7-原生类型 · Java 开源项目中文文档

# 7-原生类型 [原文链接](http://code.google.com/p/guava-libraries/wiki/PrimitivesExplained) [译文链接](http://ifeve.com/google-guava-primitives) 译者：沈义扬，校对：丁一 ## 概述 Java的原生类型就是指基本类型：byte、short、int、long、float、double、char和boolean。 _在从Guava查找原生类型方法之前，可以先查查_[**_Arrays_**](http://docs.oracle.com/javase/1.5.0/docs/api/java/util/Arrays.html)_类，或者对应的基础类型包装类，如_[**_Integer_**](http://docs.oracle.com/javase/1.5.0/docs/api/java/lang/Integer.html)**_。_** 原生类型不能当作对象或泛型的类型参数使用，这意味着许多通用方法都不能应用于它们。Guava提供了若干通用工具，包括原生类型数组与集合API的交互，原生类型和字节数组的相互转换，以及对某些原生类型的无符号形式的支持。 | **原生类型** | **Guava****工具类（都在****com.google.common.primitives****包****）** | |:--- |:--- | | byte | [`Bytes`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/primitives/Bytes.html), [`SignedBytes`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/primitives/SignedBytes.html), [`UnsignedBytes`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/primitives/UnsignedBytes.html) | | short | [`Shorts`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/primitives/Shorts.html) | | int | [`Ints`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/primitives/Ints.html), [`UnsignedInteger`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/primitives/UnsignedInteger.html), [`UnsignedInts`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/primitives/UnsignedInts.html) | | long | [`Longs`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/primitives/Longs.html), [`UnsignedLong`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/primitives/UnsignedLong.html), [`UnsignedLongs`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/primitives/UnsignedLongs.html) | | float | [`Floats`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/primitives/Floats.html) | | double | [`Doubles`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/primitives/Doubles.html) | | char | [`Chars`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/primitives/Chars.html) | | boolean | [`Booleans`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/primitives/Booleans.html) | Bytes工具类没有定义任何区分有符号和无符号字节的方法，而是把它们都放到了SignedBytes和UnsignedBytes工具类中，因为字节类型的符号性比起其它类型要略微含糊一些。 int和long的无符号形式方法在UnsignedInts和UnsignedLongs类中，但由于这两个类型的大多数用法都是有符号的，Ints和Longs类按照有符号形式处理方法的输入参数。此外，Guava为int和long的无符号形式提供了包装类，即UnsignedInteger和UnsignedLong，以帮助你使用类型系统，以极小的性能消耗对有符号和无符号值进行强制转换。在本章下面描述的方法签名中，我们用Wrapper表示JDK包装类，prim表示原生类型。（Prims表示相应的Guava工具类。） ## 原生类型数组工具原生类型数组是处理原生类型集合的最有效方式（从内存和性能双方面考虑）。Guava为此提供了许多工具方法。 | **方法签名** | **描述** | **类似方法** | **可用性** | |:--- |:--- |:--- |:--- | | List<Wrapper> asList(prim… backingArray) | 把数组转为相应包装类的List | [Arrays.asList](http://docs.oracle.com/javase/6/docs/api/java/util/Arrays.html#asList%28T...%29) | 符号无关* | | prim[] toArray(Collection<Wrapper> collection) | 把集合拷贝为数组，和collection.toArray()一样线程安全 | [Collection.toArray()](http://docs.oracle.com/javase/6/docs/api/java/util/Collection.html#toArray%28%29) | 符号无关 | | prim[] concat(prim[]… arrays) | 串联多个原生类型数组 | [Iterables.concat](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Iterables.html#concat%28java.lang.Iterable...%29) | 符号无关 | | boolean contains(prim[] array, prim target) | 判断原生类型数组是否包含给定值 | [Collection.contains](http://docs.oracle.com/javase/6/docs/api/java/util/Collection.html#contains%28java.lang.Object%29) | 符号无关 | | int indexOf(prim[] array, prim target) | 给定值在数组中首次出现处的索引，若不包含此值返回-1 | [List.indexOf](http://docs.oracle.com/javase/6/docs/api/java/util/List.html#indexOf%28java.lang.Object%29) | 符号无关 | | int lastIndexOf(prim[] array, prim target) | 给定值在数组最后出现的索引，若不包含此值返回-1 | [List.lastIndexOf](http://docs.oracle.com/javase/6/docs/api/java/util/List.html#lastIndexOf%28java.lang.Object%29) | 符号无关 | | prim min(prim… array) | 数组中最小的值 | [Collections.min](http://docs.oracle.com/javase/6/docs/api/java/util/Collections.html#min%28java.util.Collection%29) | 符号相关* | | prim max(prim… array) | 数组中最大的值 | [Collections.max](http://docs.oracle.com/javase/6/docs/api/java/util/Collections.html#max%28java.util.Collection%29) | 符号相关 | | String join(String separator, prim… array) | 把数组用给定分隔符连接为字符串 | [Joiner.on(separator).join](http://code.google.com/p/guava-libraries/wiki/StringsExplained#Joiner) | 符号相关 | | Comparator<prim[]> lexicographicalComparator() | 按字典序比较原生类型数组的Comparator | [Ordering.natural().lexicographical()](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Ordering.html#lexicographical%28%29) | 符号相关 | *符号无关方法存在于Bytes, Shorts, Ints, Longs, Floats, Doubles, Chars, Booleans。