一个 Java 的小问题

[bbbai]

看到有代码是如下这样写的，

class A{

private A(B b){...}

public static A of(B b) { return new A(b); }

}

这样写的好处是什么呢？ 如果这是一种好的风格或者手段的话，是否有一种获取途径，来了解好的风格或者手段？

[GTim]

单例模式了解下，嗯，其实，好处不仅仅是单例

[elgae]

@GTim 这跟单例有一毛钱关系？

[GTim]

@elgae 说说你的看法

[linshuang]

也是第一次看见，或许可以把 of 替换成一个句子来达到代码具备更强的语义的目的. 就模式而言，至少我没见过这种。
例如 Parent p = Parent.withChildren(children)

[maninfog]

@GTim 这个每次都是 new 和单例的确无关阿
这种写法估计是为了利用静态方法名充当注释的作用，个人感觉。

[icris]

参考示例：BigInteger.valueOf(1)

[lululau]

new A(b) VS A.of(b)

[lululau]

8 个字符 VS 7 个字符，效率提升了

[wbgbg]

Effective Java 第一条 考虑使用静态工厂方法替代构造方法
这里的主要优点就是有方法名来表示含义

[sutra]

可以参考 Instant.ofXXX() 的实现，可以看出来它并不是每次都 new 一个对象出来，有些是直接用的共享的对象。

[CasualYours]

Optional 类就是这种写法，好处就是代码语义更直接吧。

[Cbdy]

Effective Java 第三版第二章第一节有详细介绍

我简单摘录一下：

### 优点

One advantage of static factory methods is that, unlike constructors, they have names.

A second advantage of static factory methods is that, unlike constructors, they are not required to create a new object each time they ’ re invoked.

A third advantage of static factory methods is that, unlike constructors, they can return an object of any subtype of their return type.

A fourth advantage of static factories is that the class of the returned object can vary from call to call as a function of the input parameters.

A fifth advantage of static factories is that the class of the returned object need not exist when the class containing the method is written.

### 缺点

The main limitation of providing only static factory methods is that classes without public or protected constructors cannot be subclassed.

A second shortcoming of static factory methods is that they are hard for programmers to find.

### 一些常见的静态构造方法

**from**: A type-conversion method that takes a single parameter and returns a corresponding instance of this type, for example:

```java
Date d = Date.from(instant);
```

**of**: An aggregation method that takes multiple parameters and returns an instance of this type that incorporates them, for example:

```java
Set<Rank> faceCards = EnumSet.of(JACK, QUEEN, KING);
```

**valueOf**: A more verbose alternative to from and of, for example:

```java
BigInteger prime = BigInteger.valueOf(Integer.MAX_VALUE);
```

**instance** or **getInstance**: Returns an instance that is described by its parameters (if any) but cannot be said to have the same value, for example:

StackWalker luke = StackWalker.getInstance(options);

**create** or **newInstance**: Like instance or getInstance, except that the method guarantees that each call returns a new instance, for example:

```java
Object newArray = Array.newInstance(classObject, arrayLen);
```

[CasualYours]

@GTim 不是单机例，of 静态方法每次都返回新的实例。

[Raymon111111]

这是工厂, 了解一下工厂模式即可.

[lowzoom]

class A 体现不出这个写法的精髓，要 interface A

[bk201]

我认为是方便链式调用
类似 a.of(b).add(c).avg()