Java variable

Java value conversion

From low to high:

Byte, short, char-> int -> long -> float -> double

Cast: from high level to low one, e.g.,

int i = 128
  byte b = (byte)i 
  //output b = -128 memory overflow

Notes:

1. boolean value is unconversionable

2. cast may lead to memory overflow, e.g.,

int money = 10_0000_0000; // JDK new feature: numbers can be separted by "_"
int year = 20;
int total = money*year; // memory overflow 
//solution:
long total = money*((long)year);

Java variable

While define a vairable:

1. type

2. name

3. value

String temp = "Java"

Local variable

1. Declaring and initializing values

Instance variable

1. Declaring in the class

2. If there is no initilizing values, it will return defualt value (boolean:false, basic types: 0, other type: null)

3. Example for using instance variable

   public class Demo05 {
       String i1;
       int i2;
       public static void main(String[] args) {
           Demo05 instanceVariable = new Demo05();
           System.out.println(instanceVariable.i1);//out null
       }
   }

Class variable (e.g., static)

1. Can be used in another function

Constant

pulic class Demo{
static final pi = 3.14
final static pi = 3.14
public static void main(String[] args){
  System.output.println(pi)
}
} 
// No order requirement 
// They are identical 

Variable specification

1. Class variable: monthSalary (hump principle)

2. Local variable: start with lower case and hump principle

3. Constant: upper case and underscore e.g., MAX_VALUE

4. Class name: start with upper case e.g., Demo01

   public class Demo01{
     
   }

5. Function name: start with lower case and hump principle e.g., run(), runMain()