②java Basics-identifiers, keywords, and underlying data types

Java identifiers:

　　1. Consisting of English letters, numbers, _ (underscore) and $, not limited in length. Where the English letter contains uppercase letters (A~Z) and lowercase letters (a~z), numbers contain 0 to 9

2. The first character of an identifier cannot be a number (that is, the identifier cannot begin with a number)

3. Identifiers are case-sensitive

4. Identifiers cannot contain spaces

Java keywords:

The 1.Java keyword is an identifier that is specified by the Java language to have a specific meaning. Java's keywords have special meanings for Java compilers, they are used to represent a data type, or to represent the structure of a program, and the keywords are all made up of lowercase English letters.

The 2.Java keyword is similar to a function word in human language, has a specific meaning and is an essential part of composing a program (equivalent to an article in a human language).

Java Basic data Type (Introduction):

byte-type--byte--8 bit

Short-integer--short--16 bit

Integral type--int--32 bit

Long plastic--long--64 bit

Single-precision floating-point--float--32 bit

Double-precision floating-point--double--64 bit

Character type--char--16 bit

Boolean---boolean--8 bit

Integral type (Byte,short,int,long):

Range (in byte example):

Byte is 8 bits, which is made up of 8 0/1, because the machine is 2 binary, and the first bit is the sign bit (0 for positive number, 1 for negative number), so a byte type data can represent a range of: 27 to 27-1 (that is, 128 to 127, because in Java 0 is a positive number, So the positive maximum value is 127, and the minimum value of the negative number is-128)

Floating-point type (float,double):

Attention:

To add f (or F) after the number when the float data is assigned, the compiler will assume that this is a double class

　　　　Correct assignment method: float f1 = 1.3f;

Boolean Type (Boolean):

Also known as a logical type, used to represent a true/False value for a logical decision, only two character constants can be assigned to a Boolean type in Java, that is: true (true) and False (false)

Attention:

　　　　Boolean worth default initial value is False

　　　　Java binary logic, 1-bit true, 0-bit false

Character type (char):

The Java character type is Unicode encoded, char (character type) is 16 bits, and the character type is not positive or negative, so it can represent 216 numbers

Range: 0 to 216-1 (0~65535)

Example:

Char C1 = ' a ';

char C2 = 12345;

char C3 = ' Medium ';

Note:Char can store one Chinese and can use numeric values (0~65535) to assign a variable to a character type.

Basic data type Conversions (Introduction):

When assigning low-level variables to high-level variables,Java automatically uses an implicit conversion

When the strong turn (display type conversion, assign the advanced variable to a low-level variable) format: (the type to be converted) The value to be converted

such as: int x = (int) 23.89;//At this time X is 23 (strong turn results in missing precision)

②java Basics-identifiers, keywords, and underlying data types