## 1. Collections

### 1.1 排序

`List<Integer> list = new ArrayList<>();  list.add(2);  list.add(1);  list.add(3);  Collections.sort(list);//升序  System.out.println(list);  Collections.reverse(list);//降序  System.out.println(list);`

`[1, 2, 3][3, 2, 1]`

### 1.2 获取最大或最小值

`List<Integer> list = new ArrayList<>();list.add(2);list.add(1);list.add(3);Integer max = Collections.max(list);//获取最大值Integer min = Collections.min(list);//获取最小值System.out.println(max);System.out.println(min);`

`31`

### 1.3 转换线程安全集合

`List<Integer> list = new ArrayList<>();  list.add(2);  list.add(1);  list.add(3);  List<Integer> integers = Collections.synchronizedList(list);//将ArrayList转换成线程安全集合  System.out.println(integers);`

### 1.4 返回空集合

`private List<Integer> fun(List<Integer> list) {    if (list == null || list.size() == 0) {        return Collections.emptyList();    }    //业务处理    return list;}`

### 1.5 二分查找

`​binarySearch​`​​方法提供了一个非常好用的​`​二分查找​`​功能，只用传入指定集合和需要找到的key即可。例如：

`List<Integer> list = new ArrayList<>();list.add(2);list.add(1);list.add(3);int i = Collections.binarySearch(list, 3);//二分查找System.out.println(i );`

`2`

### 1.6 转换成不可修改集合

`List<Integer> list = new ArrayList<>();list.add(2);list.add(1);list.add(3);List<Integer> integers = Collections.unmodifiableList(list);integers.add(4);System.out.println(integers);`

`Exception in thread "main" java.lang.UnsupportedOperationException at java.util.Collections\$UnmodifiableCollection.add(Collections.java:1055) at com.sue.jump.service.test1.UtilTest.main(UtilTest.java:19)`

## 2. CollectionUtils

### 2.1 集合判空

`List<Integer> list = new ArrayList<>();list.add(2);list.add(1);list.add(3);if (CollectionUtils.isEmpty(list)) {    System.out.println("集合为空");}if (CollectionUtils.isNotEmpty(list)) {    System.out.println("集合不为空");}`

### 2.2 对两个集合进行操作

`List<Integer> list = new ArrayList<>();list.add(2);list.add(1);list.add(3);List<Integer> list2 = new ArrayList<>();list2.add(2);list2.add(4);//获取并集Collection<Integer> unionList = CollectionUtils.union(list, list2);System.out.println(unionList);//获取交集Collection<Integer> intersectionList = CollectionUtils.intersection(list, list2);System.out.println(intersectionList);//获取交集的补集Collection<Integer> disjunctionList = CollectionUtils.disjunction(list, list2);System.out.println(disjunctionList);//获取差集Collection<Integer> subtractList = CollectionUtils.subtract(list, list2);System.out.println(subtractList);`

`[1, 2, 3, 4][2][1, 3, 4][1, 3]`

## 3. Lists

### 3.1 创建空集合

`List<Integer> list = Lists.newArrayList();`

### 3.2 快速初始化集合

`List<Integer> list = Lists.newArrayList(1, 2, 3);`

`[1, 2, 3]`

### 3.3 笛卡尔积

`List<Integer> list1 = Lists.newArrayList(1, 2, 3);List<Integer> list2 = Lists.newArrayList(4,5);List<List<Integer>> productList = Lists.cartesianProduct(list1,list2);System.out.println(productList);`

`[[1, 4], [1, 5], [2, 4], [2, 5], [3, 4], [3, 5]]`

### 3.4 分页

`List<Integer> list = Lists.newArrayList(1, 2, 3, 4, 5);List<List<Integer>> partitionList = Lists.partition(list, 2);System.out.println(partitionList);`

`[[1, 2], [3, 4], [5]]`

### 3.5 流处理

`List<String> list = Lists.newArrayList("a","b","c");List<String> transformList = Lists.transform(list, x -> x.toUpperCase());System.out.println(transformList);`

