Note

Comment species

Single-line comments: Only comments a line of text at the beginning of the play //
Multi-line comments: Can you comment multiple lines, playing at the beginning of the line / * end of the line with * /
Documentation Comments: related JavaDoc command, / ** * beginning / end

Notes and will not be executed, but to those who write the code to see

Usually write the code must be standardized

Identifiers and Key Words

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All components of the Java name is required. Class names, variable names and method names have not become identifier.

All identifiers should begin with a letter (AZ or az), the dollar sign ($) or an underscore (_);

Illegal characters example: 123asd, -asdf, # asdf, these are illegal
After the first character may be a letter (AZ or az), any characters dollar sign ($) or an underscore (_) or digital composition
You can not use keywords as variable or method name
Identifiers are case-sensitive, that is to pay attention to case
We do not recommend the use of Chinese pinyin or name

type of data

Data types fall into two categories

Strongly typed language: the requirements of the variables used to strictly comply with the provisions, all variables must be defined before you can use (java, c ++ is a strongly typed language) safe but slow
Weakly typed language: the definition of a strongly typed language contrast, JavaScript and Visual Basic Script

Java data types

String string definitions need to add double quotation marks, he is not a keyword, he is a class, he can be used to define multiple characters

Java only be divided into two types of data

Basic types (primitive type)
1. Numeric types
  1. Integer type
    
    byte one byte: -128-127
    
    short two bytes: -32768-32767
    
    int total of four bytes: -21 one hundred million -21 million in the most commonly used
    
    long occupies eight bytes: large, will be added with an uppercase when used after the number of such values L type
  2. Floating-point type
    
    float 4 bytes, followed by one of the capital needed to use this type of F
    
    double 8 bytes long (common)
  3. Char Character type is two bytes, after the equal sign in single quotes '' enclosed, only the middle of writing a letter or word
2. boolean types: one bit, only his true and false values of two, usually with flag = true or false flag =, with the flag named

Reference types (reference type)

class

interface

Array

Spread

Integer

Binary will hexadecimal (zero x) begins with 0x to 0b (zero b) at the beginning of octal to 0 (zero) at the beginning

== represents equal, in his reference to indicate whether the output is determined before and after the equality

Float

The number of floating-point performance is limited, discrete, there is a rounding error, about, close to but not equal, all were relatively easy to make mistakes with float

Best to completely avoid using floating point numbers to compare

How about banking issues such as representation, with BigDecimal, a mathematical tool class

character

Before adding a parenthesis character output, which is a data type bracket, this is called a cast

For example:

char name = 'c';
System.out.println((int)c);

Here are the characters into digital, because all of the characters or numbers in nature

With the Unicode character encoding, occupies 2 bytes, a length of 65 536, i.e. 2 to the power 16, 97 on behalf of such a, 65 represents A

Expressed interval U0000-UFFFF, examples

char NAME1 = '\ u0061'; 
the System. OUT. the println ( NAME1); // this is the output of a

Escape character

\ T is a tab

\ N represents a newline

System.out.println("Hello\tWorld");

There are many escape character, can be found online

Extended Boolean value

Sometimes after the if statement will pick Boolean variables, but did not write anything after the variable ==, which is generally regarded as the == true

Less is more, to streamline the code easy to understand

Learning from God mad to say the Java https://www.bilibili.com/video/av68373450?p=2 4

Basics of Java 01