Long way to go ~ ~
1, Constant
1.1 Overview
Constant: is a Java program to run during the fixed data
1.2 Classification
| Types of | meaning | Data example |
|---|---|---|
| Integer constant | All integer | 0，1，2332，-12 |
| Decimal constant | All decimals | 0.0，-0.4，99.99 |
| Character constant | Single quotation marks, can only write a character, you must have content | 'A', '', 'yellow Sister' |
| Boolean constants | Only two values | true，false |
| Constant air | Only one value | null |
2, variable
- Variable: Variable data is fixed, then the amount in the program can be called a variable number of
Mathematics, letters may be used instead of the digital calculation, e.g. x = 1+ 5 or 6 = x + 5
Program can use letters stored digitally calculates, increased computing power can solve more problems, such as x save 5, x 6 can also be saved, so x saved data can be changed, that is, we explain Variables
Java, requires a variable can only hold one data must be explicitly saved data types
Create a variable and use 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，赋值交给左边的变量
一步到位的格式：
数据类型 变量名称 = 数据值; // 在创建一个变量的同时，立刻放入指定的数据值3, Data Types
3.1 data type classification
Java data types are divided into two categories:
- Basic data types (primitive type): comprising
整数,浮点数,字符,布尔 - Reference data types (reference type): comprising
类,数组,接口
3.2 Basic Data Types
Four categories of the eight basic data types:
| type of data | Keyword | Memory footprint | Ranges |
|---|---|---|---|
| Byte | byte | 1 byte | -128 ~ 127 |
| Short integer | short | 2 bytes | -32768 ~ 32767 |
| Integer | int (default) | 4 bytes | -2^31^ ~ 2^31^-1 |
| Long integer | long | 8 bytes | -2^63^ ~ 2^63^-1 |
| Single-precision floating-point | float | 4 bytes | 1.4013E-45 ~ 3.4028E+38 |
| Double-precision floating-point | double (default) | 8 bytes | 4.9E-324 ~ 1.7977E + 308 |
| Character | char | 2 bytes | 0 ~ 65535 |
| Boolean | boolean | 1 byte | true、false |
The default type in Java: integer type
int, float typedouble
3.3 reference data types
Strings, arrays, classes, interfaces ......
Precautions
Strings are not primitive data types twenty-reference type
Float knowledge may be an approximation, not a precise value
The number of bytes of data does not necessarily correlate with the range of, for example, a wider range than the data float long, but float is 4 bytes, long 8 bytes
Float among the default type is double, if you must use a float, you need to add a suffix F
If is an integer, the default type int, if we must use the long type, you need to add a suffix L. We recommend the use of capital letters suffix
4, type conversion
Because Java is a strongly typed language, all the time to perform some operation, you need to use a type conversion
In operation, to different types of data into the same type, and then computes
Automatic conversion : will
取值范围小的类型automatically promoted取值范围大的类型Cast : The
取值范围大的类型cast to取值范围小的类型Conversion format :
数据类型 变量名 = (数据类型) 被转换数据值;Features: format code requires special processing, not automatically
Precautions:
- Casts are generally not recommended because of the potential loss of precision occurs, data overflow
- byte / short / char three types can occur mathematical operations, such as addition "+"
- byte / short / char when these three types of operation, will be called the first lift type int, then calculate
- boolean type Data type conversion can not occur
5. Scope
Starting line defined variables, until the end all the way to direct your braces