Java Xiaobai introduction must learn! Notes on the most complete data types and operators, with examples

Ink rain and clouds 2021-05-04 09:09:46
java xiaobai introduction learn notes



This article is dedicated to Java The small white 、 Or want to get started Java My classmates and I'm learning Java It's arranged by my classmates , I hope it can help you . This content mainly explains Java The use of data types and operators, etc , Every knowledge point has examples .

Java Xiaobai must learn at the beginning ! Most complete notes on data types and operators , Examples are attached

One . Integer variables

1. The basic format

int Variable name = Initial value ;

Code example :

public class CSDN { public static void main(String[] args) { int a=10; System.out.println(a); }}

2.java Medium int yes 4 Bytes .

3.java in int Maximum and minimum of

If the number is greater than 0, The highest order is 0, If the number is less than 0, The highest order is 1. When judging the range of values , Regardless of the negative number, it is stored in the form of complement , Because the complement and the original code are one-to-one correspondence . When the highest is 1, Negative number , There are at most 31 individual 1, And this 31 individual 1 The sum of the added value of is 2 Of 31 Power -1, And the highest negative number is 1, This bit still satisfies the condition of negative number when it is carried again , The highest bit can be regarded as a numerical bit , So the minimum value is -2 Of 31 Power , The maximum integer must have a value of 1, And the original code equals the complement , This limits the maximum number of integers 31 Bit 1 Add up , The sum is 2^31-1.

 public class CSDN{ public static void main(String[] args) { int a = 10; System.out.println(Integer.MAX.VALUE); System.out.println(Integer.MIN.VALUE); } }

4. If a local variable , You can't use it without initialization .

Identifiers can contain : Numbers 、 Letter 、 Underscores , But you can't start with numbers , Underline is not recommended .

Two . Long integer variables

Define a long integer , The number must be followed by L

 public static void main(String[] args) { Long a = 10L; System.out.println(a); }

java And c Different language , There is no such thing as no sign ,Long Yes 8 Bytes ,64 A bit , The first is the sign bit .

If you want to print Chinese characters, use the plus sign .

public static void main(String[] args) { Long a = 10L; System.out.println(a); System.out.println("a The value of is :"+a);// Printed Chinese characters are enclosed in quotation marks and then spliced with a plus sign 

Any type of data and string to splice , The result is a string .

3、 ... and . Double precision floating point type

 public static void main(String[] args) { // Double precision floating point double d = 12.5; System.out.println(d); }

Decimal is double by default .

public static void main(String[] args) {int a = 1;int b = 2;System.out.println(a / b);}// The output is 0

because a and b It's all integers , So it's also integer type , Can't save decimals .

public static void main(String[] args) {,double num = 1.1;System.out.println(num * num)// Execution results =1.2100000000000002}

Any decimal has precision , There is no exact value , There's only one scope , To study this problem, we need to know how floating-point numbers are stored in memory .

In general , Decimal encountered , It is recommended to use double.

Four . Single precision floating point type

float yes 4 Bytes , Can't take double The type is placed in float In type , Compile errors , You need to add a... After the decimal f, Embodies the Java The safety of the .

public static void main(String[] args) { float f = 12.3; System.out.println(f); }

5、 ... and . Character data type

stay java in char Two bytes , stay c It's a byte in a language .

public static void main(String[] args) { char ch = 'a'; System.out.println(ch); char ch2 = 97; System.out.println(ch2); }

stay java Follow... In unicode Character sets are similar to c In language asii value , but unicode There's a wider range of characters .

6、 ... and . Byte type

matters needing attention :

Byte types also represent integers . It takes only one byte , The scope is small ( -128 -> +127 )

Byte type and character type are not related to each other .

 public static void main(String[] args) { byte b = 12; byte c = 21; System.out.println(b+" "+c); }

Each data type cannot be assigned beyond its range , Otherwise, an error will be reported .

7、 ... and . Short

short It's two bytes , The value range is -32768 To 32767.

The scope of this expression is small , It is generally not recommended to use .

 public static void main(String[] args) { short sh = 12; System.out.println(sh); }

8、 ... and . Boolean type

1. stay java in , Boolean types have no explicit size .

2. stay java in , Boolean type has only two values ,true and false.

3. stay java in , It doesn't matter 0 It's fake , Not 0 Is the true .

public static void main(String[] args) { boolean flag = true; System.out.println(flg); }

summary :

java in 8 The two basic data types are as follows :

1 2 2 4 8 4 8

byte char short int long float double boolean

Byte Character Short Integer Long Float Double Boolean

String type variables

String type uses String Define

matters needing attention :

Java Use Double quotes + Several characters Represents the literal value of a string .

Different from the type above , String Not a basic type , It's the type of reference ( I'll focus on it later ).

Some specific characters in a string that are not easy to express directly need to be escaped .

A string of + operation , Represents string splicing :

String a = "hello"; String b = "world"; String c = a + b; System.out.println(c);

The above code description , When one + When there is a string in an expression , Is to perform string splicing behavior .

So we can use it very easily System.out.println Print multiple strings or numbers at the same time .

Understanding numerical improvement

int and long Mixed operations

int a = 10; long b = 20; int c = a + b; // Compilation error , Prompt will long Turn into int Will lose precision long d = a + b; // Compile and pass 

Conclusion : When int and long When you're mixing , int Will be promoted to long, The result is still long type , Need to use long Type of variable to

Receive results . If you must int To receive results , You need to use cast .

byte and byte Arithmetic

byte a = 10; byte b = 20; byte c = a + b; System.out.println(c); // Compiler error Test.java:5: error : Incompatible types : from int The switch to byte There may be losses byte c = a + b;

Conclusion : byte and byte It's all the same type , But there is a compilation error . as a result of , although a and b All are byte, But calculation a + b Will be will be a and b All promoted to int, recompute , The result is also int, This is a gift to c, There will be these mistakes .

Because of the computer's CPU Usually according to 4 Read and write data from memory in bytes . For the convenience of hardware implementation , Such as byte and short This is lower than 4 The type of bytes , Will be promoted to int, And then participate in the calculation .

Summary of type promotion :

Different types of data mixing operations , The small ones will be promoted to the large ones .

about short, byte This ratio 4 A small byte type , Will be promoted to 4 Bytes of int , Re operation .

int and String Mutual conversion between

int Turn into String

int num = 10; // Method 1 String str1 = num + ""; // Method 2 String str2 = String.valueOf(num);

String Turn into int

String str = "100"; int num = Integer.parseInt(str);

Operator

1. Four basic operators + - * / %

The rules are simpler , It's worth noting that division

a) int / int The results are int, Need to use double To calculate

b) 0 Not as a divisor

2. The return types of relational operators are boolean type .

3. Shift Operators

Move left <<: I don't want the leftmost one , On the far right 0.

Move right >>: I don't want the rightmost position , The leftmost sign bit ( Positive complement 0, Negative numbers make up 1)

Be careful :


1. Move left 1 position , Equivalent to the original number * 2. Move left N position , Equivalent to the original number * 2 Of N Power .

2. Move right 1 position , Equivalent to the original number / 2. Move right N position , Equivalent to the original number / 2 Of N Power .

3. Because the efficiency of computer calculation is higher than that of multiplication and division , When a code just multiplies and divides 2 Of N We can use shift operation instead of .

4. It doesn't make sense to move negative digits or shift too many digits .

summary : Is it right after reading this article Java Data types and operators have a detailed understanding and understanding , It's not easy to create , Brush to the partner , Please support it , Thank you very much .

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