## Chapter 9. General Programming(通用程序设计)
### Item 63: Beware the performance of string concatenation(当心字符串连接引起的性能问题)
The string concatenation operator (+) is a convenient way to combine a few strings into one. It is fine for generating a single line of output or constructing the string representation of a small, fixed-size object, but it does not scale. Using **the string concatenation operator repeatedly to concatenate n strings requires time quadratic in n.** This is an unfortunate consequence of the fact that strings are immutable (Item 17). When two strings are concatenated, the contents of both are copied.
字符串连接操作符 `(+)` 是将几个字符串组合成一个字符串的简便方法。对于生成单行输出或构造一个小的、固定大小的对象的字符串表示形式,它是可以的,但是它不能伸缩。使用 **字符串串联运算符重复串联 n 个字符串需要 n 的平方级时间。** 这是字符串不可变这一事实导致的结果([Item-17](/Chapter-4/Chapter-4-Item-17-Minimize-mutability.md))。当连接两个字符串时,将复制这两个字符串的内容。
For example, consider this method, which constructs the string representation of a billing statement by repeatedly concatenating a line for each item:
例如,考虑这个方法,它通过将每个账单项目重复连接到一行来构造账单语句的字符串表示:
```
// Inappropriate use of string concatenation - Performs poorly!
public String statement() {
String result = "";
for (int i = 0; i < numItems(); i++)
result += lineForItem(i); // String concatenation
return result;
}
```
The method performs abysmally if the number of items is large. **To achieve acceptable performance, use a StringBuilder in place of a String** to store the statement under construction:
如果项的数量很大,则该方法的性能非常糟糕。**要获得能接受的性能,请使用 StringBuilder 代替 String** 来存储正在构建的语句:
```
public String statement() {
StringBuilder b = new StringBuilder(numItems() * LINE_WIDTH);
for (int i = 0; i < numItems(); i++)
b.append(lineForItem(i));
return b.toString();
}
```
A lot of work has gone into making string concatenation faster since Java 6, but the difference in the performance of the two methods is still dramatic: If numItems returns 100 and lineForItem returns an 80-character string, the second method runs 6.5 times faster than the first on my machine. Because the first method is quadratic in the number of items and the second is linear, the performance difference gets much larger as the number of items grows. Note that the second method preallocates a StringBuilder large enough to hold the entire result, eliminating the need for automatic growth. Even if it is detuned to use a default-sized StringBuilder, it is still 5.5 times faster than the first method.
自 Java 6 以来,为了使字符串连接更快,已经做了大量工作,但是这两个方法在性能上的差异仍然很大:如果 numItems 返回 100,lineForItem 返回 80 个字符串,那么第二个方法在我的机器上运行的速度是第一个方法的 6.5 倍。由于第一种方法在项目数量上是平方级的,而第二种方法是线性的,所以随着项目数量的增加,性能差异会变得越来越大。注意,第二个方法预先分配了一个足够大的 StringBuilder 来保存整个结果,从而消除了自动增长的需要。即使使用默认大小的 StringBuilder,它仍然比第一个方法快 5.5 倍。
The moral is simple: **Don’t use the string concatenation operator to combine more than a few strings** unless performance is irrelevant. Use StringBuilder’s append method instead. Alternatively, use a character array, or process the strings one at a time instead of combining them.
道理很简单:**不要使用字符串连接操作符合并多个字符串**,除非性能无关紧要。否则使用 StringBuilder 的 append 方法。或者,使用字符数组,再或者一次只处理一个字符串,而不是组合它们。
---
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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)