Good Evening,
I created this method for a class. I used a switch/case to execute depending on the value of expression. I included an if-else method for each case. I do get an error on case 1-> switch rules are a preview feature and are disabled by default. I attempted to add a : after case 1 and case 2but my results reached high numbers for the sets. I changed the : to -> and it worked appropriately. Now I am wondering if this was a proper way to set the case statements or should it be written differently.
private void playGame()
{
double winCheck = Math.random();
switch (matchServer) {
case 1 ->{
if (winCheck <= player1WinProb)
player1GamesWon++;
else
player2GamesWon++;
matchServer = 2;
}
case 2 ->{
if (winCheck <= player2WinProb)
player2GamesWon++;
else
player1GamesWon++;
matchServer = 1;
A correct switch statement must use ':'
Also, 'break' is missing. This to avoid executing next cases.
You can add 'default' that means that case 1 and case 2 were not presented.
switch (matchServer) {
case 1:
if (winCheck <= player1WinProb)
player1GamesWon++;
else
player2GamesWon++;
matchServer = 2;
break;
case 2:
if (winCheck <= player2WinProb)
player2GamesWon++;
else
player1GamesWon++;
matchServer = 1;
break;
default:
//If it was not 1 or 2
//Printing the values can help
}
If we take this code as an example :
switch (PeriodEnum.getEnum(month).getValue()) {
case 0: // Calendar.JANUARY
case 2: // Calendar.MARCH
case 4: // Calendar.MAY
case 6: // Calendar.JULY
case 7: // Calendar.AUGUST
case 9: // Calendar.OCTOBER
case 11: // Calendar.DECEMBER
nbDays = 31;
break;
case 3: // Calendar.APRIL
case 5: // Calendar.JUNE
case 8: // Calendar.SEPTEMBER
case 10: // Calendar.NOVEMBER
nbDays = 30;
break;
What is the difference between the previous code and the code below?
switch (PeriodEnum.getEnum(month).getValue()) {
case 0: // Calendar.JANUARY
nbDays = 30;
break;
case 2: // Calendar.MARCH
nbDays = 30;
break;
case 4: // Calendar.MAY
nbDays = 30;
break;
....
}
As a beginner in java , I would love to understand the difference . The main thing I don't understand is how the IDE will detect the case based on the month and associate it?
Thank you
The simple answer is that the compiler is stupid :)
In the first code snippet, the generated JVM code will happily stuff all cases you bundled together into the same branch, and in the second snippet it would, equally happily, stuff each case into its own branch. And even if a set of branches did exactly the same, the compiler doesn't care and won't do the analysis for you.
Now, there is something else to consider in Java, which is that enums are objects like any other... Which means they can have instance variables. Therefore you can do this, and avoid that switch statement altogether:
public enum Calendar
{
JANUARY(30),
FEBRUARY(28),
// etc etc
;
private final int daysInMonth;
Calendar(final int daysInMonth)
{
this.daysInMonth = daysInMonth;
}
public int getDaysInMonth()
{
return daysInMonth;
}
}
In the first part execution will come out of switch statement after break statement, while in second part the program will continue till the last case.
So for the first part whether the value will be 0, 2, 4.... or whatever the assignment nbdays = 31 will be executed and nbdays = 30 will be executed for 1, 3, 5.... Basically it is a way to minimize writing codes for multiple similar statements.
There is no grouping for switch statements.
Consider this arbitrary example:
switch (PeriodEnum.getEnum(month).getValue()) {
case 0: // Calendar.JANUARY
jan();
case 2: // Calendar.MARCH
mar();
case 4: // Calendar.MAY
may();
case 6: // Calendar.JULY
jul();
case 7: // Calendar.AUGUST
aug();
case 9: // Calendar.OCTOBER
oct();
case 11: // Calendar.DECEMBER
dec();
break;
If the switch value is 0, then jan(), mar(), may(), jul(), aug(), oct(), and dec() ALL execute.
If the switch value is 9, then only oct(), and dec() execute.
See what I mean about it not being grouping?
Just like if, while, and for, switch statements were a way of avoiding goto statements.
