1.考虑用静态工厂方法代替构造器

静态工厂方法惯用名称

• valueOf —— 类型转换方法

• of —— valueOf的一种更简洁的替代

• getInstance —— 该方法没有参数，并返回唯一的实例

• newInstance —— 确保返回的每个实例都与其他实例不同

• getType —— 与getInstance一样，但在工厂方法处于不同的类中使用,Type表示工厂方法所返回的对象类型

• newType —— 与newInstance一样，但在工厂方法处于不同的类中使用,Type表示工厂方法所返回的对象类型

• 具体方法参考Boolean，EnumSet

2.遇到多个构造器参数时考虑用构建器

**代码实例 **

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 {
        private int servingSize;
        private int servings;
        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);
        }
    }

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

    public int getServingSize() {
        return servingSize;
    }

    public int getServings() {
        return servings;
    }

    public int getCalories() {
        return calories;
    }

    public int getFat() {
        return fat;
    }

    public int getSodium() {
        return sodium;
    }

    public int getCarbohydrate() {
        return carbohydrate;
    }

    public static void main(String[] args) {
        NutritionFacts nutritionFacts = new Builder(1, 3).calories(2).carbohydrate(2).build();
        System.out.println(nutritionFacts.getCalories());
        System.out.println(nutritionFacts.getCarbohydrate());
        System.out.println(nutritionFacts.getFat());
        System.out.println(nutritionFacts.getServings());
    }
}

3.用私有构造器或枚举类型强化Singleton属性

代码实例

public class Elvis {
    private static final Elvis INSTANCE = new Elvis();
    private Elvis() {}
    private static Elvis getInstance() {
        return  INSTANCE;
    }
}

public enum  Elvis {
    INSTANCE;
}

4.通过私有构造器强化不可实例化的能力

参考Math，Array，Collections

代码实例

public class UnitityClass {
    // 避免调用构造函数实例化
    private UnitityClass() {
        throw new AssertionError();
    }
} 

5.避免创建不必要的对象

代码实例

String s = new String("stringtte"); // 不要这样做

String s = "stringtte";

6.消除过期的对象引用

一般而言，只要类是自己管理内存，程序员就应该警惕内存泄露问题。

代码实例

public Object pop() {
    if (size == 0) {
        throw new EmptyStackException();
    }
    Object result = elements[--size];
    elemtents[size] = null; //重点在这里
    return result;
}