kongkong的专栏

判断是否有重复id

Collections.sort

installsudo apt-get install build-essentialsudo apt-get install libfreetype6-devsudo apt-get install libcups2-devsudo apt-get install libx11-dev libxext-dev libxrender-dev libxrandr-dev libxtst-dev libxt-devsudo apt-get install libasound2-devsudo ap

安装openjdk8yum install java-1.8.0-openjdk我的centos7默认已经有openjdk8了，这一步省略，如果没有需要安装openjdk version “1.8.0_302”OpenJDK Runtime Environment (build 1.8.0_302-b08)OpenJDK 64-Bit Server VM (build 25.302-b08, mixed mode)下载openjdk8本步骤我是直接从网上下载的openjdk8，所以没有这

ThreadLocal.withInitialprivate static final ThreadLocal<String> CONTEXT_HOLDER = ThreadLocal.withInitial(DataSourceKey.ORDER::name);

githubhttps://github.com/openjdk/jol官网openjdk-jolmaven<dependency> <groupId>org.openjdk.jol</groupId> <artifactId>jol-core</artifactId> <version>${version}</version></dependency&...

demo1Student stu = null;// unKnownString stuName = Optional.ofNullable(stu).map(Student::getName).orElse("unKnown");demo2stu = new Student();stu.setName("king");// kingstuName = Optional.ofNullable(stu).map(Student::getName).orElse("unKnown"

demo1String[] words = {"Hello","World"};//{H,e,l,l,o},{W,o,r,l,d}Stream<String[]> stream = Arrays.stream(words).map(w-> w.split("")); //Stream<String[]>//H,e,l,l,o,W,o,r,l,dStream<String> stringStream = stream.flatMap(Arrays:.

computeIfAbsentjdk源码default V computeIfAbsent(K key, Function<? super K, ? extends V> mappingFunction) { Objects.requireNonNull(mappingFunction); V v; if ((v = get(key)) == null) { V newValue; if ((newValue = mappingFunction.a

AccountMapperpublic interface AccountMapper { public void accountAdd(Long id, String username, Date createTime);}PTestpublic class PTest { public static void main(String[] args) throws Exception { Method method = AccountMappe.

0x7fffffff = 0111 1111 1111 1111 1111 1111 1111 1111异或参与运算的两个值，如果两个相应bit位相同，则结果为0，否则为10^0 = 01^0 = 10^1 = 11^1 = 0...

TreeMap源码参见:https://blog.youkuaiyun.com/kq1983/article/details/107323231注意：新加节点，除了root节点是黑色，其他节点一开始是红色(通过变色，或者左旋，右旋变色)NIL节点默认是黑色以下通过例子演示1. 添加数字10以前没有数据，所以10是根节点2. 添加数字33比10小，添加到10的左节点3. 添加数字50由于50比10大，添加到10的右节点4. 添加数字19先跟10比较，比10大，...

红黑树是每个节点都带有颜色属性的二叉查找树，颜色或红色或黑色。红黑树特性：性质1. 节点是红色或黑色性质2. 根节点是黑色(非根节点，默认红色)性质3.所有叶子都是黑色。（叶子是NUIL节点）性质4. 每个红色节点的两个子节点都是黑色。（从每个叶子到根的所有路径上不能有两个连续的红色节点）性质5. 从任一节点到其每个叶子的所有路径都包含相同数目的黑色节点TreeMapTreeMap实现了SotredMap接口，它是有序的集合。底层是用红黑树实现。TreeMap的k...

Map<Integer, String> map = list.stream().filter(s -> s.getName() != null).collect(Collectors.toMap(Student::getAge, Student::getName,(k1,k2)-> k1)); System.out.println("map:"+map);// 多个值组成keyMap<String, String> map1 = list...

今天是星期五，2020-06-19调用nextOrSame和previousOrSame返回都是今天，2020-06-19调用previous，返回2020-06-12调用next，返回2020-06-26//上个星期五的日期 如果今天是星期五，则返回下周五的日期LocalDate previousSaturdayInThisMonth = today.with(TemporalAdjusters.previous(DayOfWeek.FRIDAY));//(往前包括当天.

DateTimeFormatter是线程安全的SimpleDateFormat线程不安全SimpleDateFormat推荐每个线程实例化该对象，如果多个线程访问1个SimpleDateFormat，必须在外部使用同步threadlocal方式static ThreadLocal<SimpleDateFormat> simpleDateFormatThreadLocal = new ThreadLocal<SimpleDateFormat>()..

Employee对象public class Employee { private String id; private String name; private int age; private String hobby; private String address; public Employee() { this....