### 3.6 颠倒顺序

Lists的有颠倒顺序的方法​`​reverse​`​。例如：

`List<Integer> list = Lists.newArrayList(3, 1, 2);List<Integer> reverseList = Lists.reverse(list);System.out.println(reverseList);`

`[2, 1, 3]`

list的原始顺序是312，使用​`​reverse​`​方法颠倒顺序之后，变成了213。

Lists还有其他的好用的工具，我在这里只是抛砖引玉，有兴趣的朋友，可以仔细研究一下。

## 4. Objects

### 4.1 对象判空

`Integer integer = new Integer(1);if (Objects.isNull(integer)) {    System.out.println("对象为空");}if (Objects.nonNull(integer)) {    System.out.println("对象不为空");}`

### 4.2 对象为空抛异常

`Integer integer1 = new Integer(128);Objects.requireNonNull(integer1);Objects.requireNonNull(integer1, "参数不能为空");Objects.requireNonNull(integer1, () -> "参数不能为空");`

### 4.3 判断两个对象是否相等

`Integer integer1 = new Integer(1);Integer integer2 = new Integer(1);System.out.println(Objects.equals(integer1, integer2));`

`true`

`Integer integer1 = new Integer(1);Long integer2 = new Long(1);System.out.println(Objects.equals(integer1, integer2));`

`false`

### 4.4 获取对象的hashCode

`String str = new String("abc");System.out.println(Objects.hashCode(str));`

`96354`

Objects的内容先介绍到这里，有兴趣的小伙们，可以看看下面更多的方法：

## 5. BooleanUtils

### 5.1 判断true或false

`Boolean aBoolean = new Boolean(true);System.out.println(BooleanUtils.isTrue(aBoolean));System.out.println(BooleanUtils.isFalse(aBoolean));`

### 5.2 判断不为true或不为false

`Boolean aBoolean = new Boolean(true);Boolean aBoolean1 = null;System.out.println(BooleanUtils.isNotTrue(aBoolean));System.out.println(BooleanUtils.isNotTrue(aBoolean1));System.out.println(BooleanUtils.isNotFalse(aBoolean));System.out.println(BooleanUtils.isNotFalse(aBoolean1));`

`falsetruetruetrue`

### 5.3 转换成数字

`Boolean aBoolean = new Boolean(true);Boolean aBoolean1 = new Boolean(false);System.out.println(BooleanUtils.toInteger(aBoolean));System.out.println(BooleanUtils.toInteger(aBoolean1));`

`10`

### 5.4 Boolean转换成布尔值

`Boolean aBoolean = new Boolean(true);Boolean aBoolean1 = null;System.out.println(BooleanUtils.toBoolean(aBoolean));System.out.println(BooleanUtils.toBoolean(aBoolean1));System.out.println(BooleanUtils.toBooleanDefaultIfNull(aBoolean1, false));`

BooleanUtils类的方法还有很多，有兴趣的小伙伴可以看看下面的内容：

## 6. StringUtils

`​字符串​`​（String）在我们的日常工作中，用得非常非常非常多。

### 6.1 字符串判空

StringUtils给我们提供了多个判空的静态方法，例如：

`String str1 = null;String str2 = "";String str3 = " ";String str4 = "abc";System.out.println(StringUtils.isEmpty(str1));System.out.println(StringUtils.isEmpty(str2));System.out.println(StringUtils.isEmpty(str3));System.out.println(StringUtils.isEmpty(str4));System.out.println("=====");System.out.println(StringUtils.isNotEmpty(str1));System.out.println(StringUtils.isNotEmpty(str2));System.out.println(StringUtils.isNotEmpty(str3));System.out.println(StringUtils.isNotEmpty(str4));System.out.println("=====");System.out.println(StringUtils.isBlank(str1));System.out.println(StringUtils.isBlank(str2));System.out.println(StringUtils.isBlank(str3));System.out.println(StringUtils.isBlank(str4));System.out.println("=====");System.out.println(StringUtils.isNotBlank(str1));System.out.println(StringUtils.isNotBlank(str2));System.out.println(StringUtils.isNotBlank(str3));System.out.println(StringUtils.isNotBlank(str4));`

