Java Enum question
sorry guys, this is IDEA unrelated but I hope this can be useful to
someone else.
Can someone explain me why is the test below not working?
@Test(groups = "util")
public class EnumTest {
private static final String RECORD = "RECORD";
private static final String PAY = "PAY";
public void testStatusEnum() {
assertEquals(RECORD, this.switchTest(Status.RECORD));
assertEquals(PAY, this.switchTest(Status.PAY));
}
private String switchTest(Status status) {
String ret = null;
switch (status) {
case RECORD:
ret = RECORD;
case PAY:
ret = PAY;
}
return ret;
}
enum Status {
PAY, RECORD;
}
}
I'm new to Enums as you may have guessed, and it seems the natural way
to use a switch.
rgds
-nodje
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You are not testing for the string constants!
Should be
switch (status) {
case Status.RECORD:
...
case Status.PAY:
...
}
You are not testing for the string constants!
Should be
switch (status) {
case Status.RECORD:
...
case Status.PAY:
...
}
didn't you forget to insert break; into switch cases?
Using multiple return points in a method often leads to clearer and less deeply indented code, plus in this case might have prevented a bug. I do not understand why some coding standards forbid them.
Also the @Test annotation is missing from your code, but I assume this is just test code, and your actual code includes it. And finally the semicolon in the enum class is redundant. I end up with:
import org.junit.Test;
import static org.junit.Assert.*;
public class EnumTest {
private static final String RECORD = "RECORD";
private static final String PAY = "PAY";
@Test
public void testStatusEnum() {
assertEquals(RECORD, this.switchTest(Status.RECORD));
assertEquals(PAY, this.switchTest(Status.PAY));
}
private String switchTest(Status status) {
switch (status) {
case RECORD:
return RECORD;
case PAY:
return PAY;
default:
throw new AssertionError();
}
}
enum Status {
PAY, RECORD
}
}
Bas
I forgot to mention, you should enable the "fallthrough in 'switch' statement" inspection, to help you detect such switch statement problems earlier.
Bas
nodje wrote:
Most of the time, you don't want to use a switch.
Sometimes its unavoidable, but usually its not.
Instead, make the behavior part of your enum:
enum Status {
PAY {
public String describe() {
return "PAY";
}
},
RECORD {
public String describe() {
return "RECORD";
}
}
;
public abstract String describe();
}
Remember, each enum value has its own class, so you can take full
advantage of polymorphism. Also, each enum value has its own instance,
so you can use constructors and member variables as well.
Most of the time, you should only use a switch statement if you are
converting from a legacy code base. Even then, you should see if it
improves the design to move the cases into polymorphic methods.
In other words, if you are writing a new switch statement, you are
likely to be doing it wrong.
Of course, there are exceptions to this rule. Its not dogma, but close
too it ;)
right Wi Wi, I forgot it
The test behaves as expected with break;.
Looking at the syntax, it didn't strike me that it was necessary.
As Daniel said it's probably a structure to avoid.
Hi Daniel, thanks for your reply.
I fell for a switch statement as I was try to replace String constants.
Enums does a good job at that, but then I faced the point where I had to find a way to do different things according to the Enum value used.
Switch appeared as an obvious way to do that.
Now that you mention it, and after having made that fall-through mistake, it seems pretty clear that it's better to avoid it.
But how do you implement condition on enum values? If condition?
if (status == Status.PAY)
//do sthig
else if (status == Status.RECORD)
//do else
else throw new AssertionException()
Switch syntax is pretty neat compare to that!
rgds
-nodje
thanks, very useful inspection!
I wish I had it enabled already, would have saved me precious time...
-nodje
nodje wrote:
Yes, switch is better than if/else/else, but polymorphism is even better ;)
In this specfic case "return status.name();" would have the same effect
as the switch.
In general to get enum-value-specific behavior you can also have an enum
implement an interface and supply an implementation for each value.
From Effective Java (highly recommended):
// Emulated extensible enum using an interface
public interface Operation {
double apply(double x, double y);
}
public enum BasicOperation implements Operation {
PLUS("+")
{
public double apply(double x, double y) {
return x + y;
}
},
MINUS("-")
{
public double apply(double x, double y) {
return x - y;
}
},
TIMES("*")
{
public double apply(double x, double y) {
return x * y;
}
},
DIVIDE("/")
{
public double apply(double x, double y) {
return x / y;
}
};
private final String symbol;
BasicOperation(String symbol) {
this.symbol = symbol;
}
@Override
public String toString() {
return symbol;
}
}
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