CODEWITHSUNDEEP
javaswitch-statementjava-7final
srk
srk

Reputation: 5136

A final constant not accepted in switch case

I would like to know the difference between the 2 declarations inside a method add() as in below.

final int c;
c = 20;

and

final int c = 20;

I think that both are final variables for which, I cannot reassign any new values. Here is the method that is treating the above declarations differently.

void add() {
        final int a = 30;
        final int b = 10;
        final int c;
        c = 20;

        switch (a) {
        case b + c:
            System.out.println("In case b+c");
            break;

        default:
            break;
        }

    }

The above method doesn't compile at all, complaining that

constant expression required case b+c

If the variable c is declared and initialized in one line, like final int c = 30;. It works.

Upvotes: 3

Views: 1142

Answers (5)

Khyathi36
Khyathi36

Reputation: 21

As you are initializing and declaring it separately you are getting that error, do it at the same time and it work

Upvotes: 0

Afzal Ahmad
Afzal Ahmad

Reputation: 626

final variables can't be re-evaluated again runtime, If you want to use switch case you can't do like (b+c) in case clause, instead of this do use this one,

void add() {
    final int a = 30;
    final int b = 10;
    final int c;
    c = 20;
    final int cmp=b+c;
    switch (a) {
    case cmp:
        System.out.println("In case b+c");
        break;
    default:
        break;
    }
}

Now this will work fine for you,

Upvotes: -1

Alok Anand
Alok Anand

Reputation: 3356

Java compiler replaces a final identifier with its value if it is declared & initialized in a single(same) statement, otherwise identifier is kept as it is and initialized by jvm at run time for blank final variable. You can see this by opening class file. switch statement permits final, but not a blank, variable to be used as a case value.

final int a = 20; // concrete final variable
switch(a){}
#=> gets changed by compiler as the following:--
switch(20){}

final int a; // blank final variable
a = 20;
switch(a){}
#=> compiler leave it for jvm to initialize 
switch(a){}

Upvotes: 2

assylias
assylias

Reputation: 328785

The JLS #4.12.4 defines constant variables as (emphasis mine):

A variable of primitive type or type String, that is final and initialized with a compile-time constant expression, is called a constant variable.

In your case, final int c = 20; is a constant variable but final int c; c = 20; is not.

Upvotes: 10

Niraj Patel
Niraj Patel

Reputation: 2208

You have to initialize local variable before use..

Upvotes: -2

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