`truetruefalsefalse=====falsefalsetruetrue=====truetruetruefalse=====falsefalsefalsetrue`

### 6.2 分隔字符串

`String str1 = null;System.out.println(StringUtils.split(str1,","));System.out.println(str1.split(","));`

`nullException in thread "main" java.lang.NullPointerException at com.sue.jump.service.test1.UtilTest.main(UtilTest.java:21)`

### 6.3 判断是否纯数字

`String str1 = "123";String str2 = "123q";String str3 = "0.33";System.out.println(StringUtils.isNumeric(str1));System.out.println(StringUtils.isNumeric(str2));System.out.println(StringUtils.isNumeric(str3));`

`truefalsefalse`

### 6.4 将集合拼接成字符串

`List<String> list = Lists.newArrayList("a", "b", "c");List<Integer> list2 = Lists.newArrayList(1, 2, 3);System.out.println(StringUtils.join(list, ","));System.out.println(StringUtils.join(list2, " "));`

`a,b,c1 2 3`

## 7. Assert

### 7.1 断言参数是否为空

`String str = null;Assert.isNull(str, "str必须为空");Assert.isNull(str, () -> "str必须为空");Assert.notNull(str, "str不能为空");`

### 7.2 断言集合是否为空

`List<String> list = null;Map<String, String> map = null;Assert.notEmpty(list, "list不能为空");Assert.notEmpty(list, () -> "list不能为空");Assert.notEmpty(map, "map不能为空");`

### 7.3 断言条件是否为空

`List<String> list = null;Assert.isTrue(CollectionUtils.isNotEmpty(list), "list不能为空");Assert.isTrue(CollectionUtils.isNotEmpty(list), () -> "list不能为空");`

## 8. IOUtils

`​IO​`​流在我们日常工作中也用得比较多，尽管java已经给我们提供了丰富的API。

### 8.1 读取文件

`String str = IOUtils.toString(new FileInputStream("/temp/a.txt"), StandardCharsets.UTF_8);System.out.println(str);`

### 8.2 写入文件

`String str = "abcde";IOUtils.write(str, new FileOutputStream("/temp/b.tx"), StandardCharsets.UTF_8);`

### 8.3 文件拷贝

`IOUtils.copy(new FileInputStream("/temp/a.txt"), new FileOutputStream("/temp/b.txt"));`

### 8.4 读取文件内容到字节数组

`byte[] bytes = IOUtils.toByteArray(new FileInputStream("/temp/a.txt"));`

IOUtils类非常实用，感兴趣的小伙们，可以看看下面内容。

## 9. MDC

`​MDC​`​​是​`​org.slf4j​`​包下的一个类，它的全称是Mapped Diagnostic Context，我们可以认为它是一个线程安全的存放诊断日志的容器。

MDC的底层是用了​`​ThreadLocal​`​来保存数据的。

`public class LogFilter implements Filter {    @Override    public void init(FilterConfig filterConfig) throws ServletException {    }    @Override    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {        MdcUtil.add(UUID.randomUUID().toString());        System.out.println("记录请求日志");        chain.doFilter(request, response);        System.out.println("记录响应日志");    }    @Override    public void destroy() {    }}`

`public class RestTemplateInterceptor implements ClientHttpRequestInterceptor {    @Override    public ClientHttpResponse intercept(HttpRequest request, byte[] body, ClientHttpRequestExecution execution) throws IOException {        request.getHeaders().set("traceId", MdcUtil.get());        return execution.execute(request, body);    }}`

`@Configurationpublic class RestTemplateConfiguration {    @Bean    public RestTemplate restTemplate() {        RestTemplate restTemplate = new RestTemplate();        restTemplate.setInterceptors(Collections.singletonList(restTemplateInterceptor()));        return restTemplate;    }    @Bean    public RestTemplateInterceptor restTemplateInterceptor() {        return new RestTemplateInterceptor();    }}`