而UnsignedInts, UnsignedLongs, SignedBytes, 或UnsignedBytes不存在。 *符号相关方法存在于SignedBytes, UnsignedBytes, Shorts, Ints, Longs, Floats, Doubles, Chars, Booleans, UnsignedInts, UnsignedLongs。而Bytes不存在。 ## 通用工具方法 Guava为原生类型提供了若干JDK6没有的工具方法。但请注意，其中某些方法已经存在于JDK7中。 | **方法签名** | **描述** | **可用性** | |:--- |:--- |:--- | | int compare(prim a, prim b) | 传统的Comparator.compare方法，但针对原生类型。JDK7的原生类型包装类也提供这样的方法 | 符号相关 | | prim checkedCast(long value) | 把给定long值转为某一原生类型，若给定值不符合该原生类型，则抛出IllegalArgumentException | 仅适用于符号相关的整型* | | prim saturatedCast(long value) | 把给定long值转为某一原生类型，若给定值不符合则使用最接近的原生类型值 | 仅适用于符号相关的整型 | *这里的整型包括byte, short, int, long。不包括char, boolean, float, 或double。 _**译者注：不符合主要是指long值超出prim类型的范围，比如过大的long超出int范围。_ 注：com.google.common.math.DoubleMath提供了舍入double的方法，支持多种舍入模式。相见第12章的”浮点数运算”。 ## 字节转换方法 Guava提供了若干方法，用来把原生类型按**大字节序**与字节数组相互转换。所有这些方法都是符号无关的，此外Booleans没有提供任何下面的方法。 | **方法或字段签名** | **描述** | |:--- |:--- | | int BYTES | 常量：表示该原生类型需要的字节数 | | prim fromByteArray(byte[] bytes) | 使用字节数组的前Prims.BYTES个字节，按**大字节序**返回原生类型值；如果bytes.length <= Prims.BYTES，抛出IAE | | prim fromBytes(byte b1, …, byte bk) | 接受Prims.BYTES个字节参数，按**大字节序**返回原生类型值 | | byte[] toByteArray(prim value) | 按**大字节序**返回value的字节数组 | ## 无符号支持 JDK原生类型包装类提供了针对有符号类型的方法，而UnsignedInts和UnsignedLongs工具类提供了相应的无符号通用方法。UnsignedInts和UnsignedLongs直接处理原生类型：使用时，由你自己保证只传入了无符号类型的值。此外，对int和long，Guava提供了无符号包装类（[UnsignedInteger](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/primitives/UnsignedInteger.html)和[UnsignedLong](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/primitives/UnsignedLong.html)），来帮助你以极小的性能消耗，对有符号和无符号类型进行强制转换。 ### 无符号通用工具方法 JDK的原生类型包装类提供了有符号形式的类似方法。 | **方法签名** | **说明** | |:--- |:--- | | [`int UnsignedInts.parseUnsignedInt(String)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/…ogle/common/primitives/UnsignedInts.html#parseUnsignedInt%28java.lang.String%29)[`long UnsignedLongs.parseUnsignedLong(String)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/…le/common/primitives/UnsignedLongs.html#parseUnsignedLong%28java.lang.String%29) | 按无符号十进制解析字符串 | | [`int UnsignedInts.parseUnsignedInt(String string, int radix)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/…ogle/common/primitives/UnsignedInts.html#parseUnsignedInt%28java.lang.String, int%29)[`long UnsignedLongs.parseUnsignedLong(String string, int radix)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/…le/common/primitives/UnsignedLongs.html#parseUnsignedLong%28java.lang.String%29) | 按无符号的特定进制解析字符串 | | [`String UnsignedInts.toString(int)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/primitives/UnsignedInts.html#toString%28int%29)[`String UnsignedLongs.toString(long)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/primitives/UnsignedLongs.html#toString%28long%29) | 数字按无符号十进制转为字符串 | | [`String UnsignedInts.toString(int value, int radix)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/primitives/UnsignedInts.html#toString%28int, int%29)[`String UnsignedLongs.toString(long value, int radix)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/primitives/UnsignedLongs.html#toString%28long, int%29) | 数字按无符号特定进制转为字符串 | ### 无符号包装类无符号包装类包含了若干方法，让使用和转换更容易。 | **方法签名** | **说明** | |:--- |:--- | | UnsignedPrim add(UnsignedPrim), subtract, multiply, divide, remainder | 简单算术运算 | | UnsignedPrim valueOf(BigInteger) | 按给定BigInteger返回无符号对象，若BigInteger为负或不匹配，抛出IAE | | UnsignedPrim valueOf(long) | 按给定long返回无符号对象，若long为负或不匹配，抛出IAE | | UnsignedPrim asUnsigned(prim value) | 把给定的值当作无符号类型。例如，UnsignedInteger.asUnsigned(1<<31)的值为2<sup>31</sup>,尽管1<<31当作int时是负的 | | BigInteger bigIntegerValue() | 用BigInteger返回该无符号对象的值 | | toString(), toString(int radix) | 返回无符号值的字符串表示 |