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.
Related
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
}
How can i check if case (for example 3 ) was choosen to make continuation of story?
I wrote this in java and I would go full detail with story but i dont know how could i .I thought I could nest cases inside each other but if I can choose options(scanner.nextInt();) also add in them.I thought if statement would work better but i dont know how can i check it .I must add that didnt found any answers to this problem before posting.Thanks for reviewing and have a good day
int choice_1 = scanner.nextInt();
switch(choice_1)
{
case 1:
System.out.println("Guard: Its a honor to meet u sir.Its a pleasure to let You through \nBut do u have coin pass?");
if(pass==1){
ending();
}
else{"You should get coin pass first. I am sure that wont be a problem for Prince *laughs*"}
break;
case 2:
System.out.println("*Guard immediately avoided attack and attack u harder that u excepted");
playerHp = playerHp -30;
playerArmorHp = playerArmorHp - 50;
break;
case 3:
System.out.println("Guard : Goodbye Sir");
plan();
break;
default:
System.out.println("Guard is impatient of your not telling anything.Choose option before making him mad.\n"+line);
break;
}
if(switch ( case:3)){
System.out.println("Do u have coin pass?");
int pass = scanner.nextInt();
}
You could change your if clause to if(choice_1 == 3) to get what you want.
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 have a variable, x.
I want to call a method m() only if x is one of two possible values.
When calling m(), I want to pass an argument to it, whose value depends on the value of x.
Is there a way to do this in Java without checking the value of x more than once, and calling/writing m() in one place only (i.e. not in multiple branches of an if statement)?
One solution I'm entertaining:
switch (x) {
case 1:
y = "foo";
break;
case 2:
y = "bar";
break;
default:
y = null;
break;
}
if (y != null) m(y);
But I can't help but feel this is technically checking x twice, just obscuring this fact by adding a "proxy" for the second check.
(To clarify why the constraints are what they are: when reading code, I have a hard time understanding logic that branches a lot when there is a high degree of duplication between branches - it becomes a game of "spot the difference" rather than simply being able to see what is happening. I prefer to aggressively refactor such duplication away, which is a habit that serves me well in Ruby, JS, and other languages; I'm hoping I can learn to do the same for Java and make code easier for me and others to understand at a glance.)
I'm not sure of what you want to do, but you can maybe use a Map to get the 'y' parameter from 'x'
Map<Integer, String> map = new HashMap<>();
map.put(1, "foo");
map.put(2, "bar");
if (map.containsKey(x)) {
m(map.get(x));
}
Use "goto" or equivalent:
void do_m_if_appropriate() {
// x and y are assumed to be eg. member variables
switch (x) {
case 1:
y = "foo";
break;
case 2:
y = "bar";
break;
default:
return; // this is the "goto equivalent" part
}
m(y);
}
Above is pretty elegant. If necessary, it's also trivial to change it to return true or false depending on if it called m(), or just y or null.
You can also do tricks with loop constructs, though some might say this is abuse of the loop construct, and you should comment it accordingly:
do { // note: not a real loop, used to skip call to m()
switch (x) {
case 1:
y = "foo";
break;
case 2:
y = "bar";
break;
default:
continue; // "goto equivalent" part
}
m(y);
} while(false);
Here's a solution with Optionals (my Java syntax might be slightly incorrect). Note that to you, the code looks like so, but implementation wise, it's similar to the example you posted (i.e. checks whether y is an exceptional value).
switch (x) {
case 1:
y = Optional<String>.of("foo");
break;
case 2:
y = Optional<String>.of("bar");
break;
default:
y = Optional<String>.empty();
break;
}
y.map((m's class)::m);
result = y.orElse( <value result should take if x was invalid> );
Actually it may be better to modify m() to return an Optional and just return empty if y is not valid, but I assume you want to do this check caller-side.
Why not
switch (x) {
case 1:
y = "foo";
m(y);
break;
case 2:
y = "bar";
m(y);
break;
}
so here is my case to make a long code short.
Let's say, we have a JOptionPane with 3 buttons.
boolean loopGameInterface = true;
while(loopGameInterface){
int chooseGame = JOptionePane........
switch(chooseGame) {
case 0:
case 1:
case 2:
System.exit(0);
}
}
So the problem is, when I click (example) second button, it goes to case 1. That's fine. But when the code inside case 1 is executed, it goes directly to case 2 and exit my program, instead of just looping the gameInterface?
You need to add break; at the end of each case. This is true for all switch statements by the way, not just when you are using a JOptionPane
switch(chooseGame) {
case 0: /* Your code */
break;
case 1: /* Your code */
break;
case 2:
System.exit(0);
default : "Give some default case too"
}
Its because you haven't added the break statement. If you are not adding break all the cases below the case which matches will be executed. For eg:
switch(ch) {
case 1:
/* some code without break */
case 2:
/* some code without break */
case 3:
System.exit(0);
}
In the above example if ch=1 then all case 2 will also be executed and then case 3.
If ch=2 then only case 2 and case 3 will be executed since case 3 is below case 2. So you need to add break after each case.