其实我用guava差不多大半年时间了,发现guava真的特别好用,又会使代码变得很简洁,最近又系统的学习了一下,大致讲一下😊

什么是guava呢?

guava就是类库,是java api的增强与扩展,里面有大量的方法供我们使用,使用之前需要引入包

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com.google.guava
guava
27.0.1


guava有哪些方法呢?我们先从以下几方面开始学习:

字符串处理:分割,连接,填充
新增的集合类型
原生类型
1.字符串的处理:分割,连接,填充

a. joiner 连接器

joiner on就是将list用,连接转成字符串

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@Test
public void joinerListTest() {
List lists = Lists.newArrayList(“a”,”b”,”g”,”8″,”9″);
String result = Joiner.on(“,”).join(lists);
System.out.println(result);
}

结果:a,b,g,8,9
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joiner skipNulls()连接跳过null元素(第一个test为了跟第二个进行比对一下)

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@Test
public void joinerListTest1() {
List lists = Lists.newArrayList(“a”,”b”,”g”,null,”8″,”9″);
String result = Joiner.on(“,”).join(lists);
System.out.println(result);
}

结果:a,b,g,null,8,9

@Test
public void joinerListTest2() {
List lists = Lists.newArrayList(“a”,”b,”g”,null,”8″,”9″);
String result = Joiner.on(“,”).skipNulls().join(lists);
System.out.println(result);
}
结果:a,b,g,8,9
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如果连接的时候list里面含有null值,会报空指针,因为join实现如下:

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1.
public final String join(Iterable parts) {
return this.join(parts.iterator());
}

2.
public final String join(Iterator parts) {
return this.appendTo(new StringBuilder(), parts).toString();
}

3.
@CanIgnoreReturnValue
public final StringBuilder appendTo(StringBuilder builder, Iterator parts) {
try {
this.appendTo((Appendable)builder, (Iterator)parts);
return builder;
} catch (IOException var4) {
throw new AssertionError(var4);
}
}

4.
@CanIgnoreReturnValue
public A appendTo(A appendable, Iterator parts) throws IOException {
Preconditions.checkNotNull(appendable);
if (parts.hasNext()) {
appendable.append(this.toString(parts.next()));

while(parts.hasNext()) {
appendable.append(this.separator);
appendable.append(this.toString(parts.next()));
}
}
return appendable;
}

5.
@CanIgnoreReturnValue
public static T checkNotNull(T reference) {
if (reference == null) {
throw new NullPointerException();
} else {
return reference;
}
}
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joiner useForNull(final String value)用value替换null元素值

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@Test
public void useNullListTest() {
List lists = Lists.newArrayList(“a”, “b”, “g”, null, “8”, “9”);
String result = Joiner.on(“,”).useForNull(“哈哈”).join(lists);
System.out.println(result);
}

结果:a,b,g,哈哈,8,9
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joiner withKeyValueSeparator(String value) map连接器,keyValueSeparator为key和value之间的分隔符

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@Test
public void withMapTest() {
Map maps = Maps.newHashMap();
maps.put(1, “哈哈”);
maps.put(2, “压压”);
String result = Joiner.on(“,”).withKeyValueSeparator(“:”).join(maps);
System.out.println(result);
System.out.println(maps);
}

结果:
1:哈哈,2:压压
{1=哈哈, 2=压压}
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b. splitter 拆分器

splitter on 拆分

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@Test
public void splitterListTest() {
String test = “34344,34,34,哈哈”;
List lists = Splitter.on(“,”).splitToList(test);
System.out.println(lists);
}

结果:[34344, 34, 34, 哈哈]
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splitter trimResults 拆分去除前后空格

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@Test
public void trimResultListTest() {
String test = ” 34344,34,34,哈哈 “;
List lists = Splitter.on(“,”).trimResults().splitToList(test);
System.out.println(lists);
}

结果:[34344, 34, 34, 哈哈]
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splitter omitEmptyStrings 去除拆分出来空的字符串

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@Test
public void omitEmptyStringsTest() {
String test = ” 3434,434,34,,哈哈 “;
List lists = Splitter.on(“,”).omitEmptyStrings().splitToList(test);
System.out.println(lists);
}

结果:[ 3434, 434, 34, 哈哈 ]
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splitter fixedLength(int lenght) 把字符串按固定长度分割

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@Test
public void fixedLengthTest() {
String test = “343443434哈哈”;
List lists = Splitter.fixedLength(3).splitToList(test);
System.out.println(lists);
}

结果:[343, 443, 434, 哈哈]
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b. charMatcher 匹配器

charMatcher is(Char char) 给单一字符匹配

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@Test
public void isTest() {
String str = “12312,agg”;
CharMatcher charMatcher1 = CharMatcher.is(‘g’);
System.out.println(charMatcher1.retainFrom(str));
}

结果:gg
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charMatcher retainFrom(String s) 在字符序列中保留匹配字符,移除其他字符

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@Test
public void charMatcherTest() {
String str = “12312,agg “;
//两个匹配符,先匹配再操作
CharMatcher charMatcher1 = CharMatcher.is(‘1’);
CharMatcher charMatcher2 = CharMatcher.is(‘2’);
//两个CharMatcher或操作
CharMatcher charMatcher3 = charMatcher1.or(charMatcher2);
System.out.println(charMatcher3.retainFrom(str));
}

结果:1212
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charMatcher matchersAllOf(Char char) 测试是否字符序列所有字符都匹配

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@Test
public void matchesAllOfTest() {
String str = “12312,agg”;
CharMatcher charMatcher1 = CharMatcher.is(‘g’);
System.out.println(charMatcher1.matchesAllOf(str));
}

结果:false
@Test
public void matchesAllOfTest() {
String str = “ggggg”;
CharMatcher charMatcher1 = CharMatcher.is(‘g’);
System.out.println(charMatcher1.matchesAllOf(str));
}

结果:true
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2.原生类型

定义list,map

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@Test
public void test() {
//JDK
List list = new ArrayList();
list.add(“a”);
list.add(“b”);
list.add(“c”);
list.add(“d”);
//guava
List lists = Lists.newArrayList(“a”, “b”, “g”, null, “8”, “9”);
List lists1 = Lists.newArrayList();
Map maps = Maps.newHashMap();
}
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3.新增集合(这里我只讲一下Mulitmap,平时用这个会使代码很方便,这里我就多讲一下)

Multimap就是将相同key的value值放在一个list里面,这样子取相同key下面的所有value值就非常简单了,不然还得for循环去匹配,把相同key值的value值找出来,在进行处理。map键值key不能重复,所以当遇到这样子场景的时候map就非常不适合了,下面我们看下例子:

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@Test
public void test1(){
//假设a表跟b表是一对多的关系,通过a表id是b表的外键,比如从b表查出了一堆数据,想把a.id相同的数据放在一起,A,B分别代表a,b表的model
List bList = BMapper.selectByExample(XXXXX);
Multimap bMultimap = ArrayListMultimap.create();
bList.forEach(v->{
bMultimap.put(v.getAId(),v);
});

}
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Multimap.get(key)即使没有key值,会返回空的list。

Multimap.keySet()返回的用set表示的不重复key;Multimap.keys()返回的是用Multiset表示的key,key数量跟value值数量一致;Multimap.containKeys()是表示是否包含这个key;

Multimap.size()返回所有值的个数,而非不同键的个数。要得到不同键的个数,要用Multimap.keySet().size()