Basic Problem record of javaSE Language

Read the basics of the javaSE language over the past two days and record some issues that have not been paid too much attention.

Question 1:

Package com. ccit. yichu;
Public class LightSpeed {
/**
* This instance mainly tests integer variable overflow.
* This program calculates the number of kilometers taken by light for 1000 days.
* Changing the second and distance types to int in this program will not return an error, but the result is-127631360 because:
* For class member variables, the default initial value of integer variables is 0, but the local variables declared in the method do not have the default value,
* The program is not initialized during compilation, so there is no overflow problem,
* When the program is running, the variables second and distance are of the int type and cannot represent 25920000000000. overflow occurs, so the result is incorrect.
* @ Param args
*/

Public static void main (String [] args ){
Int speed = 300000;
Int days = 1000;
Long second;
Long distance;
Second = days * 24*60*60*365;
Distance = speed * second;

System. out. println ("1000 days by light" + distance + "km ");
}
}


Question 2:

Package com. ccit. bijiao;
Public class Denghao {
/**
* This program mainly verifies whether the "=" and "equals ()" functions are equivalent for string comparison.
* The result is not equivalent.
* Note: String is not a basic type, but a reference type. Therefore, two strings are equal (that is, "="), which means the two references point to the same space.
* Although e and f are of the same length, type, and value, the calculation result is still false.
* E. equals (f) means that e and f are of the same length, type, and value, but not necessarily pointed to the same space.
* @ Param args
*/
Public static void main (String [] args ){
String a = "aaa ";
String B = "aaa ";
String c, d;
C = "aaa ";
D = "aaa ";
String e = new String ("aaa ");
String f = new String ("aaa ");

System. out. println ("a = B ----------" + (a = B ));
System. out. println ("a. equals (B) ----------" + a. equals (B ));
System. out. println ("c = d ----------" + (c = d ));
System. out. println ("c. equals (d) ----------" + c. equals (d ));
System. out. println ("e = f ----------" + (e = f ));
System. out. println ("e. equals (f) ----------" + e. equals (f ));
}
}


Question 3:

Public class Test {
/** I/O knowledge is required to test how to output the result to a text file.
* In the textbook, it means to run javac in the doscommand to compile the java file.
* Then run java to run the Test. class file,
* Format: java Test> F: \ result.txt
* You can output the running result of this program to F: \ result.txt.
* Note: This file can run successfully only when it is in the default package,
* The Yest file can only be compiled successfully but cannot be run,
* The exception of the class cannot be found.
* But why ?!!
*
* @ Param args
*/
Public static void main (String [] args ){
String a = "hello world! ";

System. out. printf ();
}
}