We idiomatically use them as a map, or a grouping or whatever, but that's not what they literally are. They are literally: Start where the case matches, then exit at break/continue/return/throw. The process of continuing from one case to the next is called "falling through"
To save some code, wrap your switch in a method and instead of assigning a variable and breaking, just return the value and skip the intermediate variable.
I've created a terminal emulator using a library, connect to a serial device with another library and this returns data to my terminal. I can see in the log that every time I write a character in the terminal and sent it over serial, the following three characters are returned along with the correct one. ☐[J. When writing to the terminal these characters do now show up. They are handled in this code in some way but I'm not sure which part, perhaps doEscRightSquareBracket:
private void process(byte b, boolean doUTF8) {
// Let the UTF-8 decoder try to handle it if we're in UTF-8 mode
if (doUTF8 && mUTF8Mode && handleUTF8Sequence(b)) {
return;
}
// Handle C1 control characters
if ((b & 0x80) == 0x80 && (b & 0x7f) <= 0x1f) {
/* ESC ((code & 0x7f) + 0x40) is the two-byte escape sequence
corresponding to a particular C1 code */
process((byte) 27, false);
process((byte) ((b & 0x7f) + 0x40), false);
return;
}
switch (b) {
case 0: // NUL
// Do nothing
break;
case 7: // BEL
/* If in an OSC sequence, BEL may terminate a string; otherwise do
* nothing */
if (mEscapeState == ESC_RIGHT_SQUARE_BRACKET) {
doEscRightSquareBracket(b);
}
break;
case 8: // BS
setCursorCol(Math.max(0, mCursorCol - 1));
break;
case 9: // HT
// Move to next tab stop, but not past edge of screen
setCursorCol(nextTabStop(mCursorCol));
break;
case 13:
setCursorCol(0);
break;
case 10: // CR
case 11: // VT
case 12: // LF
doLinefeed();
break;
case 14: // SO:
setAltCharSet(true);
break;
case 15: // SI:
setAltCharSet(false);
break;
case 24: // CAN
case 26: // SUB
if (mEscapeState != ESC_NONE) {
mEscapeState = ESC_NONE;
emit((byte) 127);
}
break;
case 27: // ESC
// Starts an escape sequence unless we're parsing a string
if (mEscapeState != ESC_RIGHT_SQUARE_BRACKET) {
startEscapeSequence(ESC);
} else {
doEscRightSquareBracket(b);
}
break;
default:
mContinueSequence = false;
switch (mEscapeState) {
case ESC_NONE:
if (b >= 32) {
emit(b);
}
break;
case ESC:
doEsc(b);
break;
case ESC_POUND:
doEscPound(b);
break;
case ESC_SELECT_LEFT_PAREN:
doEscSelectLeftParen(b);
break;
case ESC_SELECT_RIGHT_PAREN:
doEscSelectRightParen(b);
break;
case ESC_LEFT_SQUARE_BRACKET:
doEscLeftSquareBracket(b); // CSI
break;
case ESC_LEFT_SQUARE_BRACKET_QUESTION_MARK:
doEscLSBQuest(b); // CSI ?
break;
case ESC_PERCENT:
doEscPercent(b);
break;
case ESC_RIGHT_SQUARE_BRACKET:
doEscRightSquareBracket(b);
break;
case ESC_RIGHT_SQUARE_BRACKET_ESC:
doEscRightSquareBracketEsc(b);
break;
default:
unknownSequence(b);
break;
}
if (!mContinueSequence) {
mEscapeState = ESC_NONE;
}
break;
}
}
This is not a problem with the terminal as it is filtering it. But now I want to write the returned data into an editText and the odd characters are being written. What are they, how do I stop them? They must be some normal case of something that can happen as the terminal filters them out? You can see it here when I am typing exit and it should be mirrored on the right:
Esc-[-J is an ANSI escape code, it instructs the terminal to clear the screen from the cursor down. The problem with filtering them out is that among these commands many change how the visible text is constructed: they can move the cursor, erase parts, etc. So, just filtering them out may not give the desired result. But in this case it just seems like a precaution to make sure that the area where you normally type is cleared.
