02. Java Basics

Directory Structure

• Note: The red font is the unanswered knowledge point
• 1. Java identifier rules [rules, precautions, naming conventions]
• 2. Constants and variables [difference, types of constants]
• 3. Hex [type, conversion, complement complement]
• 4. Java data types [types, conversion rules, points to note]
• 5. Character encoding [type]
• 6. Operator [type, difference]
• 7. Java basic statement structure [type, difference description]
• 8. Method [definition, format]
• 9. Array [one-dimensional array, two-dimensional array]
• 10. Memory allocation and stack and heap

1. Java Identifier Rules

• 1.1 Composition Rules
  • English uppercase and lowercase letters
  • numeric characters
  • $ or _
• 1.2 Notes
  • cannot start with a number
  • cannot be a keyword in Java
  • case sensitive
  • no spaces
• 1.3 Naming conventions [must know the meaning of the name]
  • Underscore naming: my_name
  • CamelCase: myName
  • Class, interface name: must start with a capital letter, such as PersonActivity
  • Method, variable [string] Name: getData(), otherName
  • Constant naming: generally all uppercase, such as MAX_VALUE

2. Constants and Variables

• 2.1 What are constants and variables
  • A constant whose value cannot be changed is called a constant
  • A variable whose value changes is called a variable
• 2.2 What are the constants
  • String constant "hello world" enclosed in double quotes
  • Integer constants all integers 199
  • decimal constant all decimals 4.50
  • Character constants are 'a' enclosed in single quotes
  • boolean constants true and false
  • empty constant null
• 2.3 Variables
  • It is used to describe the amount by which the value can change, such as the outdoor temperature, the number of active users of the app, etc.

3. System

• 3.1 What are the bases?
• binary
  • 0 and 1 prefix 0B or 0b For example: 0B1001 converted to decimal is 9
• Octal
  • 0-7 prefix 0 for example: 0103
• Decimal [what we commonly use]
  • 0-9 without prefix
• hex
  • 0-9 and AF prefix 0x or 0X eg: 0x123
• Hex conversion
  • Convert any base to decimal formula: coefficient * base ^ exponent

0b1010 = 1 * 2^3 + 0 *2^2 + 1*2^1 + 0*2^0
       = 8 + 0 + 2 + 0
       = 10

0123  = 1*8^2 + 2*8^1 + 3*8^0
      = 64 + 16 + 3
      = 83

0x123 = 1*16^2 + 2*16^1 + 3*16^0
      = 256 + 32 + 3
      = 291

• 3.2's complement
  • The complement of a positive number is the same as the original code

5
原码：0000 0101
补码：0000 0101
	* 负数的补码是它的反码加1

-5：
原码：1000 0101
反码：1111 1010
    +         1
------------------
补码  1111 1011

4. Java data types

• 4.1 What are the data types

• byte type

  • 1 byte, 8 bits [between -128 and 127]

• short type

  • 2 bytes, 16 bits [-2^15 to 2^15 -1]

• int type

  • 4 bytes, 32 bits [-2^31 to 2^31 -1]

• float type [floating point]

  • 4 bytes, 32 bits

• double type [floating point type]

  • 8 bytes, 64 bits

• long type

  • 8 bytes, 64 bits [-2^63 to 2^63 -1]

• char type

  • 2 bytes, 16 bits [1 Chinese character is exactly 2 bytes]

• boolean type

  • 1 byte, true and false

• 4.2 Conversion of data

• Default conversion

  • 1：byte,short,char—int—long—float—double
  • 2: Byte, short, and char are complemented and converted to each other, and they are first converted to int type when they participate in the operation

• cast

  • Format: target type variable name = (target type) value or variable name
  • int a = (int) 15.7f;

• Points to Note

!!!!!!!注意!!!!!!!!
1:在java中，任何一个整数默认为 int 类型 (1)
2:在java种，任何一个小数，默认为 double 类型( 1.0)
3:123L 或者 1231 编译器会将该数当成long类型
4:12.345f 或者12.345F 编译器会将该数当成float类型

5. Character encoding

• common character encodings
  • ASCII code
  • GBK code
  • Unicode code [occupies 2 bytes]

