1.1 A Typical Java Development Environment

1.1.1 The steps to create and execute a Java application

Phase 1: Edit
Program is created in an editor and stored on disk in a file whose name ends with .java

Phase 2: Compile
Compiler creates bytecodes and stores them on disk in a file whose name ends with .class ( .class文件的个数取决于类的个数，而不是.java文件的个数）

Phase 3: Load
Class loader reads .class files containing bytecodes from disk and puts those bytecodes in memory
load 后，类定义以class类的对象的形式存储在内存中

Phase 4: Verify
Bytecode verifier confirms that all bytecodes are valid and do not violate Java’s security restrictions

Phase 5: Execution
To execute the program, the JVM reads bytecodes and just-in-time(JIT) compiles(translates) them into a language that the computer can understand. As the program executes, it may store data values in primary memory

1.1.2 Tips

.class 以 Java Bytecode 的形式存储
在 Ececution 阶段，JVM 既有 interpreter，也有compile。JVM 在解释的同时也有动态/即时编译，即 JIT Compile。(hot spot)

1.2 Java Characteristics

Simple
Secure
Portable
Robust
Object-oriented
Architecture-neutral
Multithreaded (多线程的)
- 进程 process —-> 线程 thread（多个线程并发）
Interpreted (解释性的)
High performance(高性能的)
Distributed(分布式的)
- Java was designed with the distributed environment. It can be transmit, run over the internet.
Dynamic(动态的)
- Class loader 将类动态加载如JVM中

1.3 Different from C++

Features	C++	Java
pointer	Yes	No
goto Statement	Yes	No
unsigned Data Types	Yes	No
Operator Overloading	Yes	No
Preprocessor Directives (#define)	Yes	No
Multiple Inheritance	Yes	No
Memory Management	Explicit ( 显式 )	Implicit ( 隐式 )
Structures Concept	Yes	No

C++支持多继承，Java不支持多继承
C++为显式的内存管理，Java为隐式的内存管理
Memory Leak（内存泄漏）: 当没有用的内存没有释放的时候，会造成内存泄漏。C++需要手动释放内存，Java中的JVM自动管理内存。Java程序仍然存在内存泄漏，但要比C++少很多
判断一个内存是否应当回收：有无指针指向这片内存（寻找GC Roots）

1.4 Others

1.4.1 Java Class Libraries

Java programs consist of pieces called classes
Classed include methods that perform tasks and return information when the tasks complete
Java class libraries
Rich collections of existing classes
Also known as the Java APIs

1.4.2 Program Languages

Imperative（命令式的）: C, C++, Java, …

下达命令时很清晰，明确指定方法的步骤
问题：冯·诺依曼瓶颈

Functional（函数式的）: SQL( 非过程化的语言）, …

这一类语言特点是将函数当变量用，类似Lambda Expressions

Logical（逻辑式的）

1.4.3 Class 类

Instance of class represent classes and interfaces in a running Java application.
class类没有构造函数，class类的对象是Class Loader在load .class 文件时产生的，不能手动new一个

Maples' Space

Java学习：1. Introduction to Java