## Chapter 2. Creating and Destroying Objects(创建和销毁对象)
### Item 4: Enforce noninstantiability with a private constructor(用私有构造函数实施不可实例化)
Occasionally you’ll want to write a class that is just a grouping of static methods and static fields. Such classes have acquired a bad reputation because some people abuse them to avoid thinking in terms of objects, but they do have valid uses. They can be used to group related methods on primitive values or arrays, in the manner of java.lang.Math or java.util.Arrays. They can also be used to group static methods, including factories (Item 1), for objects that implement some interface, in the manner of java.util.Collections. (As of Java 8, you can also put such methods in the interface, assuming it’s yours to modify.) Lastly, such classes can be used to group methods on a final class, since you can’t put them in a subclass.
有时你会想要写一个类,它只是一个静态方法和静态字段的组合。这样的类已经获得了坏名声,因为有些人滥用它们来避免从对象角度思考,但是它们确有用途。它们可以用 java.lang.Math 或 java.util.Arrays 的方式,用于与原始值或数组相关的方法。它们还可以用于对以 java.util.Collections 的方式实现某些接口的对象分组静态方法,包括工厂([Item-1](/Chapter-2/Chapter-2-Item-1-Consider-static-factory-methods-instead-of-constructors.md))。(对于 Java 8,你也可以将这些方法放入接口中,假设你可以进行修改。)最后,这些类可用于对 final 类上的方法进行分组,因为你不能将它们放在子类中。
Such utility classes were not designed to be instantiated: an instance would be nonsensical. In the absence of explicit constructors, however, the compiler provides a public, parameterless default constructor. To a user, this constructor is indistinguishable from any other. It is not uncommon to see unintentionally instantiable classes in published APIs.
这样的实用程序类不是为实例化而设计的:实例是无意义的。然而,在没有显式构造函数的情况下,编译器提供了一个公共的、无参数的默认构造函数。对于用户来说,这个构造函数与其他构造函数没有区别。在已发布的 API 中看到无意中实例化的类是很常见的。
**Attempting to enforce noninstantiability by making a class abstract does not work.** The class can be subclassed and the subclass instantiated. Furthermore, it misleads the user into thinking the class was designed for inheritance (Item 19). There is, however, a simple idiom to ensure noninstantiability. A default constructor is generated only if a class contains no explicit constructors, so **a class can be made noninstantiable by including a private constructor:**
**译注:原文 noninstantiable 应修改为 non-instantiable ,译为「不可实例化的」**
**试图通过使类抽象来实施不可实例化是行不通的。** 可以对类进行子类化,并实例化子类。此外,它误导用户认为类是为继承而设计的([Item-19](/Chapter-4/Chapter-4-Item-19-Design-and-document-for-inheritance-or-else-prohibit-it.md))。然而,有一个简单的习惯用法来确保不可实例化。只有当类不包含显式构造函数时,才会生成默认构造函数,因此**可以通过包含私有构造函数使类不可实例化:**
```
// Noninstantiable utility class
public class UtilityClass {
// Suppress default constructor for noninstantiability
private UtilityClass() {
throw new AssertionError();
} ... // Remainder omitted
}
```
Because the explicit constructor is private, it is inaccessible outside the class.The AssertionError isn’t strictly required, but it provides insurance in case the constructor is accidentally invoked from within the class. It guarantees the class will never be instantiated under any circumstances. This idiom is mildly counterintuitive because the constructor is provided expressly so that it cannot be invoked. It is therefore wise to include a comment, as shown earlier.
因为显式构造函数是私有的,所以在类之外是不可访问的。AssertionError 不是严格要求的,但是它提供了保障,以防构造函数意外地被调用。它保证类在任何情况下都不会被实例化。这个习惯用法有点违反常规,因为构造函数是明确提供的,但不能调用它。因此,如上述代码所示,包含注释是明智的做法。
As a side effect, this idiom also prevents the class from being subclassed. All constructors must invoke a superclass constructor, explicitly or implicitly, and a subclass would have no accessible superclass constructor to invoke.
