Why can I declare variables of the same name within a for loop, but not in the main program?

Question

I am playing around learning Lambda within Java and needed to create a list of variables. I created a for loop which creates a new String variable on each iteration, however each one has the same variable name which is not aloud outside of a for loop.

I see in the following post, Why is it allowed to declare a variable in a for loop? , they detail that its because of the scope of the created variable within the loop. However, I added the variable to an ArrayList outside of the scope of the for loop and then iterated through it to see that the duplicated name variables still exists with no errors.

I duplicated the code to have each variable have the same String value as well, to check if it had something to do with the values being different, but it still allowed the variables to be stored into the Array...

Code is below.

import java.util.ArrayList;

public class Lambda 
{
    public static void main(String[] args)
    {
        ArrayList<String> names = new ArrayList<String>();
        
        for(int i = 0; i<10; i++)
        {
            String name = "name: ";
            names.add(name);
        }
        
        names.forEach((name) -> {System.out.println(name);});
        
        
        System.out.println("\n***********************************\n");
        
        for(String temp: names)
        {
            System.out.println(temp);
        }
        
    }
}

Perhaps you could add actual vs. expected output to your question. — Chris, Apr 28 '23 at 18:01

score 3 · Accepted Answer · answered Apr 28 '23 at 22:02

The code fragment

ArrayList<String> names = new ArrayList<String>();
    
for(int i = 0; i<10; i++)
{
    String name = "name: ";
    names.add(name);
}

is equivalent to

ArrayList<String> names = new ArrayList<String>();
// here the `ArrayList` referenced by `names` is empty
{
    String name = "name: ";
    names.add(name);
}
// at this point there is no variable called name
// the `ArrayList` referenced by `names` contains one reference to the string "name: "
{
    String name = "name: ";
    names.add(name);
}
// at this point there is no variable called name
// the `ArrayList` referenced by `names` contains two references to the string "name: "
// some repetitions left out for brevity

I think your misunderstanding is this:

I added the variable to an ArrayList outside of the scope of the for loop

You cannot add variables to an ArrayList - you can only add the objects referenced by variables to an ArrayList

The ArrayList.add() doesn't know how you called your variable - or whether you used a variable at all - maybe you were calling it as names.add("name: ");

Basil Bourque · Answer 2 · 2023-04-29T01:20:21.500

The Answer by Thomas Kläger is correct. I'll add a bit to explain the syntax of your lambda code.

Two ways to do the same thing: Declare var, loop list

Your two pieces of code:

names.forEach( ( name ) -> { System.out.println( name ); } );

… and:

for ( String temp : names ) { System.out.println( temp ); }

… both do the very same thing: Define a local reference variable named name of type String.

We can make them even more similar by changing temp variable name to name. And your first line has unnecessary parentheses, removed here.

names.forEach( name -> { System.out.println( name ); } );
for ( String name : names ) { System.out.println( name ); }

Both lines:

Define a local variable named name, of a type to hold a reference to a String object.
Loop the object references held in the List named names.
For each time through that loop, both lines pass the object referenced by the variable named name to System.out.println.

Different syntax but same result.

Both lines contain `String name` declaration

We can make this similarity more clear by expanding the syntax of your lambda code.

The name -> portion is actually an abbreviation of ( String name ) ->. This makes more explicit the declaration of a variable named name to hold a reference to an object of the class String. Your IDE likely offers a feature to switch between the full declaration and the abbreviation.

names.forEach( ( String name ) -> { System.out.println( name ); } );

This ( String name ) -> means the same thing as String name in the second line for ( String name : names ) { System.out.println( name ); }.

Why can I declare variables of the same name within a for loop, but not in the main program?

2 Answers2

Two ways to do the same thing: Declare var, loop list

Both lines contain String name declaration

Both lines contain `String name` declaration