java基础4.数据类型扩展及面试题讲解

4.数据类型扩展及面试题讲解

这章开始可以运行代码了。
·右键运行（推荐）
·或者上面菜单栏点运行的图标（有时候运行的不是本文件）

//整数拓展:进制的头部 二进制'0b'开头 十进制 八进制0 十六进制0x
        int i=10;//十进制
        int i2=010;//八进制0
        int i3=0x10;//十六进制0x 0~9 A~F 16
        System.out.println(i);
        System.out.println(i2);
        System.out.println(i3);
        System.out.println("==================");

浮点数拓展

最好 完全避免使用 浮点数进行比较
最好 完全避免使用 浮点数进行比较
最好 完全避免使用 浮点数进行比较

//float和double是有问题的
        double d=1.0/10;
        float f=0.1f;
        System.out.println(d);//0.1
        System.out.println(f);//0.1
        System.out.println(d==f);//false

如何使用debug调试

        float q=213124124f;
        float w=q+1;
        System.out.println(q==w);//true
//浮点的字长是有限的，离散的，存在舍入误差，结果只能是一个大约数，接近不等于
//最好完全避免使用浮点数进行比较

//浮点数拓展：银行业务怎么办？
//BigDecimal数学工具类

字符拓展-强制转换

所有的字符本质还是数字

编码 Unicode表：(a=97 A=65) 2字节 最多可以表示65536(现在有更多了)

char c1='a';
char c2='中';

System.out.println(c1);
//把字符变成数字
System.out.println((int)c1);//强制转换，输出97

System.out.println(c2);
System.out.println((int)c2);//强制转换，输出20013

= = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = =

通过Unicode编码转译表示，U0000~UFFFF

char c3='\u0061'
System.out.println(c3);//a

= = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = =

转义字符

转译字符

/t 制表符

/n 换行

...

= = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = =

等以后学到对象(这一课时)再从内存分析

String sa = new String( original: "hello world");
String sb = new String( original: "hello world");
System . out . println(sa==sb);//输出false

String SC = "he1lo world"; .
String sd = "hello world"; .
System. out . println(sc==sd);//输出true

= = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = =

布尔值扩展

boolean flag = true;
if (flag==true){} //新手 
if (flag){} //老手
//代码要精简易读