第5天第3节： Java基础数据类型

==keywords
1. goto and const are keywords but have no meaning
2. assert was introduce by JDK1.4
3. enum was introduced by JDK1.5

==variable types
byte   : 1 byte  (signed -128~127): (byte)0
short  : 2 bytes ： (short)0
int      : 4 bytes: 0
long   : 8 bytes: 0L
float   : 4 bytes: 0.0f
double: 8 bytes: 0.0d
char    : 2 bytes ( unsigned): ?
boolean: : false

class
interface
array

==conversion rule
1.  short length varible to long length variable: conversion is automatic       longa = inta
2.  long length variable to short length variable: must manually convert by inta = (int) longa;
3.  byte->short->int->long->float->double;
3.  constant is by default int and double. so we have to use
      int a = 30;
      long a = 30;
      long a = 30L;
      long a = 30l;

     double a = 33.333;

     float  a = (float) 33.333;
     float  a = 33.333f;

==char type
1. char is 16 bit variable, and in Java,all char is unicode, so a char can hold a chinese char
    char a='你';
    int xa=a;

==boolean
1. Only allow "true" or "false".
2. Not allow 0 for false, and non-zero for true;

==String
1.  it is a class because its first letter is capital letter
2. it can used as intrinsic type variable
3. every type convert to String if there is a String in the expression
    String str="";
    System.out.println(str+10+20) => 1020
    System.out.println(str+(10+20)) => 30