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POLocal object creation in JAVA
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  1. POLocal object creation in JAVA
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    copied!<p>I'm new to java, so please bare with me... </p> <p>I'm writing some JAVA assignment I got in school for a few days now, and I ran into something quite weird (well, at least for me it is...).<br> Since the question has nothing to do with my project in particular, I wrote some code that presents the behaviour I wanted to ask about, so please ignore any problems you might encounter in the following code, that don't relate to this specific issue.<br> Consider the following class: </p> <pre><code>package test; import java.util.ArrayList; import java.util.List; public class Car { List&lt;Doors&gt; doors; int numOfDoors; public Car(int numOfDoors) { this.numOfDoors=numOfDoors; } public void prepare() { run(doors); } public void run(List&lt;Doors&gt; listOfDoors) { listOfDoors=new ArrayList&lt;Doors&gt;(numOfDoors); } public List&lt;Doors&gt; getDoors() { return doors; } } </code></pre> <p>And this:</p> <pre><code>package test; import java.util.List; public class TestDrive { public static void main(String[] args) { Car car=new Car(5); car.prepare(); List&lt;Doors&gt; listOfDoors=car.getDoors(); if (listOfDoors==null) { System.out.println("This is not the desired behaviour."); } else { System.out.println("This is the desired behaviour."); } } } </code></pre> <p>I agree, it's kinda stupid and has no point, but again - I wrote it only to satisfy my curiosity. </p> <p>Now, as you might have guessed, the output is "This is not the desired behaviour.", meaning, the field "doors" holds a null pointer, even though it was assigned with a new object in "run()" method. so my question is why? why is it null?<br> I mean, I know that creating a local variable - may it be a primitive, an object or a reference to an object - will result in loosing it right when we leave the method's scope, but that is not exactly the case here, since there IS a <strong>live</strong> reference to that newly created object (doors), then why would JAVA destroy it?<br> Thanks for anyone who made it through and read the whole post.</p>
 

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