Java Basics Explained-Basic Data Types and Operations

Encoding

ASCII--0~127 65-A 97-a

Western European code table---ISO-8859-1---0-255---1 character Section

gb2312----0-65535---gbk---2 bytes

Unicode encoding system---utf-8---3 bytes

中 f

bit Byte 1Byte = 8bit 1KB=1024B MB GB TB PB---Storage unit in computer

Constant

Integer constant-- -All integers 3,99,107

Decimal constants---All decimals 3.5 100.9

Character constants---Use single quotes to identify a letter, number, or symbol 'a' ' =' ' '

String constant---Use double quotes to identify one or more characters "abc" "234" "q2" ""

Boolean constant---Use Indicates logical value---true/false

Empty constant---null

5-integer, 5.0-decimal '5'-character "5"-string '5.0'- Writing error "5.0"-String

Binary system: full binary to one, 0~1 1+1=10 0b10011 0b0011, starting from JDK1.7, 0b is allowed as Beginning to identify a number is a binary number

Octal: full eights into one, 0~7, 7+1=10 It is required to start with 0 06 015

Decimal: full tenths One, 0~9

Hexadecimal: full hexadecimal one, 0~9,, A~F, 9+1=A f+1=10 It is required to start with 0x 0x5 0xad

Conversion from decimal to binary

Convert from decimal to binary: keep dividing by 2 to take the remainder, and then put the remainder in reverse order

Convert from binary to decimal: from the low bit Starting from the bit order, multiply the bit order by the power of 2, and then sum up

Convert binary to octal: starting from the low order, every three digits are divided into one group, less than three Filling the bits with 0 produces one octal number. Arrange these numbers in order to convert octal to binary: one to three --- one octal number produces three binary digits.

Convert binary to hexadecimal: the process of converting four into one

Variable

System.out.println(i);

int i = 5;---No---The variable must be declared before use

int i;

System.out.println(i);---No--The variable is in use Must be initialized before

Data type

Basic data type

Numeric type

Integer type

byte---Byte type ---1 byte--- -2^7~2^7-1 --- -128~127

byte b = 5; byte b2 = -128;

short---short integer---2 bytes--- -2^15~2^15-1 --- -32768~32767

short s = 54; short s = -900 ;

int---integer---4 bytes--- -2^31~2^31-1

int i = 100000;

int j = 100_000_000;--It is allowed starting from JDK1.7. These will be automatically ignored during compilation_ -> int j = 100000000;

int i = 00001111;---Octal

The default type of integer in Java is int

long---long integer type---8 bytes--- -2^63~2^63-1---ending with L indicates that this number is a long type number

long l = 3L;

Floating point type

float---single precision---4 bytes---must end with f

float f = 3.2f;

double---double precision---8 bytes

The default decimal type in Java is double type

double d = 3.5;

double d = 4.6D;---Yes

double d = 3.9e4; //It is scientific notation in decimal system

double d = 0x3p2; //It is hexadecimal Scientific notation -> 12

Character type

char---2 bytes--- 0 ~65535

char c = 'a';

char c = '中';

Boolean

boolean---true/false

boolean b = false;

Reference data type

Class ---class Interface ---interface Array ---[]

Data type conversion

Implicit conversion/automatic type conversion

byte b = 100;

int i = b;

long l = 63;---Yes---when the integer value is within the range of int type , you don’t need to add the ending L

Rule 1: Small types can be converted into large types---byte->short->int->long float->double

int i = 5;

float f = i;

long l = 6;

float f = l;

Rule 2: Integer Can be converted to decimal, but precision loss may occur

char c = 'a';

int i = c;

Rule 3: Character type can be converted to integer type

short s = 'a';---Yes

char c = 100;---Yes

char c = 'a' ;

short s = c;---Not possible

defines a variable c of type char. The stored data is a character. There is no need to check the specific character encoding. When assigning a value to short type, short needs to check whether the encoding corresponding to the character is within the value range of the short type. At this time, the specific encoding corresponding to the character cannot be determined. Since the value range of the short type does not completely overlap with the char type, in order to prevent If it exceeds the range, assignment is not allowed.

