Creation of java objects (1)

1. Give priority to static factory methods instead of constructors

    When a class needs to have multiple constructors, consider using static factory methods instead of these constructors.

    The benefits of doing this are:

    First: The static factory method can have a clearer description of the object to be created through the method signature, while the constructor method cannot

    Second: static factory methods can avoid creating the same object repeatedly (Boolean example can illustrate)

    Third: static factory methods can return any subclass of the desired return type (this is very useful in frameworks, especially APIs)

    Fourth: Code can be simplified when creating parameterized instances (HashMap example can illustrate)

 

Example:

    1. Boolean example:

publice static Boolean valueOf(boolean b){
	return b ? Boolean.TRUE : Boolean.FALSE;
}

    2. HashMap example:

    If you want to construct such an instance:

Map<String, List<String>> map = new HashMap<String, List<String>> ();

    It's cumbersome to implement through constructors, but relatively simple through static factory methods.

publice static <K, V>Map<K, V> newInstance(){

    return new HashMap<K, V>();

}

Map<String, List<String>> map = HashMap.newInstance();

 

Second, use the Builder pattern to handle the creation of multi-construction parameter objects

    1. When an object has multiple construction parameters and most of them are optional, you should consider using the Builder pattern.

    2. After a class is created, if you want to extend member variables later, you should consider using the Builder pattern.

 

    Why not use cascading constructors and static factory methods?

    answer:

        Too many cascading constructors are not user-friendly, and will bring certain maintenance costs.

        Static factory methods are more suitable for constructing objects with few parameters.

 

    Example :

public classCat{

    public static void main(String[] args) {
        Cat cat = new CatBuilder().name("小花").sex('母').color("❀").weight(2.5).build();
        System.out.println(cat.getColor());
    }

    private String color;
    private Integer age;
    private Character sex;
    private String name;
    private Double weight;

    public static classCatBuilder{
        private String color;
        private Integer age;
        private Character sex;
        private String name;
        private Double weight;

        public CatBuilder color(String color) {
            this.color = color;
            return this;
        }

        public CatBuilder age(Integer age) throws Exception {
            if (0 > age) throw new Exception("年龄输入错误");
            this.age = age;
            return this;
        }

        public CatBuilder sex(Character sex) {
            this.sex = sex;
            return this;
        }

        public CatBuilder name(String name) {
            this.name = name;
            return this;
        }

        public CatBuilder weight(Double weight) {
            this.weight = weight;
            return this;
        }

        public Cat build() {
            Cat cat = new Cat();
            cat.color = this.color;
            cat.age = this.age;
            cat.sex = this.sex;
            cat.name = this.name;
            cat.weight = this.weight;
            return cat;
        }

    }

    public String getColor() {
        return color;
    }

    public Integer getAge() {
        return age;
    }

    public Character getSex() {
        return sex;
    }

    public String getName() {
        return name;
    }

    public Double getWeight() {
        return weight;
    }

    @Override
    public String toString() {
        return "Cat{" +
                "color='" + color + '\'' +
                ", age=" + age +
                ", sex=" + sex +
                ", name='" + name + '\'' +
                ", weight=" + weight +
                '}';
    }
}