性感的 Guava

转自:性感的 Guava

What’s Guava

Google 的一个开源项目,包含许多 Google 核心的 Java 常用库。

很多类似 Apache Common 的功能,但实现更优雅,项目也更活跃。

试图弥补 Java 语言的不足,很多特性被加入到最新的 Java 语言规范中。

Objects#equal

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Objects.equal("a", "a"); // return true
Objects.equal(null, "a"); // return false
Objects.equal("a", null); // return false
Objects.equal(null, null); // return true

Java7 引入的 Objects 类提供了一样的方法 Objects.equals。

Objects#hashCode

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public int hashCode() {
    return Objects.hashCode(firstName, lastname, address);
}

Java7 引入的 Objects 类提供了一样的方法 Objects.hash(Object…)。

Objects#toString

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public String toString() {
    return Objects.toStringHelper(this)
            .add("firstName", firstName)
            .add("lastName", lastName)
            .add("age", age)
            .toString();
}

高版本的 Guava 里的 toStringHelper 方法移到了 MoreObjects 里了。

Comparable

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public int compareTo(User that) {
    return CComparisonChain.start()
            .compare(this.firstName, that.firstName)
            .compare(this.lastName, that.lastName)
            .compare(this.age, that.age)
            .compareFalseFirst(this.locked, that.locked)
            .result();
}

PreConditions

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//Preconditions.checkNotNull(arg1, "参数 arg1 不正确,不能为空。");
checkNotNull(arg1, "参数 arg1 不正确,不能为空。");
checkArgument(arg2 > 18, "参数 arg2 不正确,值是%s,必须大于18。", arg2);
方法声明描述检查失败时抛出的异常
checkArgument(boolean)检查 boolean 是否为 true,用来检查传递给方法的参数。IllegalArgumentException
checkNotNull(T)检查 value 是否为 null,该方法直接返回 value,因此可以内嵌使用 checkNotNull。NullPointerException
checkState(boolean)用来检查对象的某些状态。IllegalStateException
checkElementIndex(int index, int size)列表、字符串或数组是否有效。index>=0 && index<sizeIndexOutOfBoundsException
checkPositionIndex(int index, int size)检查index作为位置值对某个列表、字符串或数组是否有效。index>=0 && index<=sizeIndexOutOfBoundsException
checkPositionIndexes(int start, int end, int size)检查[start, end]表示的位置范围对某个列表、字符串或数组是否有效IndexOutOfBoundsException

Optional#or

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String username = null;
// String name = username != null ? username ? "游客";
// String name = Objects.firstNonNull(username, "游客");
String name = Optional.of(username).or("游客");
System.out.println(name + "你好!"); // 游客你好!

相当于 JavaScript 中: var a = x || y;

Return null — What’s mean?

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// 可能会返回 null
User user = User.getUserByName("tom");
if(user != null) {
    user.doSomething();
} else {
    admin.doSomething();
}
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// 可能会返回 null
Optional<User> user = User.getUserByName("tom");
user.or(admin).doSomething();

Java8 里提供了 Optional:http://blog.jhades.org/java-8-how-to-use-optional/

Strings#split

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Iterable<String> strs = Splitter.on(",")
            .trimResults()
            .omitEmptyStrings()
            .split(",a,,           b,");
System.out.println(strs); // [a, b]

Strings#utils

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Strings.padStart("521", 5, '0'); // 00521
Strings.padEnd("33", 4, '#'); // 33##
Strings.repeat("hello ", 3); // hello hello hello

Strings#join

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Joiner.on(",")
    .skipNulls()
    .join(new String[] {"1", "5", null, "7"});
// return "1,5,7"

CharMatcher

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// 移除 control 字符
CharMatcher.JAVA_ISO_CONTROL.removeFrom(string);
// 只保留数字字符
CharMatcher.DIGIT.relationFrom(string);
// 去除两端的空格, 并把中间的连续空格替换成单个空格
CharMatcher.WHITESPACE.trimAndCollapseFrom(string, ' ');
// 用*号替换所有数字
CharMatcher.JAVA_DIGIT.replaceFrom(string, '*');
// 只保留数字和小写字母
CharMatcher.JAVA_DIGIT.or(CharMatcher.JAVA_LOWER_CASE).retainFrom(string);
// returns "constantName"
CaseFormat.UPPER_UNDERSCORE.to(CaseFormat.LOWER_CAMEL, "CONSTANT_NAME");

Collection – Multiset

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Multiset<String> votes =HashMultiset.create();
votes.add("张三");
votes.add("李四");
votes.add("张三");
int count = votes.count("张三"); // 返回2
Set<String> strings = votes.elementSet(); // 不重复元素的Set

