Type system val list: List<String> = java.util.ArrayList() Why does it work? java.util.ArrayList is not a subclass of kotlin.List, and kotlin.List is actually read-only. Because in bytecode level, kotlin.List is just java.util.List! I decompile the class Kotlin compiled: import java.util.ArrayList; import java.util.List; import kotlin.Metadata; @Metadata(mv={1, 1, 6}, bv={1, 0, 1}, k=2, d1={"\000\b\n\000\n\002\020\002\n\000\032\006\020\000\032\0020\001��\006\002"}, d2={"a", "", "kotlin_demo"}) public final class _1Kt { public static final void a() { List list = (List)new ArrayList(); } } You can think as if java.

Pseudo random number Given a seed, and you will always get the next number, which is predictable. So that is reason why it is called pseudo random. A simple algorithm is Linear congruential generator. next random = (a * random + c) mod 2^32 The initial random is seed. Java implementation In next(bits) method: nextseed = (oldseed * multiplier + addend) & mask; here, multiplier is 0x5DEECE66DL, addend is 0xBL, mask is (1L << 48) - 1.

Basic Operators Assignment Operator let b = 10 // constant var a = 5 // variable a = b let is not the same as that in ES6. It is more or like final in Java. It means the variable (value or reference) cannot be changed once initialization. But let in Swift has stronger semantics, which enable immutability or constant. It means not only the reference cannot be mutable but also the object it refers cannot be mutable.

In Angular2, there are many *Ref class: ElementRef, ViewContainerRef, TemplateRef. What do they means? Nearly every components (Components, Attribute Directives, Structural Directives) have ViewContainerRef, which is the host of its view. ViewContainerRef has two important methods: createEmbeddedView(templateRef: TemplateRef<C>, context?: C, index?: number) : EmbeddedViewRef<C> This method is to create a view from a HTML template. createComponent(componentFactory: ComponentFactory<C>, index?: number, injector?: Injector, projectableNodes?: any[][]) : ComponentRef<C> This method is to initialize a component and insert its host view to the view container.

how to use? Basically, ThreadLocal is used for these scenarios: whenever you set a value/reference within this thread, you can get it from this thread. But it cannot guarantee you can ONLY get it from this thread. When wrongly used, it may cause something counter-intuitive. For example, when you set a value/reference to a ThreadLocal, you also share it with other threads (say other threads can access the reference and mutable it.

JMM is an advanced and abstract topic in Java. It provides well-defined semantics for synchronization and help us reason the code execution in a multi-thread enviorionment. Keywords: main memory, local memory, reordering, visibility, memory barrier/fence What do the issues come from? References The JSR-133 Cookbook for Compiler Writers Memory Barriers and JVM Concurrency //TODO

double-checked locking idiom (aka. DCL) broken versions Here we follow the evolution process of how to create a lazy-initialization singleton. // naive version public class Singleton { private static Singleton DEFAULT; public static Singleton getDefault() { if(DEFAULT == null) { DEFAULT = new Singleton(); } return DEFAULT; } private Singleton() {} // other instance methods } Race condition: - when thread 1 has executed if(DEFAULT == null) and then is preempted by thread 2.

To begin with, I issue some questions: - what does Thread#interrupt() do? - what is the difference between Thread#isInterrupted() and Thread.interrupted? - do I have to respond to interruption event? - when an InterruptedException is thrown? - how should I handle the caught InterruptedException? - how do I implement a cancelable mechanism for my task by polling interruption event? - apart from wait(), sleep(), join(), are there other scenarios throwing InterruptedException?

Today, a coworker asked me in which part of object header the length of an array object is stored. Since in my impression, the object header is of two machine words. But a book said for array, there must be length information stored in the object header. But where? array is special There is no “class definition” of an array (you can’t find it in any .class file), they’re a part of the language itself.

Several simple code snippets // Sample1 char[] chars = {'h', 'e', 'l', 'l', 'o'}; String str1 = new String(chars); System.out.println(str1 == str1.intern()); print: true // Sample2 String str1 = new String("hello"); System.out.println(str1 == str1.intern()); print: false // Sample3 String a = "hello"; String b = new String("hello"); String c = new String("h" + "e" + "l" + "l" + "o"); String d = b.intern(); How many String objects is created?

