Variables and data types

1. Variables

1. Memory is the data space in the computer. Different data are stored in the space with different memory addresses. Independent of each other.

2. How to find the stored data?

Variable name not found

3. Variable naming rules

Word, down, beauty, man, number, camel

Words: Letters

Bottom: underline

US dollar sign $

Person: RMB symbol ¥

Must start above

The follow-up can only be words, words, beauty, people and numbers

Number: number

Camel: Camel nomenclature or hump nomenclature. Capitalize the first letter of each word after the beginning.

When a variable name consists of multiple words, the first word is capitalized, and then the first letter of each word is capitalized.

Variable names should be meaningful

Do not use Chinese for variable names

Keywords cannot be used

2.java common data types

1. Data types are divided into numeric and non numeric

The values are divided into

Integer (int)

Non integer (double double precision floating point number, folat single precision floating point number)

Non numerical

char single character

boolean (true or false)

String string

 

3. Variable declaration and use

1. Declare variable: give the variable a name int money

2. Assignment: money=1000;

=Assign the following to the front for

It can be combined into one sentence int money=1000

4. Constant

1. Constants that do not change should be capitalized.

2. Add final before the variable to become a constant.

final double PAI=3.14

In double PAI=3.14; Add final to make it a constant

3. The two characters are separated by an underscore

final double GOOD_UP=100;

5. Use of scanner

1. It is necessary to open an entry for the user to enter the program through the keyboard

Scanner input = new Scanner(System.in);
//a key

The program keyboard needs to enter the opening: Scanner

Entry name input (variable name can be changed freely)

But now it is in the state of error reporting.

You need to write in front of class 👇

import java.util.*;

This sentence means that the Scanner above is located in the java.util package. It is equivalent to importing the java.util. The following * means importing everything in the package.

Then write below

String name =input.next();

Next is next. This sentence means next. The next input is what needs to be output

//Import a package

import java.util.*; 

​
// You need to open an entry input for the user to enter the program through the keyboard
        Scanner input = new Scanner(System.in);
        //Get string using input
        System.out.println("Please enter your name:");
        String name =input.next();

Scanner can only output strings, not characters.

nextInt can output integers. When outputting integers, change next to nextInt.

The age entered is an integer

System.out.println("Please enter age:");
        int age =input.nextInt();

Change String to int and next to nextInt.

The input score is small floating point

    System.out.println("Please enter your grade:");
        double score = input.nextDouble();

Change String to double and next to nextDouble.

Input steps using user keyboard:

1. Import the package import java.util. *;

1. //You need to open an entry for the user to enter the program through the keyboard. input Scanner input = new Scanner(System.in);

3. Use iinput to obtain the data entered by the user's keyboard

character string

System.out.println("Please enter your name:");
	String name =input.next();

integer

System.out.println("Please enter age:");
		int age =input.nextInt();

Floating point number

System.out.println("Please enter your grade:");
	double score = input.nextDouble();

No nextChar.

6. Use of data types

//Data type: / / numeric value (integer int float\double)

//Non numeric value (String double quotation mark character char single quotation mark boolean value: true, false)

//All data types except String are called basic data types

//String is called a reference data type

//Floating point numbers are double by default

double num2=1.0；

float num3=1.0f；

double is more widely used than float. If you want to use float to represent floating point numbers, you need to add f after it.

int num1 = 10;
double num2 = 9.8;
float num3 = 1.8f;
//When one of the expressions is double, the whole result is promoted to double
//Force type conversion, report the error reason, and assign double to int
int num4=num1-num2

When one of the expressions is double, the whole result is promoted to double

int num4=num1-num2 error

Error reason, assign double to int

To change double into int, add (int) before it to complete a cast

int num4=num1-(int)num2

After the cast, double becomes int, and all after the decimal point is erased.

Forced conversion is to change a data type with a large range (double) into a data type with a small range (int)

Automatic conversion

//The automatic type conversion int is directly assigned to double
		//The assignment with a large range itself is directly converted to a small range
	   num2 = 100;

Those with a large range can be directly assigned to those with a small range

num2 = 100;

7. Conversion rules

double firstAvg=81.29;//First average score
double secondAvg:   //Second average score
int rise=2；
secoundAvg=firstAvg+rise;
System.out.println("The average score for the second time is"+secondAvg)
 

double firstAvg=81.29； Assign a value of 81.29 to firstavg, because it is a decimal, so use double.

double secondAvg: the final required formula.

System.out.println("the average score of the second time is" + secondAvg) the final output is secondavg+ secondAvg

int rise=2； It's an integer

secoundAvg=firstAvg+rise; The following int integer automatically becomes double.

