Java

学习笔记

Java 编程思想

Everything is an object

reference

where storage lives?

    registers.     inside the processor

    the stack.    has direct support from the processor via its stack pointer.—RAM

reference exists on the stack. objects are not placed on the stack.

    the heap.     flexibility,take more time.

    constant storage. ROM

    Non-RAM storage.    lightweight persistence.

primitive types

new objects on the heap. primitive types on the stack.

size invariance

all numeric types are signed, so don’t look for unsigned types.

wrapper classes(包装类) make a non_primitive object on the heap. Java SE5 autoboxing.

high-precision(高精度) numbers: BigInteger BigDecimal

arrays in java

reference isn’t pointing to an object.----null

you never need to destroy an object

confusion over variable lifetimes?

scoping 代码块

the reference vanishes at the end of the scope. However ,the object is still occupying memory.

what keeps them from filling up memory and halting your program?

garbage collector

it release the memory for those objects that there’s no reference.

creating new data types: class

fields and methods.

default values 默认数值 only used as a member of a class.

returnType methodName(/*argument list*/){

    /*method body*/

}

signature of the method: name and argument list.

name visibility

clashing

namesapces

domain name----net.mindview.utility.foinles

static key word

using the class name is the preferred way to refer to a static variable.

your first java program

public static void main(String [] args){}

the name of the class name is the same as the file name .

the argument to main() is an array of String objects. the args won’t be used in this program, but the java complier insists that they be there because they hold the arguments from the command line.

compiling and running

javac HelloDate.java

java HelloDate

comments

javadoc

doc tags ’@'  embed(嵌入) HTML

/**……*/

for only public and protected members.

@see

 {@link package.class#member label}

{@docRoot}

{@inherit Doc}

@version

@author

@since

@param

@return

@throws

@deprecated

coding style

camel-casing

operators

simpler print statements

the static import

assignment(赋值)

from one object to another.  copy a reference from one place to another.

aliasing

aliasing during method calls

mathematic operators

create a Random object with no arguments. java use the current time as a seed for the random number generator.

call the methods nextInt().  set the upper bound. the lower bound is zero.

auto increment and decrement(自增自减)

relational operators

object equivalence.   equals().  the default behavior of equals() is to compare references.    override

logic operators

&& || !

short-circuiting(惰性)

a&&b&&c

literals

0x L D F

Exponential notation

1.39e-43f

bitwise operators

& |  not ^(xor)

shift operators

<<< >>> unsigned.  zero extension

<<     >> signed        sign extension

ternary if-else operator

boolean-exp ? value0 : value1

string operator + and +=

if an expression begins with a string, then all operands that follows must be strings.  convert

common pitfalls

while(x=y),in java, the complier won’t convert to a boolean, so a compile-time error.

casting operators(类型转换)

warning:this can be dangerous thing to do-if you want  me to do it anyway you must make the cast explicit.

java allows you to cast any primitive type, except for boolean, which doesn’t allow any casting at all.

truncation and rounding

java.lang.Math    Math.round()(四舍五入)

一般丢掉小数部分

java has no “sizeof”

all the data types are the same size on all machines.

Controlling Execution

java don’t allow you use a number as a boolean.

if-else

if(boolean-expression)

    statement

else

    statement

iteration(迭代)

do

    statement

while(boolean-expression);

 

for(initialization ; boolean-expression ; step)

    statement

the comma operator

those statements will be evaluated sequentially.

foreach syntax

for use with arrays and containers

you can easily iterate through the characters in a string.

break and continue

break quits the loop without executing the rest of the statements in the loop. continue stops the execution of the current iteration and goes back to begin the next iteration.

the infamous “goto”

label

switch

selection statement. enums are designed to work nicely with switch.

 

Initialization and cleanup

<<Java 核心技术>>

chapter 1

1.1    Java程序设计平台

    多种优势

1.2    Java"白皮书"的关键术语

    简单性. 面向对象. 分布式. 健壮性. 安全性. 体系结构中立. 可移植性. 解释型. 高性能. 多线程. 动态性

1.3    Java applet 与 Internet

chapter 2

JDK: Java Development Kit    JRE: Java Runtime Environment    SE: Standard Edition    EE: Enterprise Edition    ME: Micro Edition

SDK: Software Development Kit

Chapter 3        Java 的基本程序设计结构

3.1    一个简单的Java应用程序

类名以大写字母开头的名词

源代码的文件名==公共类的类名

3.2    注释

/** ...... **/自动生成文档

3.3    数据类型

    整型 浮点类型 char Unicode boolean

3.4    变量

初始化<可与声明同时进行>

   int i=10;

常量 final 指示 final double a =3.14;

3.5    运算符 NaN

数值之间的类型转换

double x=9.97;

int nx =(int)x;

位运算符 and &    or |    not ~    xor ^    <<    >>    <<<    >>>

枚举类型

3.6    字符串

子串 substring

String greeting =”Hello”;

String s=greeting.substring(0,3);

“Hel”

不可变字符:    编译器可以让字符串共享

3.6.4    检测字符串是否相等

Hello.equals(greeting);

3.7    输入输出

3.7.1 输入

读取输入----要想通过控制台进行输入,首先需要构造一个Scanner对象,并与标准输入流System.in关联

String name = in .nextLine();

要想读取一个单词用    String firstName=in.next();

整数    int age =in.nextInt();    Double型:    nextDouble();

Scanner类定义在java.util包中,不存在java.lang包中

注释:    因为输入是可见的,所以Scanner类不适用于从控制台读取密码. Java SE 6特别引入了 Console类.

Console cons =System.console();

String name =cons.readLine(”Username:    ”);

char [] password=cons.readPassword(“Password:    ”)

为了安全起见,返回的密码存放在一维数组中,而不是字符串中.在对密码进行处理之后, 应该马上用一个填充值覆盖元素.

3.7.2格式化输出    沿用C的模式

3.7.3 文件输入与输出

读取文件    Scanner in =new Scanner(Path.get(“myfile.txt”), “UTF-8”);

写入文件    PrintWriter ou =new PrintWriter(“myfile.txt”, “UTF-8”);    如果文件不存在,创建.

可以使用print命令

3.8    控制流程

3.8.1 block 块作用域

块确定了变量的作用域,但是,不能在嵌套中的两个块中声明同名变量.

3.8.2

条件语句    jf(condition)   statement

循环语句    while(condition)    statement

确定循环    for

多重选择    switch (choice)    {case 1:    }

3.9   大数值