Introduction Five forms of method invocation in Java: - invokevirtual - invokeinterface - invokestatic - invokespecial - invokedynamic invokestatic and invokevirtual Of these, invokestatic and invokevirtual are easy to understand. invokevirtual is the default behaviour when we invoke an instance method. invokestatic is used to invoke the class method based on the type of the reference, not the class of the object when we invoke the static method through the instance rather than the class name.

Dynamic Proxy in JDK Proxy is a class which can get the proxy class and create the proxy instance. It has a private default constructor and a protected constructor with a contractor argument: InvocationHandler. InvocationHandler is an interface which we can apply our code into the generated proxy class. It has only one method: Object invoke(Object proxy, Method method, Object[] args) Proxy has four static methods: static Class<?> getProxyClass(ClassLoader loader, Class<?

DbVisualizer Tips How do we execute stored procedure? use @call stored_procedure_name; Parameterize SQL with variables Write your SQL as select * from table1 where a = ${param}$ Execute it and the “Enter data for variables” window will pop up. Parameterize SQL with markers Write your SQL as select * from table1 where a = :marker Execute it and the “Enter data for markers” window will pop up.

Prepare At first, I create a table: CREATE TABLE "SYSTEM"."TEST" ( "ID" NUMBER(*,0), "NAME" VARCHAR2(200 BYTE), CONSTRAINT "TEST_PK" PRIMARY KEY ("ID") ); Then, create a stored procedure: CREATE PROCEDURE sp_insert (x NUMBER) IS BEGIN -- Do some inserts here. INSERT INTO test VALUES (x, 'insert by stored procedure'); -- Sometimes there might be an error. IF x = 2 THEN RAISE_APPLICATION_ERROR(-20000, 'Wooops...'); END IF; EXCEPTION WHEN OTHERS THEN --Rollback all the changes and then raise the error again.

Oracle Statement-Level ROLLBACK (undo of a transaction) Purpose Use the ROLLBACK statement to undo work done in the current transaction or to manually undo the work done by an in-doubt distributed transaction. Articles Related Oracle Database - Deadlock Oracle Database - Distributed Transactions Oracle Database - Locks Oracle Database - Row Locks (TX) Oracle Database - TableSpace Oracle Database - Transactions Oracle Database - UNDO (Rollback Segment) PL/SQL - Autonomous Transactions (Pragma)

security manager vs. access controller Historically, before the access controller existed, the security manager relied on its internal logic to determine the security policy that should be in effect, and changing the security policy required changing the security manager itself. Starting with Java 2, the security manager defers these decisions to the access controller. Since the security policy enforced by the access controller can be specified by using policy files, this allows a much more flexible mechanism for determining policies.

How do you implement a Stack using Queue? As is known to all, the difference between Stack and Queue is the order to add and remove elements. So it’s possible to implement a Stack using Queue by controlling the operation logic. push 3 elements to the queue | | | 3 | <- top | 2 | | 1 | pop a element for the elements other than the top, repush into the queue

Chapter 1 Getting Started The focus of this book: locking, concurrency control, multiversioning, transactions, redo/undo. These concepts are all interrelated. Locking The database uses locks mechanism that allows for concurrency. Each database implements locking differently: some have page-level locking, others row-level some implementations escalate locks from row level to page level, some do not some use read locks, others don’t some implement serializable transactions via locking, others via read-consistent views of data (no locks) Oracle’s locking policy:

Some concepts metacharacters <([{\^-=$!|]})?*+.> these characters have special meaning which can affect the way a pattern is matached. character classes a set of characters enclosed within square brackets. Construct Description [abc] a, b, or c (simple class) [^abc] Any character except a, b, or c (negation) [a-zA-Z] a through z, or A through Z, inclusive (range) [a-d[m-p]] a through d, or m through p: a-dm-p [a-z&&[def]] d, e, or f (intersection) [a-z&&[^bc]] a through z, except for b and c: ad-z [a-z&&[^m-p]] a through z, and not m through p: a-lq-z predefined character classes convenient shorthands for commonly used regular expressions

New year has come and I need a book reading plan now. Database in-depth knowledge for a programmer (February) read the document of Postgres and see how it implements transactions and locking Security knowledge for a Java web developer (March) The Art of encryption and decryption about Java 白帽子讲Web安全 Java I/O (April) Java IO, NIO and NIO.2(Apress,2015) Pro Java NIO.2 Java IO.2nd.Edition.May.2006