这个习惯用法也防止了类被子类化,这是一个副作用。所有子类构造函数都必须调用超类构造函数,无论是显式的还是隐式的,但这种情况下子类都没有可访问的超类构造函数可调用。
---
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- Chapter 2. Creating and Destroying Objects(创建和销毁对象)
- Item 1: Consider static factory methods instead of constructors(考虑以静态工厂方法代替构造函数)
- Item 2: Consider a builder when faced with many constructor parameters(在面对多个构造函数参数时,请考虑构建器)
- Item 3: Enforce the singleton property with a private constructor or an enum type(使用私有构造函数或枚举类型实施单例属性)
- Item 4: Enforce noninstantiability with a private constructor(用私有构造函数实施不可实例化)
- Item 5: Prefer dependency injection to hardwiring resources(依赖注入优于硬连接资源)
- Item 6: Avoid creating unnecessary objects(避免创建不必要的对象)
- Item 7: Eliminate obsolete object references(排除过时的对象引用)
- Item 8: Avoid finalizers and cleaners(避免使用终结器和清除器)
- Item 9: Prefer try with resources to try finally(使用 try-with-resources 优于 try-finally)
- Chapter 3. Methods Common to All Objects(对象的通用方法)
- Item 10: Obey the general contract when overriding equals(覆盖 equals 方法时应遵守的约定)
- Item 11: Always override hashCode when you override equals(当覆盖 equals 方法时,总要覆盖 hashCode 方法)
- Item 12: Always override toString(始终覆盖 toString 方法)
- Item 13: Override clone judiciously(明智地覆盖 clone 方法)
- Item 14: Consider implementing Comparable(考虑实现 Comparable 接口)
- Chapter 4. Classes and Interfaces(类和接口)
- Item 15: Minimize the accessibility of classes and members(尽量减少类和成员的可访问性)
- Item 16: In public classes use accessor methods not public fields(在公共类中,使用访问器方法,而不是公共字段)
- Item 17: Minimize mutability(减少可变性)
- Item 18: Favor composition over inheritance(优先选择复合而不是继承)
- Item 19: Design and document for inheritance or else prohibit it(继承要设计良好并且具有文档,否则禁止使用)
- Item 20: Prefer interfaces to abstract classes(接口优于抽象类)
- Item 21: Design interfaces for posterity(为后代设计接口)
- Item 22: Use interfaces only to define types(接口只用于定义类型)
- Item 23: Prefer class hierarchies to tagged classes(类层次结构优于带标签的类)
- Item 24: Favor static member classes over nonstatic(静态成员类优于非静态成员类)
- Item 25: Limit source files to a single top level class(源文件仅限有单个顶层类)
- Chapter 5. Generics(泛型)
- Item 26: Do not use raw types(不要使用原始类型)
- Item 27: Eliminate unchecked warnings(消除 unchecked 警告)
- Item 28: Prefer lists to arrays(list 优于数组)
- Item 29: Favor generic types(优先使用泛型)
- Item 30: Favor generic methods(优先使用泛型方法)
- Item 31: Use bounded wildcards to increase API flexibility(使用有界通配符增加 API 的灵活性)
- Item 32: Combine generics and varargs judiciously(明智地合用泛型和可变参数)
- Item 33: Consider typesafe heterogeneous containers(考虑类型安全的异构容器)
- Chapter 6. Enums and Annotations(枚举和注解)
- Item 34: Use enums instead of int constants(用枚举类型代替 int 常量)
- Item 35: Use instance fields instead of ordinals(使用实例字段替代序数)
- Item 36: Use EnumSet instead of bit fields(用 EnumSet 替代位字段)
- Item 37: Use EnumMap instead of ordinal indexing(使用 EnumMap 替换序数索引)
- Item 38: Emulate extensible enums with interfaces(使用接口模拟可扩展枚举)
- Item 39: Prefer annotations to naming patterns(注解优于命名模式)
- Item 40: Consistently use the Override annotation(坚持使用 @Override 注解)
- Item 41: Use marker interfaces to define types(使用标记接口定义类型)
- Chapter 7. Lambdas and Streams(λ 表达式和流)
- Item 42: Prefer lambdas to anonymous classes(λ 表达式优于匿名类)
- Item 43: Prefer method references to lambdas(方法引用优于 λ 表达式)
- Item 44: Favor the use of standard functional interfaces(优先使用标准函数式接口)
