CODEWITHSUNDEEP
java
user5725767
user5725767

Reputation:

Why can you have a duplicate variable name in java for a variable outside of a method?

I am learning Java and I understand that you cannot name a variable declared within an inner scope the same name as a variable declared in an outer scope as shown below

public class Practice {
    public static void main(String[] args){
        int x = 10;
        if (x == 10){
            int x = 10;
        }           
    }   
}

However, I noticed that the following is not illegal

public class Practice {
    int x = 10;
    public static void main(String[] args){
        int x = 10;
        if (x == 10) {
        }           
    }   
}

Is this not a variable that is declared twice??

Upvotes: 1

Views: 2816

Answers (4)

hagrawal7777
hagrawal7777

Reputation: 14668

Is this not a variable that is declared twice??

No, it is not. Because they both are in different scope. x outside of main function has class level scope while x inside of main has method/function level scope.

It is legal for 2 variables in different scope to have same name.

Please DO read §6.3. Scope of a Declaration from JLS. Below are few of the statement from that section.

The scope of a declaration is the region of the program within which the entity declared by the declaration can be referred to using a simple name, provided it is visible (§6.4.1).

A declaration is said to be in scope at a particular point in a program if and only if the declaration's scope includes that point.

The scope of a local variable declaration in a block (§14.4) is the rest of the block in which the declaration appears, starting with its own initializer and including any further declarators to the right in the local variable declaration statement.

There are lot of concepts related to scope like shadowing, so do read §6.4. Shadowing and Obscuring.

JLS is the best place to learn what is allowed and what is not allowed in Java. Feel free to read sections there.

Upvotes: 5

Stephen C
Stephen C

Reputation: 719310

Why can you have a duplicate variable name in java for a variable outside of a method?

It is not illegal because it is allowed by the Java Language Specification. (Yes ... circular explanation)

As with all such questions, we cannot tell you for sure why it is allowed, because the Java language was designed behind closed doors. However, a couple of possible reasons are:

  • Java was originally designed to be "palatable" to C and C++ programmers, and this is a case where doing it differently (banning shadowing) would have caused objections.

  • A lot of people actually like it that they don't have to think of different names for variables; e.g.

     public class Foo {
     private Bar bar;

     public Foo() {
        Bar bar = new Bar();
        this.bar = bar;
     }
 }

Admittedly, there is a downside. Sometimes, people mistakenly declare local variables that shadow instance (or static) variables. Particularly newbies :-)

Is this not a variable that is declared twice??

No. It is actually two >>distinct<< variables. Both exist. Within the method, the local declaration of x shadows the instance declaration of x. But you can still refer to the latter as this.x.

Upvotes: 0

saurav
saurav

Reputation: 5936

The first code snippets shows the variables are in method's scope. So two variable are belonging to same method which is not allowed.

But in the second example one variable is on instance level and the other one in method's scope, so there is no duplicate. In the second example if you want to access the instance variable, you have to do it using this.x.

Check more on scope and lifetime of variables here

http://www.javawithus.com/tutorial/scope-and-lifetime-of-variables

Upvotes: 1

Atri
Atri

Reputation: 5831

In the second case when x is declared outside the method, it is an instance variable of the class Practice. Within the method, scope of x(declared inside main) is limited to the main method.

In the first case when you declare x within the method, you cannot declare it inside if because it is already declared in the scope of the method outside if.

Upvotes: 1

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