Crazy God said java study notes --- operator

JAVA basic grammar

5. Operator
6. Package mechanism, JavaDoc

Operator

• The Java language supports the following operators:Priority: best use ()

  • Arithmetic operators: +, -, *, /,% (remainder), ++ (increment), – (decrement);

  • Assignment operator: =;

  • Relational operators:>, <, >=, <=, ==, !=, instanceof;

    The calculation result must be true or false; it will be used with if in the future

  • Logical operators: && (and), || (or),! (non);

    The calculation result must be true or false

  • Bit operators: &, |, ^, ~, >>, <<, >>>;

  • Conditional operator:? ,:;

  • Extended assignment operators: +=, -=, *=, /=;

  Arithmetic Operator

  ++ (increment)

  a++: When ++ is behind, assign value first, then increment;

  ++a: ++ is first incremented and then assigned when it comes before;

  int a = 5;
  int b = a++;//a++加在后，先给b赋值，再自增；
  System.out.println(a);//a=6
  System.out.println(b);//b=5
  ================================================
  int c = ++a;//++a加在前，先自增，再赋值给c；
  System.out.println(a);//a=6;
  System.out.println(c);//c=6;
  

  – (Decrement) the same

• Precautions:

  • One of the arithmetic operators is of type long or double, and the result is of type long or double, otherwise it is of type int.
  • Many operations in Java will use many tools to operate

  Logical Operators

  • && (and): logical AND operation, both variables are true, the result is true
  • || (or): logical OR operation, one of the two variables is true, the result is true
  • ! (Not): If it is true, it becomes false, if it is false, it becomes true

  int q = 10;
  int  w = 20;
  System.out.println(q<w && w>30);//（false)，q<w正确，w>30错误,
  // 可解释为:q<w与w>30同时两条成立，结果为true，所以最后结果为"false"
  System.out.println(q<w || w>30);//（true),q<w正确，w>30错误,
  // 可以解释为:q<w或w>30其中一条成立，结果就为true，所以最后结果为"true"
  System.out.println(!(q<w));//（false),q<w正确。
  // 可以解释为:q应该不（非）小于w，但是实际上q<w，所以为”false"
  

  Short-circuit calculation

  When the former is "false", the latter will not be executed, and only "true" will run. This is called short-circuit operation.

  int z = 5;
  boolean x = (z<4)&&(z++<4);//由于（z<4）不成立为false，（z++<4)就不运行
  System.out.println(x);//(false)
  System.out.println(z);//(5)由于（z++<4)没有运算，所以z没有自增
  

  Bitwise operator

  & (1): All 1s are 1, otherwise 0

  | (0): All 0 is 0, otherwise it is 1

  ^ (Same as 0): the same is 0, otherwise it is 1

  ~ (Inverted): take the opposite number in binary

   A = 1100 0011
   B = 1001 0110
  ---------------
  A&B= 1000 0010
  A|B= 1101 0111
  A^B= 0101 0101
  ~B = 0110 1001
  

  "<<"Binary left shift=represent *2

  ">>" Binary Right Shift = Representation/2

  Spread operator

  +=，-=，*=，/=;()

  int a = 5, b = 10;
  a+=b;//a=a+b(15)
  a-=b;//a=a-b(-5)
  a*=b;//a=a*b(50)
  a/=b;//a=a/b(0.5)
  

  String concatenation

  When the string type appears before the "+" sign, he will convert an operand after the "+" into a steing type for splicing.

   int a = 5, b = 20;
  System.out.println("yes"+a+b);//(yes520)
  System.out.println(a+b+"yes");//(25yes)
  System.out.println("yes"+(a+b));//(yes25)
  System.out.println(""+a+b+"yes");//(520yes)
  System.out.println(""+a+b+(a+b)+"yes");//(52025yes)
  

  Conditional operator

  x? y: z //If the value of x is true = y, if it is false = z;

  int score = 60;
  int xm = 70;
  int xg = 49;
  int xh = 60;
  System.out.println("xm:"+(xm>=score ?"及格":"不及格"));//(xm:及格)
  System.out.println("xg:"+(xg>=score ?"及格":"不及格"));//(xg:及格)
  System.out.println("xh:"+(xh>=score ?"及格":"不及格"));//(xh:及格)
  

  You can use "if" to do it later

  Packet mechanism

"The essence of a package is a folder"

• In order to organize classes better, Java provides a package mechanism for distinguishing the namespace of class names.

• The syntax format of the package statement is:

  package pkg1[.pkg2[.pkg3...]]//定义包
  

• Generally use the inversion of the company domain name as the package name;

• In order to be able to use a member of a package, we need to explicitly import the package in the Java program. Use "import" to complete this function

import package1[.package2...].(classname|*);//导入包，*导入这个包下所以的类

Try not to repeat the package name

JavaDoc

• The javadoc command is used to generate your own API documentation

• Parameter information

  • @author author name
  • @version version number
  • @since indicates the jdk version that needs to be used earliest
  • @param parameter name
  • @return Return value situation
  • @throws exception throw situation

  /**
   * @author ssxxz
   * @version 1.0
   * @since 1.8
   */
  

  Production documents: javadic+parameters+java files

  javadoc -encoding UTF-8 -charset UTF-8 Doc.java