variable
During the running of the program, the value in the space where the variable is stored changes, and this memory space is called a variable.
For ease of operation, give this space a name, called a variable name.
Syntax for variable names:
Data type variable name; for example: char sex; //declare character variable sex to store gender
variable name = value; eg: sex = 'male'; // store 'male'
data type variable name=value; for example: char sex='male';
Variable naming rules: 
Common mistakes in variable names:
1. The variable is used first without assignment
2. Using an illegal variable name
3. The variable name has the same name (the variable name cannot be the same name)
operator
Operators are divided into:
1. Assignment operator In java, "=" is called assignment and "==" is a numerical comparison
2. Arithmetic operators Arithmetic operators are the arithmetic we learned before + addition - subtraction * multiplication / division, the difference is that % is the remainder
3. Relational operators
type of data
Commonly used data types in java are: int integer; double double-precision floating-point type; char character type; String string type
Conversion of data types:
There are automatic conversions and forced conversions
1. Automatic conversion is the conversion from low precision to high precision
byte<short<int<long<float<double
2. Forced conversion
int a=（int）1.2；