Day 5 : Variables( 1 )

Chi-Tse Chiang
2022-09-20 22:20:00

#Java Variable

In programming languages, we call the value of the data stored Variable. There are mainly the following types:

String stores a piece of text, the content must be between double quotes

int stores an integer without decimal point

float stores a number with decimal point (the declared number must be followed by the letter f)

char stores a single character or ASCII value, e.g. j, C (contents must be enclosed in single quotes ' ')

boolean stores the two states of a piece of data: true or false

The way to declare a variable in a program:

type variableName = value;
/* Example:
	  int number = 1206;
	  char grade = 'A';
	  String name = "Chi";    */

The usage modes and restrictions of each variable will be introduced one by one below!!!
(Markdown is included in a class, and only part of the code is displayed for instructional purposes.)

#Num

Integer

1. byte (1 byte)
Number range: -128 to 127

2. short (2 bytes)
Number range: -32768 to 32767

3. int (4 bytes)
Number range: 2147483648 to 2147483647

4. long (8 bytes)
Number range: 9223372036854775808 ~ 9223372036854775807 (declared value must be followed by the L identifier)

byte number1 = 126;
short number2 = 12060;
int number3 = 120600;
long number4 = 12060000000L

Floating Point

1. float (4 bytes)
Range of decimal places stored: approximately 6 to 7 decimal places

2. double (8 bytes)
Range of decimal places stored: approximately 15 decimal places

The respective identifier, f or d, must be added after the declared value.

float flnum = 12.6f;
double dounum = 12.666d;

Character

Occupied 2 bytes (bits) and can be used to store not only a single character, but also the values in the ASCII table.

char grade = 'S';
char first = 67, second = 72, third = 73;     /*也可以一次宣告多個字串*/
System.out.println(grade);           /* Outputs : S   */
System.out.print(first);
System.out.print(second);
System.out.print(third);             /* Outputs : CHI */

String

String is considered a relatively special Non-Primitive Data Type, and this section will introduce it in detail on Day 6.
Declare the use of the keyword String, and enclose the content in double quotation marks .

String name = "Chi";
String greeting = "Hello";
System.out.println(name + " " + greeting);  /* Outputs : Chi Hello */
																				    /*可以用 + 號把字串給串起來並印出*/

Boolean

Boolean type variables are declared using the keyword boolean, and their contents can only be accessed as true or false (lowercase)

boolean myNameIsChi = true;
boolean isYourNameYu = false;
System.out.println(myNameIsChi);        //Outputs : true
System.out.println(isYourNameYu);       //Outputs : false

Supplementary concept:
We will learn about the use of loops in the future, and whether to execute the loop is determined by a conditional judgment.
Boolean is also very useful here, because true and false can also be considered 1&0 (1->true, 0->false).
Declaring a boolean value allows the loop to continue executing!!! (It's okay if you don't understand it here, I'll introduce it later.)

int num = 0;
boolean infinity = true;
while(infinity){           /*infinity內容為true，所以迴圈會一直執行進入*/
 if(num == 10){            /*直到num = 10時才停止跳出*/
		break;
	}
	num++;
}