What I consider the best solution, but quite probably overkill in your case, is to integrate a VT100 interpreter in your program (for example this one) that converts a stream of text and command codes into a memory view of the screen, and save that memory. It will be a perfect representation of what the sending program would like to see on the screen at any time.
So I want to do a certain action 60 % of the time and another action 40% of the time. And sometimes have it doing neither. The best way I can think to do this is through switches and making a bunch of cases. An example is below if this doesn't make any sense to ya'll.
My question is, is there a better way?
Is there a way to just do Case 0-5 does action 1 all in one statement?
Random rand = new Random(50);
switch(rand.nextInt())
{
case 1:
{
do action 1
}
break;
case 2:
{
do action 1
}
break;
case 3:
{
do action 1
}
break;
case 4:
{
do action 1
}
break;
case 5:
{
do action 1
}
break;
case 6:
{
do action 1
}
break;
case 7:
{
do action 2
}
break;
case 8:
{
do action 2
}
break;
case 9:
{
do action 2
}
break;
case 10:
{
do action 2
}
break;
}
Something like this would be much more readable IMO:
if( Math.random() >= probabilityOfDoingNothing ){
if( Math.random() < 0.6 ){
action1;
}else{
action2;
}
}
Re. your question about cases, the following is equivalent to your code:
Random rand = new Random(50);
switch(rand.nextInt())
{
case 1:
case 2:
case 3:
case 4:
case 5:
case 6:
{
do action 1
}
break;
case 7:
case 8:
case 9:
case 10:
{
do action 2
}
break;
}
Random rand = new Random(50);
int n = rand.nextInt(11);
if(n<=6)
do action 1
else
do action 2
You need to use nextInt(n) to generate a number between 0 (inclusive) and n (exclusive). In this case we use 11 which gives us a number between 0 and 10. Anything below 6 (60% chance) we do action 1 otherwise do action 2.
See this for more details on the Random class.
Using a switch statement is only useful if you have a lot of actions you want to perform, where the action performed depends on something. For example a different action is performed based on the current month. Its quicker than writing if-else statements.
If you want same action to happen for multiple cases, then don't put break. For example
case 1:
case 2:
do action 1; break;
In this case, action 1 will happen for both case 1 and 2.
All you can do is do not apply break in between like
case 1:
case 2:
case 3:
case 4:
case 5:
do action 1;
break
case 6:
case 7:
do action 2;
break
default : break;
or
use if-else if you have range of value ..
There are many approaches to "random" behavior. Some are easier to implement than others but these sacrifice the entropy bucket. Random() is an expensive operation. Switch is useful for complicated signalling, but for a binary decision if is what you want:
int signal = (int)(System.currentTimeMillis() % 5);
if(signal==0 || signal == 1){
doActionTwo();//one third of the time
}else{
doActionOne();//two thirds of the time
}
Why doesn't the compiler automatically put break statements after each code block in the switch? Is it for historical reasons? When would you want multiple code blocks to execute?
Sometimes it is helpful to have multiple cases associated with the same code block, such as
case 'A':
case 'B':
case 'C':
doSomething();
break;
case 'D':
case 'E':
doSomethingElse();
break;
etc. Just an example.
In my experience, usually it is bad style to "fall through" and have multiple blocks of code execute for one case, but there may be uses for it in some situations.
Historically, it's because the case was essentially defining a label, also known as the target point of a goto call. The switch statement and its associated cases really just represent a multiway branch with multiple potential entry points into a stream of code.
All that said, it has been noted a nearly infinite number of times that break is almost always the default behavior that you'd rather have at the end of every case.
Java comes from C and that is the syntax from C.
There are times where you want multiple case statements to just have one execution path.
