Variable, "+" sign, data type

3.1 Concept ： Variable is equivalent to the representation of a data storage space in memory , You can think of variables as the house number of a room , We can find the house by the house number between , You can access variables through their names ( value )

3.1 Basic steps of variable usage

1) Declare variables

int a;

2) assignment

a = 60; // I should say so : hold 60 Assign to a

Use System.out.println(a);

public class VarDetail{
	// Write a main Method 
	public static void main(String[] args) {
		// Variables must first declare , After use , There is order 
		int a = 50;//int
		System.out.println(a);//50
		// The data value of this area can constantly change within the same type 
		//a = "jack";//x
		a = 88;//√
		System.out.println(a);//88
		// Variables cannot have the same name in the same scope 
		//int a = 77;//×
	}
}
class Dog {
	public static void main(String[] args) {
		int a = 666;//√		
	}
}

3.3  In the program + Use of numbers

  3.4 data type  

 

3.5 The type of integer  

public class IntDetail{
	// Write a main Method 
	public static void main(String[] args){

	//java The integer constant of （ Specific value ) The default is  int  type , Statement long Type constant must be added later ‘l’ or ‘l’
	int n1 = 1;// Bytes 
	//int n2 = 1L;// incorrect , reason L = long byte , Occupy 8 Memory , and int Account for only a 4 Memory 
	long n3 = 1L;// Yes 
	}
}

3.5.1 Case presentation ：

byte n1 = 10;（ Memory allocated 1 Bytes ）

short n2 = 10;（ Memory allocated 2 Bytes ）

int n3 = 10;（ Memory allocated 4 Bytes ）

long n4 = 10; （ Memory allocated 8 Bytes ）

3.6 Floating point usage details  

3.6.1 explain  

1) A brief description of the storage form of floating point numbers in the machine , Floating point numbers = Sign bit + Exponential position + mantissa

2) The mantissa may be lost , Loss of accuracy ( Decimals are approximations ).