Guava基本操作
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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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
@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
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)
@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
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：
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;
}
}
joiner useForNull(final String value)用value替换null元素值
@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
joiner withKeyValueSeparator(String value) map连接器，keyValueSeparator为key和value之间的分隔符
@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}
b. splitter 拆分器
splitter on 拆分
@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]
splitter trimResults 拆分去除前后空格
@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]
splitter omitEmptyStrings 去除拆分出来空的字符串
@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 ]
splitter fixedLength(int lenght) 把字符串按固定长度分割
@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]
b. charMatcher 匹配器
charMatcher is(Char char) 给单一字符匹配
@Test
public void isTest() {
String str = "12312,agg";
CharMatcher charMatcher1 = CharMatcher.is('g');
System.out.println(charMatcher1.retainFrom(str));
}
结果：gg
charMatcher retainFrom(String s) 在字符序列中保留匹配字符，移除其他字符
@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
charMatcher matchersAllOf(Char char) 测试是否字符序列所有字符都匹配
@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
2.原生类型
定义list，map
@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();
}
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
@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);
});
}
Multimap.get(key)即使没有key值，会返回空的list。
Multimap.keySet()返回的用set
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()
