java-based -2 constants, data types, data type conversion, and variable

First, the constant

1, constant: the program is running, a fixed amount of unchanged

2, constant classification

• String constants: partially double quotation marks, 'abc'
• Integer constants: integers, no decimal point, 100
• Float constants: decimal points, 2.5
• Character Constants: single quotation marks single character, 'a'
• Boolean constants: true, false
• Empty constants: null. No data on behalf of

3 Notes

• The middle two single quotes must have one and only one character
• Empty constants can not be printed out directly

Second, the data type

1, the data type classification

• The basic data types: integers, floats, characters, boolean
• Reference data types: type, an array, an interface

2, the basic data types (four eight kinds)

• Integer: byte, short, int, long
• Float: float, double
• Character: char
• Boolean: boolean

type of data	Keyword	Memory footprint	Ranges
Byte	byte	1 byte	-128~127
Short integer	short	2 bytes	-32768~32767
Integer	int	4 bytes	-231~1
Long integer	long	8 bytes	-263~263-1
Single-precision floating-point number	float	4 bytes	1.4013E~3.4028E+38
Double-precision floating-point number	double	8 bytes	4.9E-324 ~ 1.7977E + 308
Character	char	2 bytes	0~65535
Boolean	boolean	1 byte	true false

3 Notes

• Basic string data type is not, but the type of reference
• Float may be an approximation, not a precise value
• Range data does not necessarily correlate with the number of bytes, such as a wider range than the data float long, but float is 4 bytes, long 8 bytes
• Float Default is double. To use the float, you need to add the suffix f. To
• The default integer int, to use long, need to add the suffix l.

Third, variable

1, variable

- 程序运行期间，内容可以发生变化

2, create a variable format

数据类型    变量名称;//创建了一个变量
变量名称    数据值;//赋值，将右边的数据值，赋值交给左边


数据类型    变量名称=数据值;//创建一个变量同时赋值

4 Notes

• When you create multiple variables, names between variables can not be repeated.
• with long float type suffix f and l can not lose
• When using the short type variable byte data value can not exceed the range of the left side of the right type
• No variable assignment, it can not be used directly.
• Using Variables can not exceed the scope of scope
  • [Scope] start by defining variables, braces belongs to the end

Fourth, data type conversion

• When the data type is not the same, data type conversion occurs
  • Automatic type conversion (implicit type conversions)
  • Casts (explicit)

1, automatic type conversions

• Features: Code does not require special handling, automatically
• Rules: data range from small to large

2, cast

• Features: Code need for special formatting can not be done automatically

• Format: Small small range of types of variable name = (small type) had a large range of data

  public class demo2 {
      public static void main(String[] args) {
          int num = (int)100l;
          System.out.println(num);
      }
  }

  3 Notes

  • Cast is not recommended, loss of precision may occur, data overflow
  • byte, short, char when these three types of data operation occurs, it will first lift an int, then the calculation.