Collection – Multimap

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Multimap<String, String> tags = ArrayListMultimap.create();
tags.put("《红楼梦》", "好看");
tags.put("《红楼梦》", "有内容");
tags.put("《红楼梦》", "价格太贵");
tags.put("《水浒》", "打打杀杀");

结构相当于Map<String, List

>但操作更便利。

Collection – Bimap

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BiMap<String, String> couple = HashBiMap.create();
couple.put("梁山伯", "祝英台");
couple.put("罗密欧", "朱丽叶");
couple.inverse().get("朱丽叶"); // 返回 罗密欧
couple.put("马文才", 祝英台"); // 抛出 IllegalArgumentException 异常

Collection – Table

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Table<String, String, Double> table = HashBasedtable.create();
table.put("张三", "数学", 95.0);
table.put("张三", "语文", 88.5);
table.put("李四", "数学", 90.0);
table.put("李四", "语文", 92.0);
table.get("张三", "数学"); // 返回 95.0

 

Collection – utils

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// Java7 里的 List<String> list = new ArrayList<>();
List<String> list1 = Lists.newArrayList();
List<String> list2 = Lists.newArrayList("alpha, "beta", gamma");
List<String> list3 = Lists.newArrayListWithExpectedSize(100);
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List countUp = Ints.asList(1, 2, 3, 4, 5);
List countDown = Lists.reverse(countUp); // {5, 4, 3, 2, 1}
List<List> parts = Lists.partition(countUp, 2); // {{1,2}, {3,4}, {5}}
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Map<String, Integer> left = ImmutableMap.of("a", 1, "b", 2, "c", 3);
Map<String, Integer> right = ImmutableMap.of("b", 2, "d", 4);
MapDifference<String, Integer> diff = Maps.difference(left, right);
diff.entriesInCommon(); // {"b" => 2}
diff.entriesOnlyOnLeft(); // {"a" => 1, "c" => 3}
diff.entriesOnlyOnRight(); // {"d" => 4}

Collection – Fuctional

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boolean hasYoung = Iterables.any(users, new Predicate<User>() {
    @Override
    public boolean apply(@Nullable User user) {
        return user.getAge() < 18;
    }
});
boolean hasYoung = Iterables.all(users, new Predicate<User>() {
    @Override
    public boolean apply(@Nullable User user) {
        return user.getSex() == MALE;
    }
});
boolean hasYoung = Iterables.find(users, new Predicate<User>() {
    @Override
    public boolean apply(@Nullable User user) {
        return "Tom".equals(user.getName());
    }
});
boolean hasYoung = Iterables.removeIf(users.iterator(), new Predicate<User>() {
    @Override
    public boolean apply(@Nullable User user) {
        return user.getAge() > 80;
    }
});

Java8 Stream API

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Optional<String> reduced = stringCollection
        .stream()
        .sorted()
        .reduce((s1, s2) -> s1 + "#" + s2);
 
reduced.ifPresent(System.out::println);
// "aaa1#aaa2#bbb1#bbb2#bbb3#ccc#ddd1"

Supplier

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static Supplier<DateAndDuration> yearEndSupplier;
static {
    Supplier<DateAndDuration> supplier = new Supplier<DateAndDuration>() {
        @Override
        public DateAndDuration get() {
            Calendar calendar = Calendar.getInstance();
            int year = calendar.get(Calendar.YEAR);
            calendar.clear();
            calendar.set(year, 11, 31, 23, 59, 59);
            long duration =calendar.getTimeInMills() - new Date().getTime();
            return new dateAndDuration(calendar.getTime(), duration);
        }
    };
    yearEndSupplier = Suppliers.memoizeWithException(supplier, supplier.get()
                            .getDuration(), TimeUnit.MICROSECONDS);
}
public static Date getYearEnd() {
    return yearEndSupplier.get().getDate();
}

Java8 中引入了 Supplier 接口:http://www.byteslounge.com/tutorials/java-8-consumer-and-supplier

Cache

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witerCache = CacheBuilder.newBuilder()
            .maximumSize(this.fileCacheSize).removalListener((RemovalListener) {
    try {
        objectObjectRemovalNotification.getValue().flush();
        objectObjectRemovalNotificaiont.getValue().close();
        if(log.isDebugEnabled()) {
            log.debug("释放文件写入的write。");
        }
    } catche(IOException e) {
        log.error("关闭文件出错", e);
    }
}).build();

Other

Math – 最大公约数、各种舍入、指数计算等等

IO – 文件拷贝、URL 访问、遍历文件目录等等 EventBus / 多线程 / Range

 

转载请并标注: “本文转载自 linkedkeeper.com (文/张松然)”

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