short s = 97;

char c = s;--not possible

Explicit conversion/forced type Conversion

long l = 54;

int i = (int)l;

double d = 3.5;

int i = (int)d ;---When converting a decimal to an integer, the decimal part is discarded directly

double type cannot store decimals accurately

Hexadecimal--Hexadecimal

Decimal-- Decimal

Octal---Octal

Binary--Binary

Operator

Arithmetic operator

+addition-subtraction*multiplication/division% modulo++auto-increment--auto-decrement+string concatenation

int i = 5210 / 1000 * 1000;--->i = 5000;

Note:

1. After the integer operation is completed, the result must be an integer

2. Integer When dividing by 0, the compilation passes and the error is reported when running---ArimeticException---Arithmetic exception

3. The result of dividing a decimal by 0 is Infinity

4. The result of 0/0.0 is NaN---Not a Number---Not a number

5. The byte/short type will be automatically promoted to the int type during operation

%Remainder operation

-5%3=-2 -4%3=-1 -3%7=-3

5%-3=2 7%-2=1 2%-8= 2

-5%-3=-2 -9%-3=0

For the remainder of a negative number, first follow the remainder operation of a positive number and look at the number to the left of the remainder sign The sign of #++/--

For ++, increment by 1 on the original basis

int i = 5;

int j = ++i;--- > i increments by 1, and then assigns the value of i to j---increments first, and then operates

int j = i++;--->Get the value of i first, 5, and increments i becomes 6, and then assign the obtained value 5 to j---operate first, then increment

int i = 3;

int j = ++i * 2;-> ; j = 8;

int j = i++ * 2;->j = 6

int i = 6;

int j = i++ + ++i;->i = 8; j = 14;

int j = ++i + i++;->i = 8; j = 14

byte b = 5;

b++;---JVM will perform forced type conversion on the result at the bottom level, and then convert the result into byte type

char c = 'a';

System.out.println(c + 4);--can

char c2 = 'd';

System.out. println(c + c2);---Promote to int type and then perform operation

+ String concatenation operation

"a" + "b"-- -> "ab"

"a" + 3---> "a3"

"a" + true-> "atrue"

2 + 4 + “f”-> “6f”

“f” + 2 + 4-> “f24”

Assignment operator

= += -= *= /= %= &= |= ^= <<= >>= >>>= ~=

int i= 5;

i + = 3; -> i = i + 3; -> i = 8;

i -= 2;-> i = i - 2;-> i = 3;

int j;

j += 4;---No

int i = 5;

i += i -= i *= 5;--> i = -15;

i = 5 + ( 5 - (5 * 5)) ;

i += i -= i *= ++ i;--->i = -20;

i += i*= i-= (i++ + --i);---> i = -20;

i = 5 + ( 5 * (5 - (5 + 5)));

byte b = 5;

b += 3;--- Yes

byte b = 125;

b += 3;---Yes--- -128

Comparison/relational operator

= =Equal!=Not equal> < >= <= instanceof

##3 == 4;-> false

instanceof---Judges the relationship between objects and classes- -Can only be used for reference data types

String s = “abd”;

System.out.println(s instanceof String);---true

System. out.println(“def” instanceof String);---true

Logical operator

is used to operate logical values

&AND|OR! NOT^XOR && short circuit with || short circuit or

true&true=true true&false=false false&true=false false&false=false

true|true=true true|false=true false|true=true false|false= false

!true=false !false=true

true^true=false true^false=true false^true=true false^false=false

For &&, If the value of the previous expression is false, then it can be determined that the value of the entire expression is false, and the operation after && will no longer be performed.

ternary/ternary/conditional operator

Logical value? Expression 1: Expression 2

If the logical value is true, execute expression 1; otherwise, execute expression 2

int i = 5, j = 7;

i > j ? System.out.println(i): System.out.println(j);---No! There must be a result after the ternary operator operation is completed!

double d = i > j ? i * 2.5 : j;---The return value types of the two expressions are either consistent or compatible

Get data from the console

import java.util.Scanner; //Written under package, above class

Scanner s = new Scanner(System.in);

int i = s. nextInt();

double d = s.nextDouble();

String str = s.nextLine();

String str2 = s.next();

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