6. Operators

• 6.1 What are the operators?
  • arithmetic operators
  • assignment operator
  • relational operator
  • Logical Operators
  • Ternary operator
  • Special Operators: Bitwise Operators
• 6.2 Arithmetic operators
  • • , - , * , / , % [remainder] , ++ [self-addition], --[self-subtraction]
• 6.3 Assignment Operators
  • = ， += ， -= ， *= ， /= ， %=
• 6.4 Relational Operators
  • == ， > , < , >= , <= , !=
• 6.5 Logical Operators
  • & ，^ ，| ，&& ，|| ，！ ， ^
• 6.6 Ternary operator [many used in Android]
  • a == b ？ c ： d
  • Format: (conditional expression)? Expression 1: Expression 2
• 6.7 Bitwise operators
  • & , | , ^ , ~ , << , >> , >>>

7. Java basic statement structure

• 7.1 Types of Statement Structures
  • sequential structure
  • choose structure
  • loop structure
• 7.2 Sequence structure
  • Execute from top to bottom
• 7.3 Select structure [can be nested]
  • if ， if else ， if else if …… else
  • switch case【支持byte，short，char，int，String】
• 7.4 Looping Structures
• for

for(初始化语句;判断条件语句;控制条件语句) {
         循环体语句;
}
第一步：先执行初始化语句，只执行一次
第二步：执行判断条件语句
第三部：执行循环体语句
第四步：执行控制条件语句，然后再执行循环体语句【循环】

• while

初始化语句;
while(判断条件语句) {
    循环体语句;
    控制条件语句;
}

• do while

do {
         循环体语句;
}while((判断条件语句);

8. Java methods

• 8.1 Definitions

• A method is a block of code that performs a specific function

  • 1: System method, only need to be able to use, do not need to know the internal structure
  • 2: Custom method: The internal implementation of the method requires us to write
  • 3: Methods cannot be nested inside methods

• 8.2 Format

• method format

修饰符   返回值类型   方法名(参数类型 参数 , ……){
     函数体
     return 返回值
}

• Modifier: public static or directly public or other private
• Return type: all data types (basic types: int, char, float, long, reference type: String)
• Method name: 1: See the name and know the meaning 2: getMax
• Parameter types: all data types, if a method has no return value, the return value type is void
• parameter name: variable name
• Return value: is the result to be returned (this result must be consistent with the return type)

9. Arrays

• 9.1 Array Concepts

  • An array is a container that can store multiple variables, and the data types of these variables must be consistent
  • Arrays can store both primitive and reference data types

• 9.2 One-dimensional arrays

• array definition format

  • Format 1: data type [] array name
  • Format 2: data type array name []

int[] a；     定义了一个int类型的数组a；
int a[];    定义了一个int类型的a数组；
推荐使用第一种定义方式。

• array initialization

  • Arrays in Java must be initialized before they can be used.
  • The so-called initialization: is to allocate memory space for the array elements in the array, and assign values ​​to each array element.

• Initialize the classification:

  • a: Dynamic initialization: only the length is specified, and the initialization value is given by the system
  • b: static initialization: give the initialization value, the length is determined by the system
  • Note: Only one of these two methods can be used, and the combination of dynamic and static cannot be used.

• 9.3 Two-dimensional arrays

• array definition format

  • datatype[][] variablename = new datatype[m][n];
  • m indicates how many one-dimensional arrays there are in this two-dimensional array
  • n represents the number of elements in each one-dimensional array

• Example:

int[][] arr = new int[3][2];
定义了一个二维数组arr
这个二维数组有3个一维数组，名称是arr[0],arr[1],arr[2]
每个一维数组有2个元素，可以通过arr[m][n]来获取,表示获取第m+1个一维数组的第n+1个元素

10. Java memory allocation and the difference between stack and heap

• 10.1 First understand a few concepts
  • stack
  • heap
  • method area
  • native method to
  • register

A:栈: 存放的是局部变量
局部变量：在方法定义或者方法声明上的变量都是局部变量。
B:堆: 存放的是所有new出来的东西
特点：
    a: 每一个new出来的东西都会为其分配一个地制值。
    b: 每一个变量都有一个默认的值
        byte，short，int，long  -- 0
        float，double            -- 0.0
        char                    -- '\u0000'
        boolean                -- false
        引用数据类型               -- null       
    c: 使用完毕就变成了垃圾，等待垃圾回收器对其回收
C:方法区:(面向对象部分讲解)
D:本地方法区:(和系统相关)
E:寄存器:(cpu使用)