guava 实用操作集合

[size=large][b][url=http://code.google.com/p/guava-libraries/]guava[/url][/b][/size]是 google 几个java核心类库的集合,包括集合、缓存、原生类型、并发、常用注解、基本字符串操作和I/O等等。

大家平时经常遇到某些相同的问题，自己写代码也都能解决。但是久而久之会感觉到很痛苦，因为我们一而再，再而三的重复发明轮子。为了不再忍受痛苦，也许我们可以总结自己的类库，但是新的问题来了。自己总结的类库很难与大家分享，不能帮助到更多人。同时自己的类库要不断的进行维护。guava 正是出于这样的目的而来的。

只说不练不行啊，让我们举上一两个例子
[b]判断 String不为null,且不为空[/b]

String str=...;
//use java
if(str !=null && !str.isEmpty()){
    //do something
}


//use guava
if(!Strings.isNullOrEmpty(str)){
    //do something
}

上而的例子还不是很给力，让我们举一个更给力的例子。[b]复制文件[/b]

File from=...;
File to=...;

//use java
FileInputStream in=new FileInputStream(from);
FileOutputStream out=new FileOutputStream(to);
byte[] buff=new byte[1024];
int readLength=-1;
while((readLength = in.read(buff)) > 0){
    out.write(buff, 0, readLength);
}
in.close();
out.close();

//use guava
Files.copy(from,to); //注意，只用了一行代码噢

通过上面的例子，已经能感觉到guava的强大。接下来，让我们更深入看看guava的其他功能。[url=http://guava-libraries.googlecode.com/svn/tags/release09/javadoc/index.html]guava(r09-api)[/url]分为这几个包
[list]
[*][b]base[/b] 基本的工具类与接口
[*][b]io[/b] io流相关的工具类与方法
[*][b]net[/b] 网络地址相关的工具类与方法
[*][b]primitives[/b] 原始类型的工具类
[*][b]collect[/b] 通用集合接口与实现，与其集合相关工具类
[*][b]util.concurrent[/b] 并发相关工具类
[/list]

[size=large][b][url=http://guava-libraries.googlecode.com/svn/tags/release09/javadoc/com/google/common/base/package-summary.html]base[/url]包[/b][/size]

[b]字符串相关工具类[/b]
[url=http://guava-libraries.googlecode.com/svn/tags/release09/javadoc/com/google/common/base/Strings.html]Strings[/url]

public class StringsTest {
    @Test
    public void test() {
        //将空字符串转换为null
        Assert.assertEquals(null, Strings.emptyToNull(""));
        //将null转换为空字符串
        Assert.assertEquals("", Strings.nullToEmpty(null));
        //判断字符串为null或者为空
        Assert.assertTrue(Strings.isNullOrEmpty("") && Strings.isNullOrEmpty(null));
        //将字符串重复
        Assert.assertEquals("javajavajava", Strings.repeat("java", 3));
    }
}

[url=http://guava-libraries.googlecode.com/svn/tags/release09/javadoc/com/google/common/base/CharMatcher.html]CharMatcher[/url]

public class CharMatcherTest {
    @Test
    public void test() {
        String source = "a1b2c3";
        CharMatcher matcher = CharMatcher.DIGIT; //预定义的 DIGIT 类型
        Assert.assertTrue(mathcer.match('8'));
        Assert.assertEquals("123", matcher.retainFrom(source));
        Assert.assertEquals(3, matcher.countIn(source));
        Assert.assertEquals("abc", matcher.removeFrom(source));
        Assert.assertEquals("a2b3c", matcher.trimFrom("1a2b3c4"));
    }
}

[url=http://guava-libraries.googlecode.com/svn/tags/release09/javadoc/com/google/common/base/Joiner.html]Jioner[/url]

public class JoinerTest {
    @Test
    public void test() {
        Assert.assertEquals("2011-08-04", Joiner.on("-").join("2011", "08", "04"));
    }
}

[url=http://guava-libraries.googlecode.com/svn/tags/release09/javadoc/com/google/common/base/Splitter.html]Splitter[/url]

public class SplitterTest {
    @Test
    public void test() {
        Splitter.on(',').split("a,b"); //结果返回Iterable<String>,包含 "a" and "b"
        //将结果中的元素trim
        //结果依然包含  "a" 和 "b" ,而不是 "a " 和 " b"
        Splitter.on(',').trimResults().split("a , b"); 
        //忽略空字符串
        //结果必须是"a" 和 "b",而不是 "a" ,"" 和 "b"
        Splitter.on(',').omitEmptyStrings().split("a,,b"); 
    }
}

[url=http://guava-libraries.googlecode.com/svn/tags/release09/javadoc/com/google/common/base/CaseFormat.html]CaseFormat[/url]

public class CaseFormatTest {

    @Test
    public void test() throws IOException {
        // helloGuava => HELLO_GUAVA
        Assert.assertEquals("HELLO_GUAVA", CaseFormat.LOWER_CAMEL.to(CaseFormat.UPPER_UNDERSCORE, "helloGuava"));
        // hello-guava => HelloGuava
        Assert.assertEquals("HelloGuava", CaseFormat.LOWER_HYPHEN.to(CaseFormat.UPPER_CAMEL, "hello-guava"));
    }
}

[b]其他[/b]
[url=http://guava-libraries.googlecode.com/svn/tags/release09/javadoc/com/google/common/base/Preconditions.html]Preconditions[/url]
有些方法在执行前，先要检查传入的参数是否正确，或者类的状态是否正确。通常会这样做

if (count <= 0) {
       throw new IllegalArgumentException("must be positive: " + count);
}

guava就可以这样，达到相同的效果

Preconditions.checkArgument(count > 0, "must be positive: %s", count);

[b][size=large][url=http://guava-libraries.googlecode.com/svn/tags/release09/javadoc/com/google/common/io/package-summary.html]I/O[/url]包[/size][/b]
[url=http://guava-libraries.googlecode.com/svn/tags/release09/javadoc/com/google/common/io/ByteStreams.html]ByteStreams[/url]提供了针对字节流的工具方法

InputStream from=...;
OutputStream to=...;
ByteStreams.copy(from,to); //复制

[url=http://guava-libraries.googlecode.com/svn/tags/release09/javadoc/com/google/common/io/CharStreams.html]CharStreams[/url]提供了针对字符流的工具方法

Reader from =...;
Writer to =...;
CharStreams.copy(from, to); //复制

[url=http://guava-libraries.googlecode.com/svn/tags/release09/javadoc/com/google/common/io/Files.html]Files[/url]提供了针对文件的工具方法

File from=...;
File to=...;
Files.copy(from, to); //复制

Files.deleteDirectoryContents(File directory); //删除文件夹下的内容(包括文件与子文件夹)
Files.deleteRecursively(File file); //删除文件或者文件夹
Files.move(File from, File to); //移动文件

[url=http://guava-libraries.googlecode.com/svn/tags/release09/javadoc/com/google/common/io/Resources.html]Resources[/url]提供了针对classpath下资源操作的工具方法

URL url = Resources.getResource("config.xml"); //获取classpath根下的config.xml文件url