考虑使用静态代码块

/**
 * 静态代码块
 *
 * @author iHelin
 * @since 2017/11/27 14:21
 */
public class BabyBoomer {
    private static Date birthDate;

    private static final Date BOOM_START;
    private static final Date BOOM_END;

    static {
        Calendar gmtCal = Calendar.getInstance(TimeZone.getTimeZone("GMT"));
        gmtCal.set(1946, Calendar.JANUARY, 1, 0, 0, 0);
        BOOM_START = gmtCal.getTime();
        gmtCal.set(1965, Calendar.JANUARY, 1, 0, 0, 0);
        BOOM_END = gmtCal.getTime();
    }

    public static boolean isBabyBoomer() {
        return birthDate.compareTo(BOOM_START) >= 0 && birthDate.compareTo(BOOM_END) < 0;
    }

    public static void main(String[] args) {
        Calendar cal = Calendar.getInstance();
        cal.set(1947, Calendar.FEBRUARY, 1, 0, 0, 0);
        birthDate = cal.getTime();
        System.out.println(isBabyBoomer());
    }
}

BOOM_START和BOOM_END不变，考虑使用静态常量，然后使用静态代码块初始化。

使用枚举实现单例

/**
 * 单例 - 使用枚举实现
 *
 * @author iHelin
 * @since 2017/11/27 14:27
 */
public class Elvis {

    private static final Elvis INSTANCE = new Elvis();

    private Elvis() {

    }

    public static Elvis getInstance() {
        return INSTANCE;
    }

    public Object readResolve() {
        return INSTANCE;
    }

}

遇到多个构造器参数时考虑用构建器

/**
 * 遇到多个构造器参数时考虑用构建器
 *
 * @author iHelin
 * @since 2017/11/27 14:24
 */
public class NutritionFacts {

    private final int servingSize;
    private final int servings;
    private final int calories;
    private final int fat;
    private final int sodium;
    private final int carbohydrate;

    public static class Builder {
        /**
         * Required parameters
         */
        private final int servingSize;
        private final int servings;

        /**
         * Optional parameters
         */
        private int calories = 0;
        private int fat = 0;
        private int sodium = 0;
        private int carbohydrate = 0;

        public Builder(int servingSize, int servings) {
            this.servingSize = servingSize;
            this.servings = servings;
        }

        public Builder calories(int val) {
            calories = val;
            return this;
        }

        public Builder fat(int val) {
            fat = val;
            return this;
        }

        public Builder carbohydrate(int val) {
            carbohydrate = val;
            return this;
        }

        public Builder sodium(int val) {
            sodium = val;
            return this;
        }

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

    private NutritionFacts(Builder builder) {
        servingSize = builder.servingSize;
        servings = builder.servings;
        calories = builder.calories;
        fat = builder.fat;
        sodium = builder.sodium;
        carbohydrate = builder.carbohydrate;
    }

    public static void main(String[] args) {
        NutritionFacts cocaCola = new NutritionFacts.Builder(240, 8).calories(100).sodium(35).carbohydrate(27).build();
        System.out.println(cocaCola);
    }

    @Override
    public String toString() {
        return ToStringBuilder.reflectionToString(this);
    }
}