Java -- 基础

基础知识

• float和double的不同

  • 1. Memory :

    float takes 4 bytes (32 bits) where as double takes 8 bytes(64 bits).

  • 2. Suffix :

    By default, float numbers are double in java. In order to store them into float variable, you need to explicitly add the suffix 'f' or 'F'.

    float num = 1.23456f
    // or
    float num1 = 1.23456F
    

    For double you need to explicitly add the suffix 'd' or 'D'.

    0.5 , 0.5d or 0.5D
    

  • 3. Precision :

    float: 6-7 digits decimal points precision

    double: 15-16 decimal points precision

  • 4. Data loss :

    float-> double 会有精度损失， double-> float 不会又精度损失

  • 5. Default data type :

    double是浮点数的默认类型，float不是

  • 6. Wrapper class :

    Wrapper class for float is java.lang.Float.
    Wrapper class for double is java.lang.Double.

  • 7. Defaut value :

    float default value is 0.0f.
    double default value is 0.0d.

• 防止数字越界

  System.out.println(Float.MAX_VALUE);
  System.out.println(Float.MIN_VALUE);
  System.out.println(Integer.MAX_VALUE);
  System.out.println(Integer.MIN_VALUE);
  System.out.println(Double.MAX_VALUE);
   System.out.println(Double.MIN_VALUE);
  

  输出：

  3.4028235E38
  1.4E-45
  2147483647
  -2147483648
  1.7976931348623157E308
  4.9E-324

问题

• 怎么在别的包内访问该包内数据？