A hands-on brain

1. Analysis Code

Package Example; 

public  class Exam { 

    public  static  void main (String [] args) {
         // the TODO Auto-Generated Method Stub 
        Size = S Size.SMALL; 
        Size t = Size.LARGE;
         // S and t refer to the same object? 
        System.out.println (S == t);   // 
        // primitive data type? 
        System.out.println (s.getClass () isPrimitive ().);
         // convert from a string 
        Size Size.valueOf U = ( "the SMALL" ); 
        System.out.println (S == U);   // to true
         // list all of its value 
        for(Size value:Size.values()){
            System.out.println(value);
        }

    }

}
enum Size{SMALL,MEDIUM,LARGE};

operation result:

Enumerated type common usage:

https://blog.csdn.net/testcs_dn/article/details/78604547

2. The principle of the same name variables shield

In java, local variables can override the value of a global variable.

　　　 byte: 1 byte -128 to 127

short: 2 bytes ~ 2 ^ 15 ^ -2 15-1
int: ~. 4 byte 31 is ^ 2 ^ -2 31-1
Long:. 8 ~ byte 63 is 2 ^ ^ -2 63-1
Boolean:. 1 byte true false (java can not be replaced by 0 or 0)
a float: 3.403E38. 4 ~ byte -3.403E38
Double: ~. 8 bytes -1.798E308 - 4.9E324
char: 2-byte '\ u0000' ~ '' \ uffff '(hexadecimal, i.e. recalculated from 0 to 65535)
(1 byte is equal to 8)

Note: low-accuracy conversion without losing the accuracy of precision, accuracy is lost from the low steering precision.

public class TestDouble {

    public static void main(String args[]) {
        System.out.println("0.05 + 0.01 = " + (0.05 + 0.01));
        System.out.println("1.0 - 0.42 = " + (1.0 - 0.42));
        System.out.println("4.015 * 100 = " + (4.015 * 100));
        System.out.println("123.3 / 100 = " + (123.3 / 100));
    }
}

Double inaccurate calculation, loses precision; String type can be accurately calculated.

int X = 100;

int Y = 200;

System.out.println("X+Y=" + X + Y);

// string plus the latter plays the role of connection, i.e., "X + Y" after the plus sign is automatically converted to a string of 100 to X, so that X has become a string

// the same token, Y plus sign will later converted to a string of X 200, the final output is "X + Y =" 100200

System.out.println(X + Y + "=X+Y");

X // because no string in front, so that X and Y are first subjected to ordinary summation, the final output is 300 = X + Y

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