JAVA basic concepts (two)

A, java modifiers and usage scenarios

Modifier is used to define the class, method or variable access rights, divided into two categories:

Access modifiers:

Whether defined classes, properties, methods in other parts of the program can be accessed and invoked.

private<default<protect<public
Non-access modifiers used to modify or assist:

For example static, final, abstract, synchronize, etc.

The main Remember:

External class modifiers: public or default
Method, property modifiers: private, default, protected, public
- public disclosure of the externally visible
- protected package and subclasses visible
- Internal private class only visible

Modifiers	The current class	The same intracapsular	Different subclasses package	Different classes package nonsubtyped
public	AND	AND	AND	AND
protected	AND	AND	AND	N
default	AND	AND	N	N
private	AND	N	N	N

Eight basic types of data (each data required to store the data from the application space of the computer)
- byte
  - 8th
  - Maximum 127, minimum -128
  - Save space, occupied a quarter of an int
  - Default: 0
- short
  - # 16
  - Maximum 32767, minimum -32768
  - One-half of type int
  - Default: 0
- int
  - # 32
  - Min -2147483648, maximum 2147483647
  - The default integer type is int
  - Default: 0
- long
  - # 64
  - Min -9223372036854774808, maximum 9223372036854774807
  - Default value: 0L
- float
  - Single precision 32-bit
  - 0.0f
- double
  - 64-bit double precision
  - The default double floating-point type
  - The default value 0.0d
- boolean
  - One
  - true or false
  - The default is false
- char
  - 16-bit Unicode characters, two bytes represent a character
  - The minimum \ u0000 that is 0, the biggest \ ufff namely 65535
- Type Conversion
  - double>float>long>int>short>byte
  - Small turn big can be directly converted, while descending the need to cast, there will be loss of accuracy
Reference data types: array of Class objects or reference data types are created
- String: string object, but also reference data types