- Item 45: Use streams judiciously(明智地使用流)
- Item 46: Prefer side effect free functions in streams(在流中使用无副作用的函数)
- Item 47: Prefer Collection to Stream as a return type(优先选择 Collection 而不是流作为返回类型)
- Item 48: Use caution when making streams parallel(谨慎使用并行流)
- Chapter 8. Methods(方法)
- Item 49: Check parameters for validity(检查参数的有效性)
- Item 50: Make defensive copies when needed(在需要时制作防御性副本)
- Item 51: Design method signatures carefully(仔细设计方法签名)
- Item 52: Use overloading judiciously(明智地使用重载)
- Item 53: Use varargs judiciously(明智地使用可变参数)
- Item 54: Return empty collections or arrays, not nulls(返回空集合或数组,而不是 null)
- Item 55: Return optionals judiciously(明智地的返回 Optional)
- Item 56: Write doc comments for all exposed API elements(为所有公开的 API 元素编写文档注释)
- Chapter 9. General Programming(通用程序设计)
- Item 57: Minimize the scope of local variables(将局部变量的作用域最小化)
- Item 58: Prefer for-each loops to traditional for loops(for-each 循环优于传统的 for 循环)
- Item 59: Know and use the libraries(了解并使用库)
- Item 60: Avoid float and double if exact answers are required(若需要精确答案就应避免使用 float 和 double 类型)
- Item 61: Prefer primitive types to boxed primitives(基本数据类型优于包装类)
- Item 62: Avoid strings where other types are more appropriate(其他类型更合适时应避免使用字符串)
- Item 63: Beware the performance of string concatenation(当心字符串连接引起的性能问题)
- Item 64: Refer to objects by their interfaces(通过接口引用对象)
- Item 65: Prefer interfaces to reflection(接口优于反射)
- Item 66: Use native methods judiciously(明智地使用本地方法)
- Item 67: Optimize judiciously(明智地进行优化)
- Item 68: Adhere to generally accepted naming conventions(遵守被广泛认可的命名约定)
- Chapter 10. Exceptions(异常)
- Item 69: Use exceptions only for exceptional conditions(仅在确有异常条件下使用异常)
- Item 70: Use checked exceptions for recoverable conditions and runtime exceptions for programming errors(对可恢复情况使用 checked 异常,对编程错误使用运行时异常)
- Item 71: Avoid unnecessary use of checked exceptions(避免不必要地使用 checked 异常)
- Item 72: Favor the use of standard exceptions(鼓励复用标准异常)
- Item 73: Throw exceptions appropriate to the abstraction(抛出能用抽象解释的异常)
- Item 74: Document all exceptions thrown by each method(为每个方法记录会抛出的所有异常)
- Item 75: Include failure capture information in detail messages(异常详细消息中应包含捕获失败的信息)
- Item 76: Strive for failure atomicity(尽力保证故障原子性)
- Item 77: Don’t ignore exceptions(不要忽略异常)
- Chapter 11. Concurrency(并发)
- Item 78: Synchronize access to shared mutable data(对共享可变数据的同步访问)
- Item 79: Avoid excessive synchronization(避免过度同步)
- Item 80: Prefer executors, tasks, and streams to threads(Executor、task、流优于直接使用线程)
- Item 81: Prefer concurrency utilities to wait and notify(并发实用工具优于 wait 和 notify)
- Item 82: Document thread safety(文档应包含线程安全属性)
- Item 83: Use lazy initialization judiciously(明智地使用延迟初始化)
- Item 84: Don’t depend on the thread scheduler(不要依赖线程调度器)
- Chapter 12. Serialization(序列化)
- Item 85: Prefer alternatives to Java serialization(优先选择 Java 序列化的替代方案)
- Item 86: Implement Serializable with great caution(非常谨慎地实现 Serializable)
- Item 87: Consider using a custom serialized form(考虑使用自定义序列化形式)
- Item 88: Write readObject methods defensively(防御性地编写 readObject 方法)
- Item 89: For instance control, prefer enum types to readResolve(对于实例控制,枚举类型优于 readResolve)
- Item 90: Consider serialization proxies instead of serialized instances(考虑以序列化代理代替序列化实例)