
    private final String walls;
    private final String roof;
    private final int windows;

    private House(HouseBuilder builder) {
        this.walls = builder.walls;
        this.roof = builder.roof;
        this.windows = builder.windows;
    }

    @Override
    public String toString() {
        return "House [walls=" + walls + ", roof=" + roof + ", windows=" + windows + "]";
    }

    public static class HouseBuilder {
        private String walls;
        private String roof;
        private int windows;

        public HouseBuilder setWalls(String walls) {
            this.walls = walls;
            return this;
        }

        public HouseBuilder setRoof(String roof) {
            this.roof = roof;
            return this;
        }

        public HouseBuilder setWindows(int windows) {
            this.windows = windows;
            return this;
        }

        public House build() {
            return new House(this);
        }
    }
}