שאלה ב-JAVA

[_alon_]

שאלה ב-JAVA

שאלה ב-JAVA, העברת פרמטרים בפונקציות. פרמטרים עוברים כ-refrence, ולכן אם אשנה את האוביקט בתוך הפונקציה גם מחוצה לה הוא ישתנה. למה בתוכנית הבאה זה לא כך? public class X { public static void main(String[] args) { X myX = new X(); Y myY = null; myX.f(myY); if (myY == null) System.out.println("myY is null here. why???"); // ^^^ why did i get here? passing by refrence means that if the function changed the object it´s changed here also, right? ^^^^ } // end of main func public void f(Y myY) { myY = new Y(); // myY is no longer null. } } // end of class X

 

[_alon_]

lets try again but this time in eng

can anyone tell me why, if parameters pass as refrence, do myY turns back to null, after returning from function f thanks, alon

public class X { public static void main(String[] args) { X myX = new X(); Y myY = null; myX.f(myY); if (myY == null) System.out.println("myY is null here. why???"); // ^^^ why did i get here? passing by refrence means that if the function changed the object it´s changed here also, right? ^^^^ } // end of main func public void f(Y myY) { myY = new Y(); // myY is no longer null. } } // end of class X​

 

[albanetc]

wrong!

you don´t pass object by reference, what you pass is a handle and you pass it by value. in your example you can do f(null) as well, just don´t expect that after this call ´null´ would point to anything. here´s what´s going on: Y myY = null; // a new handle is created on stack. myX.f (myY); // here the handle (null) is copied and placed on stack. // in myX.f (Y myY): new handle myY is created and initialised // by what is on the stack (null). { myY = new Y(); // new Y object is created. local copy of handle myY is initialized // by its address. } // local myY got out of scope, it is cleaned from stack. // Y object created in function has no handle that points to it. // it will be garbage-collected sometime. if () etc... // myY in main stayed null. take a look at bruce eckel´s thinking in java, chapter 12