Below is a sample that will tell you how many days in a month.
class SwitchDemo2 {
public static void main(String[] args) {
int month = 2;
int year = 2000;
int numDays = 0;
switch (month) {
case 1:
case 3:
case 5:
case 7:
case 8:
case 10:
case 12:
numDays = 31;
break;
case 4:
case 6:
case 9:
case 11:
numDays = 30;
break;
case 2:
if ( ((year % 4 == 0) && !(year % 100 == 0))
|| (year % 400 == 0) )
numDays = 29;
else
numDays = 28;
break;
default:
System.out.println("Invalid month.");
break;
}
System.out.println("Number of Days = " + numDays);
}
}
I think it is a mistake. As a language construct it is just as easy to have break as the default and instead have a fallthrough keyword. Most of the code I have written and read has a break after every case.
You can do all sorts of interesting things with case fall-through.
For example, lets say you want to do a particular action for all cases, but in a certain case you want to do that action plus something else. Using a switch statement with fall-through would make it quite easy.
switch (someValue)
{
case extendedActionValue:
// do extended action here, falls through to normal action
case normalActionValue:
case otherNormalActionValue:
// do normal action here
break;
}
Of course, it is easy to forget the break statement at the end of a case and cause unexpected behavior. Good compilers will warn you when you omit the break statement.
Why doesn't the compiler automatically put break statements after each code block in the switch?
Leaving aside the good desire to be able to use the identical block for several cases (which could be special-cased)...
Is it for historical reasons? When would you want multiple code blocks to execute?
It's mainly for compatibility with C, and is arguably an ancient hack from the days of old when goto keywords roamed the earth. It does enable some amazing things, of course, such as Duff's Device, but whether that's a point in its favor or against is… argumentative at best.
The break after switch cases is used to avoid the fallthrough in the switch statements. Though interestingly this now can be achieved through the newly formed switch labels as implemented via JEP-325.
With these changes, the break with every switch case can be avoided as demonstrated further :-
public class SwitchExpressionsNoFallThrough {
public static void main(String[] args) {
Scanner scanner = new Scanner(System.in);
int value = scanner.nextInt();
/*
* Before JEP-325
*/
switch (value) {
case 1:
System.out.println("one");
case 2:
System.out.println("two");
default:
System.out.println("many");
}
/*
* After JEP-325
*/
switch (value) {
case 1 ->System.out.println("one");
case 2 ->System.out.println("two");
default ->System.out.println("many");
}
}
}
On executing the above code with JDK-12, the comparative output could be seen as
//input
1
// output from the implementation before JEP-325
one
two
many
// output from the implementation after JEP-325
one
and
//input
2
// output from the implementation before JEP-325
two
many
// output from the implementation after JEP-325
two
and of course the thing unchanged
// input
3
many // default case match
many // branches to 'default' as well
So you do not have to repeat code if you need several cases to do the same thing:
case THIS:
case THAT:
{
code;
break;
}
Or you can do things like :
case THIS:
{
do this;
}
case THAT:
{
do that;
}
In a cascade fashion.
Really bug/confusion prone, if you ask me.
As far as the historical record goes, Tony Hoare invented the case statement in the 1960s, during the "structured programming" revolution. Tony's case statement supported multiple labels per case and automatic exit with no stinking break statements. The requirement for an explicit break was something that came out of the BCPL/B/C line. Dennis Ritchie writes (in ACM HOPL-II):
For example, the endcase that escapes from a BCPL switchon statement was not present in the language
when we learned it in the 1960s, and so the overloading of the break keyword to escape
from the B and C switch statement owes to divergent evolution rather than conscious change.
I haven't been able to find any historical writings about BCPL, but Ritchie's comment suggests that the break was more or less a historical accident. BCPL later fixed the problem, but perhaps Ritchie and Thompson were too busy inventing Unix to be bothered with such a detail :-)
Java is derived from C, whose heritage includes a technique known as Duff's Device .
It's an optimization that relies on the fact that control falls through from one case to the next, in the absence of a break; statement. By the time C was standardized, there was plenty of code like that "in the wild", and it would have been counterproductive to change the language to break such constructions.
As people said before, it is to allow fall-through and it is not a mistake, it is a feature.
