enum and switch case

[Antoine Junod]

Hello group,

Is there a way to simulate the C enum type without Java 1.5 such that
i can use it in a switch case?

Thanks for your reply
-AJ

 

[Stefan Schulz]

Sure, usually i would implement it like this:

class Foo {
private int ord;

public final Foo ONE_CASE = new Foo(0);
public final Foo ANOTHER_CASE= new Foo(1);
// ...
public final Foo MOGRIFY = new Foo(717);

private Foo(int ord) { this.ord = ord; }

public int getOrdinal(){return ord; }
}

Switch it like this:

switch (anEnum.getOrdinal()) {
case ONE.getOrdinal();
//...
}

 

[=?ISO-8859-15?Q?Tobias_Schr=F6er?=]

Sure, usually i would implement it like this:

class Foo {
private int ord;

public final Foo ONE_CASE = new Foo(0);
public final Foo ANOTHER_CASE= new Foo(1);
// ...
public final Foo MOGRIFY = new Foo(717);

private Foo(int ord) { this.ord = ord; }

public int getOrdinal(){return ord; }
}

Switch it like this:

switch (anEnum.getOrdinal()) {
case ONE.getOrdinal();
//...
}

For me, this results in a compiler error, which says, that case
expressions must be constant expressions (Tested with JDK 1.5.0 and
Eclipse 3.1.2).

I don't know an exact way to do the simulation, only a rough workaround:

- declare a number of int based constants in your class
- provide a static getInstance member for an int value

<code>
public class EnumType {

public static final int VALUE_1 = 1;
public static final int VALUE_2 = 2;
// ... more

public static EnumType getEnumType(int val) {
switch (val) {
case EnumType.VALUE1 : return new EnumType(1);
// ... and so on
default : throw new IllegalArgumentException("Unpupportet type: "
+ val);
}
}

public EnumType(int type) {
// construcor code
}
}
</code>

Use it like follows:

<code>
// ...
int myType = ...
EnumType eType = EnumType.getEnumType(mytype);
</code>

The actual switching is done in the getEnumType member.
This is, as said, a rough workaround, not an exact emulation.

hth,
Tobi

 

[Jeffrey Schwab]

Is there a way to simulate the C enum type without Java 1.5 such that
i can use it in a switch case?

Use a bunch of integer constants, or google the Typesafe Enum Pattern.

 

[Oliver Wong]

For me, this results in a compiler error, which says, that case
expressions must be constant expressions (Tested with JDK 1.5.0 and
Eclipse 3.1.2).

I don't know an exact way to do the simulation, only a rough workaround:

- declare a number of int based constants in your class
- provide a static getInstance member for an int value

<code>
public class EnumType {

public static final int VALUE_1 = 1;
public static final int VALUE_2 = 2;
// ... more

public static EnumType getEnumType(int val) {
switch (val) {
case EnumType.VALUE1 : return new EnumType(1);
// ... and so on
default : throw new IllegalArgumentException("Unpupportet type: " +
val);
}
}

public EnumType(int type) {
// construcor code
}
}
</code>

Use it like follows:

<code>
// ...
int myType = ...
EnumType eType = EnumType.getEnumType(mytype);
</code>

The actual switching is done in the getEnumType member.
This is, as said, a rough workaround, not an exact emulation.

You should make the constructor of EnumType private so that clients cna
only get instances via the factory method, rather than directly invoking the
constructor.

- Oliver

 

[Oliver Wong]

You should make the constructor of EnumType private so that clients cna
only get instances via the factory method, rather than directly invoking
the constructor.

Sorry, one more thing. Don't create a new instance each time in the
getEnumType method. Instead, always return the same instance, given the same
ordinal index. This will allow your custom workaround to behave more like
1.5's enum class. If you *DO* decide to return a new instance every time,
you should probably override the equals() (and thus the hashcode()) method
so that two different instances with the same ordinal are considered equal.

- Oliver

 

[Antoine Junod]

For me, this results in a compiler error, which says, that case
expressions must be constant expressions (Tested with JDK 1.5.0 and
Eclipse 3.1.2).

Same here.

I don't know an exact way to do the simulation, only a rough
workaround:
[snipped code]

It works for me. Thanks to all for your answers.

-AJ

 

[=?ISO-8859-15?Q?Tobias_Schr=F6er?=]

Sorry, one more thing. Don't create a new instance each time in the
getEnumType method. Instead, always return the same instance, given the
same ordinal index. This will allow your custom workaround to behave
more like 1.5's enum class. If you *DO* decide to return a new instance
every time, you should probably override the equals() (and thus the
hashcode()) method so that two different instances with the same ordinal
are considered equal.

- Oliver

Yes, you're right. The solution above was rather a quick shot :\