Java multiple variable Initialisation - How Does it work?

Question

package com.mypackage;

import java.util.List;
import java.util.Map;

public class InitializationDemo {    

   public static void main(String[] args) {

       List<String> a, b = null;
       List<String> c = null, d = null;
       Map<String, String> e, f = null;
       Map<String, String> g = null, h = null;

       if(c == null){ //line $38: Works no compilation error
           // Do Something here
       }

       if(a == null) { //line #40: compilation error
           // Do Something here            
       } 

       if(e == null) { //line #44: compilation error
           // Do Something here            
       }

       if(g == null) { //line #46 Works no compilation error
           // Do Something here            
       } 
   }
}

Get the "The local variable a may not have been initialised." compilation error at line #40 and line #44:

I am trying to understand under the wood how does it work so that line #38 and #46 does not signal a compilation error however #40 and #44 signals compilation error.

Why on earth would you write code this way? Awful. Let's hope this is a badly conceived example, not something you'd ever write. — duffymo
– duffymo, Commented Sep 28, 2015 at 21:37
Any initializer you provide in conjunction with a local variable declaration applies only to the variable directly left of the = sign. It does not apply to any other variables declared in the same statement, either before or after. — John Bollinger
– John Bollinger, Commented Sep 28, 2015 at 21:38

akhil_mittal · Accepted Answer · 2015-09-28 21:37:48Z

4

You are not initializing a and that is why it is complaining that it might not have been initialized. You should do:

List<String> a =null, b = null;

rather than:

List<String> a, b = null;

as the later one is equivalent to:

List<String> a;
List<String> b = null;

answered Sep 28, 2015 at 21:37

akhil_mittal

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Comments

ControlAltDel · Accepted Answer · 2015-09-28 21:37:21Z

4

List<String> a, b = null;

this is equivalent to

List<String> a;
List<String> b = null;

Does that answer your question?

answered Sep 28, 2015 at 21:37

ControlAltDel

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2 Comments

Shivakumar Over a year ago

Okay. By declaring in this way List<String> a, b = null; why would someone expect variable "b" to be assigned null and variable "a" just declared (and letting it to signal compilation error)? Doesn't one really wants both variable "a" and "b" assigned null? On the other hand I think if it is not the right way to do multiple variable initialisation. I would expect the language not support such initialisation in first place (its my thought by the way!)

duffymo Over a year ago

When I write code it's one variable declaration per line. I initialize references in the constructor, where they should be initialized. You way is not the right thing to do.

Jean Jung · Accepted Answer · 2015-09-28 21:37:54Z

1

You should do a = null, b = null;. The initialization of the variables is like on the C language, each variable with your own value.

answered Sep 28, 2015 at 21:37

Jean Jung

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Comments

Parker_Halo · Accepted Answer · 2015-09-28 21:44:26Z

1

Your code would become more readable and easier to handle with if you'd choose one notation. So either declare your variables as:

List<String> a = null, b = null, c = null, d = null;

or as

List<String> a = null;
List<String> b = null;
List<String> c = null;
List<String> d = null;

Another possiblitly to do this would be:

List<String> a, b, c, d;
a = b = c = d = null;

answered Sep 28, 2015 at 21:44

Parker_Halo

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Comments

subash · Accepted Answer · 2015-09-28 21:42:31Z

0

because you should initialize the local variable, local variables are not in the object level. it only having the local scope. so its not getting initialize on object creation. but in the code execution, jvm expect value for the each fields. so compiler tells you should initialize the local variable.

answered Sep 28, 2015 at 21:42

subash

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