If too many break statements annoy you, you can easily get rid of them by using return statements instead. This is actually a good practice, because your methods should be as small as possible (for the sake of readability and maintainability), so a switch statement is already big enough for a method, hence, a good method should not contain anything else, this is an example:
public class SwitchTester{
private static final Log log = LogFactory.getLog(SwitchTester.class);
public static void main(String[] args){
log.info(monthsOfTheSeason(Season.WINTER));
log.info(monthsOfTheSeason(Season.SPRING));
log.info(monthsOfTheSeason(Season.SUMMER));
log.info(monthsOfTheSeason(Season.AUTUMN));
}
enum Season{WINTER, SPRING, SUMMER, AUTUMN};
static String monthsOfTheSeason(Season season){
switch(season){
case WINTER:
return "Dec, Jan, Feb";
case SPRING:
return "Mar, Apr, May";
case SUMMER:
return "Jun, Jul, Aug";
case AUTUMN:
return "Sep, Oct, Nov";
default:
//actually a NullPointerException will be thrown before reaching this
throw new IllegalArgumentException("Season must not be null");
}
}
}
The execution prints:
12:37:25.760 [main] INFO lang.SwitchTester - Dec, Jan, Feb
12:37:25.762 [main] INFO lang.SwitchTester - Mar, Apr, May
12:37:25.762 [main] INFO lang.SwitchTester - Jun, Jul, Aug
12:37:25.762 [main] INFO lang.SwitchTester - Sep, Oct, Nov
as expected.
It is an old question but actually I ran into using the case without break statement today. Not using break is actually very useful when you need to combine different functions in sequence.
e.g. using http response codes to authenticate user with time token
server response code 401 - token is outdated -> regenerate token and log user in.
server response code 200 - token is OK -> log user in.
in case statements:
case 404:
case 500:
{
Log.v("Server responses","Unable to respond due to server error");
break;
}
case 401:
{
//regenerate token
}
case 200:
{
// log in user
break;
}
Using this you do not need to call log in user function for 401 response because when the token is regenerated, the runtime jumps into the case 200.
Not having an automatic break added by the compiler makes it possible to use a switch/case to test for conditions like 1 <= a <= 3 by removing the break statement from 1 and 2.
switch(a) {
case 1: //I'm between 1 and 3
case 2: //I'm between 1 and 3
case 3: //I'm between 1 and 3
break;
}
because there are situations where you want to flow through the first block for example to avoid writing the same code in multiple blocks but still be able to divide them for mroe control. There are also a ton of other reasons.
You can makes easily to separate other type of number, month, count.
This is better then if in this case;
public static void spanishNumbers(String span){
span = span.toLowerCase().replace(" ", "");
switch (span){
case "1":
case "jan": System.out.println("uno"); break;
case "2":
case "feb": System.out.println("dos"); break;
case "3":
case "mar": System.out.println("tres"); break;
case "4":
case "apr": System.out.println("cuatro"); break;
case "5":
case "may": System.out.println("cinco"); break;
case "6":
case "jun": System.out.println("seis"); break;
case "7":
case "jul": System.out.println("seite"); break;
case "8":
case "aug": System.out.println("ocho"); break;
case "9":
case "sep": System.out.println("nueve"); break;
case "10":
case "oct": System.out.println("diez"); break;
}
}
I am now working on project where I am in need of break in my switch statement otherwise the code won't work. Bear with me and I will give you a good example of why you need break in your switch statement.
Imagine you have three states, one that waits for the user to enter a number, the second to calculate it and the third to print the sum.
In that case you have:
State1 - Wait for user to enter a number
State2 - Print the sum
state3 - Calculate the sum
Looking at the states, you would want the order of exaction to start on state1, then state3 and finally state2. Otherwise we will only print users input without calculating the sum. Just to clarify it again, we wait for the user to enter a value, then calculate the sum and prints the sum.
Here is an example code:
while(1){
switch(state){
case state1:
// Wait for user input code
state = state3; // Jump to state3
break;
case state2:
//Print the sum code
state = state3; // Jump to state3;
case state3:
// Calculate the sum code
state = wait; // Jump to state1
break;
}
}
If we don't use break, it will execute in this order, state1, state2 and state3. But using break, we avoid this scenario, and can order in the right procedure which is to begin with state1, then state3 and last but not least state2.
Exactly, because with some clever placement you can execute blocks in cascade.