Java Fundamentals (Chapter two)

Java Fundamentals (Chapter two)

One, variable

1. A variable is an identifying symbol in memory that is used to store the data

2. Variable naming rules

L must start with a letter, underscore _, dollar sign $

l variables, which can include numbers

L variable, cannot appear special symbol, space

L keywords in Java (red font), cannot do variable name

3. Variable naming, followed by camel nomenclature (variables are made up of more than 1 words, the first letter of the 1th Word is lowercase, and the first letter of the following word is capitalized)

Second, the data type

A) number ( int(integer), double-precision floating-point double (decimal))

b) Character type (char), storing a single character

c) string character (string), storing multiple characters

Iii. How to declare variables and assign values to variables (Assignment operator =)

Method 1: Declare first, then assign value

Syntax: data type variable name; ( int i ;)

Variable name = value; ( i=0; )

Method 2: Assign a value directly when declaring a variable

Syntax: data type variable name = value; ( int i=0 ;)

Remember: Variables are declared first, then assigned, and then used

Iv. input data from the keyboard

Step 1: In the first line of the current code file, import the package

Import java. util.*; or Import java. util. Scanner;

Step 2: Write the statement (write only once)

Scanner input=new Scanner (system.in);

Step 3: Define the variables and save the data entered by the keyboard

Data type variable name =input.next data type ();

┉┉┉┉┉┉┉┉┉┉┉┉┉┉┉┉┉┉┉┉┉┉┉┉┉┉┉┉┉┉┉┉┉┉┉┉┉┉┉┉

Arithmetic operators

+,-,*(multiplication sign),/(division sign, pick-up),%(take remainder, modulo)

Special operators: ++ ( gaga ) , -- ( minus )

int a = 1;

a+=10; equivalent to a=a+10;

a-=5; equivalent to a=a-5;

int a = 1;

Scene 1:system.out.print (a++);//Output 1

int a = 1;

Scene 2:system.out.print (++a);//Output 2

Relational operators

Greater than:> less than:< greater than or equal to:>= less than equals:<= equals sign:= = Not equal to: /c6>!=

┉┉┉┉┉┉┉┉┉┉┉┉┉┉┉┉┉┉┉┉┉┉┉┉┉┉┉┉┉┉┉┉┉┉┉┉┉┉┉┉

Five, Boolean data types

Boolean: A value of TRUE or False

Assignment: The boolean variable name =true or false or an expression;

Data type Conversions ( for numeric types )

1, divided into 2 major categories:

numeric type (int, double),

Non-numeric type (character ', String ', Boolean type bool)

2. Data type Conversion classification

A) automatic type conversion--"P41 page"

I. Rule 1: If one operand is of type double, the entire expression can be promoted to a double type

Ii. Rule 2: Conditions for automatic type conversions are met.

Two types are compatible: numeric types (int and floating-point double)

Destination type is greater than Source type: Double type can hold int type,(int-->double"enlarge transform")

b) Coercion type conversion: Target type variable name = (target type) variable or expression;

"A value of type double, saved to the INT type variable"

int variable name =(int)a value or variable of type double;

Java Fundamentals (Chapter two)