CODEWITHSUNDEEP
javaswitch-statementjava-7
Rakesh KR
Rakesh KR

Reputation: 6527

Switch statement with string

I tried the following code:

String str = "Str1";

switch(str) {
    case Constants.First_String  : System.out.println("First String");
                                   break;
    case Constants.Second_String : System.out.println("Second String");
                                   break;
    default : System.out.println("Default String");
}

And my Constants class is,

public class Constants {
    public static String First_String  = "Str1";
    public static String Second_String = "Str2";
    public static String Third_String  = "Str3";
}

And I got a compilation error as,

Exception in thread "main" java.lang.Error: Unresolved compilation problems: case expressions must be constant expressions

But when I tried with following code,

switch(str){
    case "Str1" : System.out.println("First String");
                  break;
    case "Str2" : System.out.println("Second String");
                  break;
    default : System.out.println("Default String");
}

No Compilation errors, and prints the output as,

First String

My question is, why in the first case compliation error occurs. And how can I resolve it.

Upvotes: 4

Views: 1092

Answers (5)

Anders R. Bystrup
Anders R. Bystrup

Reputation: 16050

A constant expression is not the same as a static member. Even a static member can be changed by code... It needs to be final to be considered a constant expression:

From the JLS:

A compile-time constant expression is an expression...

  • Simple names (§6.5.6.1) that refer to constant variables (§4.12.4).

So

case "Something":

is OK. As is

public static final String ME = "Other";
...
case ME:

Finally, enum's are also OK to use in switch-case statements.

Cheers,

Upvotes: 4

Jordi Castilla
Jordi Castilla

Reputation: 26961

You must declare Constants as final 

public static final String First_String  = "Str1";
public static final String Second_String = "Str2";
public static final String Third_String  = "Str3";

Or transform switch in if-else statement:

In Eclipse
You can quickly convert a switch statement to an if-else statement using the following:

Move your cursor to the switch keyword and press Ctrl + 1 then select

Convert 'switch' to 'if-else'.

Upvotes: 1

Tom Jonckheere
Tom Jonckheere

Reputation: 1648

Change

public static String First_String  = "Str1";

to 

public static final String First_String  = "Str1";

Now it is a constant.

Upvotes: 2

khelwood
khelwood

Reputation: 59095

The strings in your Constants class must be declared final to be regarded as constants.

If they are declared final, the compiler knows they can never change, so can treat them as constant expressions. If they are not declared final, the possibility exists they might be reassigned to different strings during the execution of the program, so they are not constants.

Upvotes: 1

Konstantin Yovkov
Konstantin Yovkov

Reputation: 62864

"Str1" is a compile-time constant and that's why case "Str" is fine.

However, the from the definition of First_String we can see that it is not a constant, because it can change it's value at any time.

You can try setting it as final:

public static final String First_String  = "Str1";

Upvotes: 7

Related Questions