`public class MdcUtil {    private static final String TRACE_ID = "TRACE_ID";    public static String get() {        return MDC.get(TRACE_ID);    }    public static void add(String value) {        MDC.put(TRACE_ID, value);    }}`

## 10. ClassUtils

spring的​`​org.springframework.util​`​​包下的​`​ClassUtils​`​类，它里面有很多让我们惊喜的功能。

### 10.1 获取对象的所有接口

`Class<?>[] allInterfaces = ClassUtils.getAllInterfaces(new User());`

### 10.2 获取某个类的包名

`String packageName = ClassUtils.getPackageName(User.class);System.out.println(packageName);`

### 10.3 判断某个类是否内部类

`System.out.println(ClassUtils.isInnerClass(User.class));`

### 10.4 判断对象是否代理对象

`System.out.println(ClassUtils.isCglibProxy(new User()));`

ClassUtils还有很多有用的方法，等待着你去发掘。感兴趣的朋友，可以看看下面内容：

## 11. BeanUtils

spring给我们提供了一个​`​JavaBean​`​​的工具类，它在​`​org.springframework.beans​`​​包下面，它的名字叫做：​`​BeanUtils​`​。

### 11.1 拷贝对象的属性

`User user1 = new User();user1.setId(1L);user1.setName("苏三说技术");user1.setAddress("成都");User user2 = new User();BeanUtils.copyProperties(user1, user2);System.out.println(user2);`

### 11.2 实例化某个类

`User user = BeanUtils.instantiateClass(User.class);System.out.println(user);`

### 11.3 获取指定类的指定方法

`Method declaredMethod = BeanUtils.findDeclaredMethod(User.class, "getId");System.out.println(declaredMethod.getName());`

### 11.4 获取指定方法的参数

`Method declaredMethod = BeanUtils.findDeclaredMethod(User.class, "getId");PropertyDescriptor propertyForMethod = BeanUtils.findPropertyForMethod(declaredMethod);System.out.println(propertyForMethod.getName());`

## 12. ReflectionUtils

### 12.1 获取方法

`Method method = ReflectionUtils.findMethod(User.class, "getId");`

### 12.2 获取字段

`Field field = ReflectionUtils.findField(User.class, "id");`

### 12.3 执行方法

`ReflectionUtils.invokeMethod(method, springContextsUtil.getBean(beanName), param);`

### 12.4 判断字段是否常量

`Field field = ReflectionUtils.findField(User.class, "id");System.out.println(ReflectionUtils.isPublicStaticFinal(field));`

### 12.5 判断是否equals方法

`Method method = ReflectionUtils.findMethod(User.class, "getId");System.out.println(ReflectionUtils.isEqualsMethod(method));`

## 13. Base64Utils

`String str = "abc";String encode = new String(Base64Utils.encode(str.getBytes()));System.out.println("加密后：" + encode);try {    String decode = new String(Base64Utils.decode(encode.getBytes()), "utf8");    System.out.println("解密后：" + decode);} catch (UnsupportedEncodingException e) {    e.printStackTrace();}`

`加密后：YWJj解密后：abc`

## 14. StandardCharsets

`String str = "abc";String encode = new String(Base64Utils.encode(str.getBytes()));System.out.println("加密后：" + encode);String decode = new String(Base64Utils.decode(encode.getBytes()), StandardCharsets.UTF_8);System.out.println("解密后：" + decode);`

## 15. DigestUtils

### 15.1 md5加密

`String md5Hex = DigestUtils.md5Hex("苏三说技术");System.out.println(md5Hex);`

### 15.2 sha256加密

`String md5Hex = DigestUtils.sha256Hex("苏三说技术");System.out.println(md5Hex);`

## 16. SerializationUtils

`Map<String, String> map = Maps.newHashMap();map.put("a", "1");map.put("b", "2");map.put("c", "3");byte[] serialize = SerializationUtils.serialize(map);Object deserialize = SerializationUtils.deserialize(serialize);System.out.println(deserialize);`

## 17. HttpStatus

`private int SUCCESS_CODE = 200;private int ERROR_CODE = 500;private int NOT_FOUND_CODE = 404;`