Variables in different switch cases can't have the same name?

Question

I was refactoring some code to make it easier to read and I ran into something that I find weird and I was wondering if anyone could explain this to me.

Original code:

if(tokensLeft == 3) { String id = tokens.nextToken(); String value = tokens.nextToken(); String trailerId = tokens.nextToken(); rawListener.binaryInfo(id, Integer.parseInt(value), trailerId, this); } else if(tokensLeft == 2) { String id = tokens.nextToken(); String value = tokens.nextToken(); rawListener.binaryInfo(id, Integer.parseInt(value), this); } else { System.out.println("Method call binaryInfo could not be done because: \"Wrong number of parameters\""); }

After refactoring:

switch(tokensLeft) { case 3: String id = tokens.nextToken(); String value = tokens.nextToken(); String trailerId = tokens.nextToken(); rawListener.binaryInfo(id, Integer.parseInt(value), trailerId, this); break; case 2: String id = tokens.nextToken(); // Syntax error String value = tokens.nextToken(); // Syntax error rawListener.binaryInfo(id, Integer.parseInt(value), this); break; default: System.out.println("Method call binaryInfo could not be done because: \"Wrong number of parameters\""); break; }

At first glance this looks perfectly reasonable, but this gives me a syntax error.

Link all references for a local rename (does not change references in other files)

It turns out that for some reason in a switch statement, I am unable to use the String id and String value again in a different case.

This makes naming my variables rather awkward.

Now you could say: "Just declare your variables above your switch statement." But that would mean that I always create my variables, even if tokensLeft is neither 3 or 2 and I wouldn't need my variables. That just feels like using unnecessary memory.

Can anyone explain to me why the switch case does this and how I could solve my problem?

Use methods - it's good practice anyway and your scope will be sorted for you. If you really want to write spaghetti code then use explicit blocks ({}). — Boris the Spider
– Boris the Spider, Commented Jul 12, 2013 at 7:29

yushulx · Accepted Answer · 2013-07-12 07:31:39Z

add {}. try this:

switch(tokensLeft) { case 3: { String id = tokens.nextToken(); String value = tokens.nextToken(); String trailerId = tokens.nextToken(); rawListener.binaryInfo(id, Integer.parseInt(value), trailerId, this); } break; case 2: { String id = tokens.nextToken(); // Syntax error String value = tokens.nextToken(); // Syntax error rawListener.binaryInfo(id, Integer.parseInt(value), this); } break; default: System.out.println("Method call binaryInfo could not be done because: \"Wrong number of parameters\""); break; }

pretty clever. this should be the right answer. it achieves op's goal to use same variable name in multiple switch cases.

tashuhka · Accepted Answer · 2015-09-15 09:46:31Z

You are redefining variables , that is duplicate variable declaration . case doesn't make a block.

As per JLS 14:

A block is a sequence of statements, local class declarations and local variable declaration statements within braces.

You have two options here:

Define an explicit block under each case using { .. }, though it looks pretty odd I must say.

OR
In each case you can delegate the logic to a method call .

You can then enclose the logic in a method call for each case ?

Sanjaya Liyanage · Accepted Answer · 2013-07-12 07:54:59Z

Do it as below

 String id; String value ; switch(tokensLeft) { case 3: id = tokens.nextToken(); value = tokens.nextToken(); String trailerId = tokens.nextToken(); rawListener.binaryInfo(id, Integer.parseInt(value), trailerId, this); break; case 2: id = tokens.nextToken(); // Syntax error value = tokens.nextToken(); // Syntax error rawListener.binaryInfo(id, Integer.parseInt(value), this); break; default: System.out.println("Method call binaryInfo could not be done because: \"Wrong number of parameters\""); break; }

Here it will not cause memory issues as you say. When it executes the method it will keep id and value in the stack and no relevant object in heap for those references. Thus no issue with memory instead the small memory it take for two ref.

morpheus05 · Accepted Answer · 2013-07-12 07:32:13Z

You can use curly brackets {} after the case:

int aInt = 3; switch (aInt) { case 0: { String text = ""; break; } case 1: { String text = ""; break; } }

Boris the Spider · Accepted Answer · 2013-07-12 07:40:23Z

How about not declaring the variables at all?

switch (tokensLeft) { case 3: rawListener.binaryInfo( tokens.nextToken(), parseInt(tokens.nextToken()), tokens.nextToken(), this); break; case 2: rawListener.binaryInfo( tokens.nextToken(), parseInt(tokens.nextToken()), this); break; default: throw new IllegalArgumentException("Method call binaryInfo could not be done because: \"Wrong number of parameters\""); }

I added a static import for Integer.parseInt.

Better yet call your logic in well named methods from the switch and declare whatever variables you want:

public void parseTokens() { switch (tokensLeft) { case 3: parseThreeTokens(rawListener, tokens); break; case 2: parseTwoTokens(rawListener, tokens); break; default: throw new IllegalArgumentException("Method call binaryInfo could not be done because: \"Wrong number of parameters\""); } } public void parseThreeTokens(final RawListener rawListener, final Tokens tokens) { final String id = tokens.nextToken(); final String value = tokens.nextToken(); final String trailerId = tokens.nextToken(); rawListener.binaryInfo(id, parseInt(value), trailerId, this); } public void parseTwoTokens(final RawListener rawListener, final Tokens tokens) { final String id = tokens.nextToken(); final String value = tokens.nextToken(); rawListener.binaryInfo(id, parseInt(value), this); }

I guess it would work, but it seems to make things less easy to read when my original goal is to make it easier.
In that case create methods. Your code is not easy to read - best practice (and Uncle Bob) suggests that methods should be no more than 10 lines long. This switch block is already almost 20.
Do you have any suggestions on how I can differentiate between the 2 methods I'd create because the only difference is that one of them will use a 3rd parameter. Other than that, they're identical. And the methods would both accept only 1 paremeter tokens.

Steve P. · Accepted Answer · 2013-07-12 07:33:13Z

case in switch statements are not blocks; hence, by declaring the same variable in multiple switch cases, you're attempting to redefine the variable. This works in if statements because they form blocks.

Either declare them before the switch, or put blocks in your cases.

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