8. Assignment operator

=The value or expression on the right of the action assignment is assigned to the left in the order from right to left.

int rise=2, 2 is assigned to rise

public static void main(String[] args) {
		int num1=8;
		int num2=9;
		System.out.println("Before exchange:");
		System.out.println("num1:"+num1);
		System.out.println("num2:"+num2);

System.out.println("num1:"+num1);

The string itself is output in "double quotation marks", and the value of assignment is output in num1 linked with + outside.

int num3=num1;
		num1=num2;
		num2=num3;
		
		
		System.out.println("After exchange:");
		System.out.println("num1:"+num1);
		System.out.println("num2:"+num2);

Find an intermediate temporary variable num3 and store the value of num1 first

Then exchange

9. Arithmetic operator

 int num1 =9;
	 int num2=2;
	 int num3=num1/num2;
	 int num4=num1%num2;
	 System.out.println("num1/num2:"+num3);
	 System.out.println("num1%num2:"+num4);

/As long as the quotient is used for division, the% remainder takes the remainder.

 int num1 =9; 
System.out.println(++num1);
//num1=num1+1 Result 10
	 

++For self addition, the original value is + 1.

int num2=2; 
 System.out.println(--num2);
//num1=num1-1 result 1

--It is self subtraction, and the original value is - 1.

System.out.println(num1++);//num1=num1+1  
System.out.println(num2--);//num1=num1-1  

++And -- can be written after variables or expressions.

System.out.println(num1++);//num1=num1+1 result 9
System.out.println(num2--);//num1=num1-1 result 2

There is a difference between before and after writing

++Or --, self addition or self subtraction before operation

++Or --, after, first calculate, then add or subtract, and then output first, and then add or subtract, so the output is the original value.

+=, - = are all operators

 int num1 =9;
 num1 += 2； // After + 2 on its own basis, it is assigned to num1
    System.out.println(num1);

5%2=1 5/2=2

num1=6 num2=1

10. Comprehensive application of arithmetic operators

import java.util.*;

public static void main(String[] args) {
		//Enter a four digit membership card number, and the console calculates each digit of the membership card number and the sum of the numbers.
		//Scanner usage: make a cut first
		Scanner input = new Scanner(System.in);
		//Please enter a four digit membership card number for the output statement
		System.out.println("Please enter a four digit membership card number:");
		//The input card number is an integer. Use int and nextInt input to obtain the input card number and output
		int no =input.nextInt();
		//Output card number
		System.out.println("The membership card number is:"+no);
	   //8369 take out ten thousand values
		int num1=no%10;
		int num2=no/10%10;
		int num3=no/100%10;
		int num4=no/1000;
		int num5=num1+num2+num3+num4;
	   //Use the remaining 10 methods to get a bit
		System.out.println("Single digits of card number:"+num1);
	   //Divide by 10 and then take the remainder to get ten
		System.out.println("Ten digits of card number:"+num2);
	   //Divide by 100 and then take the remainder to get the hundred
		System.out.println("Hundred digits of card number"+num3);
		//Get the thousands by dividing by 1000
		System.out.println("Thousand digits of card number"+num4);
		System.out.println("Number of members:"+num1+"Ten digits:"+num2+"Hundreds:"+num3+"thousands :"+num4);
		System.out.println("Total number of four digits of card number:"+num5);
 

11. Relational operators

The result of the relational expression is a Boolean value

==Is the equal operator

12. Logical operators

Logical operators are used to connect two expressions or operands

Do not continue after short circuit, and the second + + X will not operate, so x in the second formula is 8 + 1 = 9

12. Conditional operator

Conditions? Expression 1: expression

If the condition is true, the first expression is output; if it is wrong, the second expression is output.

import java.util.*;

//Judge whether a genus is even (a number divisible by 2)

	public static void main(String[] args) {
		Scanner input = new Scanner(System.in);
		System.out.print("Please enter a number other than 0:");
		int num = input.nextInt();

		String result =(num%2==0)?"even numbers":"Odd number";
		System.out.println(num+"yes"+result);
	}

}

import java.util.*;

	public static void main(String[] args) {
		//Enter a four digit membership card number, the console calculates the digits of the membership card number, and calculates the sum of the numbers, and then calculates whether there is a winner according to the size of the sum

		Scanner input = new Scanner(System.in);
	
		System.out.println("Please enter a four digit membership card number:");

		int no =input.nextInt();
		//Output card number
		System.out.println("The membership card number is:"+no)；
		int num1=no%10;
		int num2=no/10%10;
		int num3=no/100%10;
		int num4=no/1000;
		int num5=num1+num2+num3+num4;

		System.out.println("Single digits of card number:"+num1);
	 
		System.out.println("Ten digits of card number:"+num2);
	   
		System.out.println("Hundred digits of card number"+num3);
		
		System.out.println("Thousand digits of card number"+num4);
		System.out.println("Number of members:"+num1+"Ten digits:"+num2+"Hundreds:"+num3+"thousands :"+num4);
        
		System.out.println("Total number of four digits of card number:"+num5);
		
		String result=(num5>10)?"You won the prize":"You didn't win the prize";
		System.out.println("Card number is"+no+"Member of"+result);
	}