Java Syntax format

Syntax format

1.1 Constants 1.1.1 Constants Overview
– The amount of value that cannot be changed during program execution
1.1.2 Constant classification
– string constants are enclosed in double quotes ("HelloWorld")
– Integer constant all integers (12,-23)
– Decimal constant All decimals (12.34)
– Character constants are enclosed in single quotes (' A ', ' a ', ' 0 ')
– Boolean constants are more specific, only true and false
– Null constant Null (part of the array explained)
1.1.3 Case Code Four:

 /* Constant: The amount of the value that cannot be changed during program execution class: A: String constant "HelloWorld" B: integer constant 12,-23 C: decimal constant    12.34 D: Character constant ' a ', ' 0 ' E: Boolean constant true,false F: null constant Null (explained later) */public class Changliang {        public static void Main (string[] args) {//String constant System.out.println ("HelloWorld");        Integer constant System.out.println (12);        System.out.println (-23);        Decimal constant System.out.println (12.34);        Character constant System.out.println (' a ');        System.out.println (' 0 ');        Boolean constant System.out.println (true);    System.out.println (FALSE); }}

1.1 Variable 1.1.1 Variable Overview
– In the course of a program execution, the amount of its value can change in a range
– essentially, a variable is actually a small area in memory
1.1.2 Variable Definition Format
– data type variable name = initialization value;
– Note: Format is fixed, remember format, Status Quo
1.1.3 variable plot

1.2 data type 1.2.1 Computer storage unit
variable is a small container in memory that is used to store data. So how does computer memory store data? Whether it is a memory or a hard disk, the smallest unit of information in a computer's storage device is called a bit, which we also call "bit", usually in lowercase letter B. The smallest storage unit of the computer is called "byte", usually denoted by a capital letter B, which consists of 8 consecutive bits.
In addition to the bytes there are some common storage units, we may be familiar with, we look at:
–1b (bytes) = 8bit
–1kb = 1024B
–1MB = 1024KB
–1GB = 1024MB
–1TB = 1024GB
1.2.2 Overview and classification of data types
A: Why is there a data type
The Java language is a strongly typed language that defines specific data types for each data and allocates different sizes of memory space in memory
classification of data types in B:java
Base data type
Reference data type
Object-oriented section explanation


1.3 identifier 1.3.1 Identifier Overview
A effect
– Give the package, class, method, variable, etc. name
B constituent rules
– by character, underscore _, Dollar sign $ composition
? The characters here are in the Unicode character set, so they include English case letters, Chinese characters, numeric characters, and so on.
– Considerations
– Cannot start with a number
– cannot be a keyword in Java
C: Naming principle: See the name of the meaning
a package
preferably the domain name upside down, require all the letters lowercase
Class B or interface
If it is a word with the first letter capitalized
If it is more than one word each word is capitalized (Hump identification)
C method or variable
If it is a word all lowercase
If it is more than one word, capitalize from the second word
D constant
If it is a word, all letters are capitalized
If you have more than one word, all the words are capitalized, and each word is distinguished by an underscore
1.3.2 Case Code five

 /* Identifier: Is the name of the package, class, method, variable. Composition rule: A:unicode character numeric characters, English case, kanji (Chinese characters not recommended) B: Underscore _ C: Dollar sign $ note A: Cannot start with a number B: no Can be a keyword in Java common naming rules: A: Basic requirements See the name of the understanding B: Common Naming A: the package (in fact, is the folder for the management of the class) all lowercase, many                Pack. Examples of separation: Com,com.itheima B: Class One Word capitalize example: Student,car Capitalize the first letter of each word in multiple words example: HelloWorld C: Methods and variables An example of a word first letter lowercase:  Age,show () Multiple words start with the first letter of each word from the second word example: Maxage,getage () */public class Biaozhifu {public        static void Main (string[] args) {//define variable//data type variable name = initialize value;        int a = 10;        correct int b2 = 20;        Error//int 2b = 30;    Cannot be a keyword in Java//error//int public = 40; }}

1.1 Defining variables 1.1.1 The definition and use of basic data type variables
Define the format of the variable:
Variable name of data type = initialization value;
Basic data type:
Byte,short,int,long,float,double,char,boolean
Attention:
integers are type int by default, and when you define a long type of data, you add l after the data.
Floating-point numbers are double by default, and when you define data of type float, you add f after the data.
1.1.2 Case Code VI

public class VariableDemo {    public static void main(String[] args) {        //定义byte类型的变量        byte b = 10;        System.out.println(10);        System.out.println(b);        //定义short类型的变量        short s = 100;        System.out.println(s);        //定义int类型的变量        int i = 10000;        System.out.println(i);        //定义long类型的变量        long l = 1000000000000000L;        System.out.println(l);        //定义float类型的变量        float f = 12.34F;        System.out.println(f);        //定义double类型的变量        double d = 12.34;        System.out.println(d);        //定义char类型的变量        char c = ‘a‘;        System.out.println(c);        //定义boolean类型的变量        boolean bb = false;        System.out.println(bb);    }}

Considerations for 1.1.1 Variable Definitions
? The variable is not assigned to a value and cannot be used directly
– The second use format of the derivation variable
? A variable is valid only within the range to which it belongs.
– Where the variable is in curly braces, the variable belongs to the curly brace
? Multiple variables can be defined on a single line, but not recommended
1.1.2 Case Code VII

/*      变量定义注意事项：        1:变量未赋值,不能直接使用        2:变量只在它所属的范围内有效            变量属于它所在的那对大括号        3:一行上可以定义多个变量,但是不建议*/public class VariableDemo2 {    public static void main(String[] args) {        //定义变量        int a = 10;        System.out.println(a);        int b;        b = 20; //变量在使用前赋值都是可以的        System.out.println(b);        {            int c = 100;            System.out.println(c);        }        //System.out.println(c);        /*        int aa,bb,cc;        aa = 10;        bb = 20;        cc = 30;        */        /*        int aa = 10;        int bb = 20;        int cc = 30;        */        int aa=10,bb=20,cc=30;    }}

1.1 Data-type conversions 1.1.1 Implicit data type conversions
A data type with a small range of values and a data type with a large range of values will first be promoted to a large, and then a small data type
1.1.2 Case Code Eight

/*    +:是一个运算符，做加法运算的。    我们在做运算的时候，一般要求参与运算的数据类型必须一致。    类型转换：        隐式转换        强制转换    隐式转换            byte,short,char -- int -- long -- float -- double*/public class TypeCastDemo {    public static void main(String[] args) {        //直接输出了运算的结果        System.out.println(3 + 4);        //定义两个int类型的变量        int a = 3;        int b = 4;        int c = a + b;        System.out.println(c);        //定义一个byte类型,定义一个int类型        byte bb = 2;        int cc = 5;        System.out.println(bb + cc);        //我能不能不直接输出，用一个变量接受呢?        //用变量接受，这个变量应该有类型        //可能损失精度        //byte dd = bb + cc;        int dd = bb + cc;        System.out.println(dd);    }}

1.1.1 Forcing type data conversion
Format of the cast

• b = (byte) (A + B);
  Considerations for Casting
• If you exceed the value range of the assigned data type, the result will be different from the result you expect.
  1.1.2 Case Code IX

  /*强制转换：    目标类型 变量名 = (目标类型) (被转换的数据);    不建议强制转换，因为会有精度的损失。*/public class TypeCastDemo2 {public static void main(String[] args) {    int a = 3;    byte b = 4;    int c = a + b;    //byte d = a + b;    byte d = (byte) (a + b);}}

Java Syntax format