CS 123 - Memory model for pictureInAPictureVer2 method

Below are two illustrations of the memory map for the following code fragment. 

   > Picture x = new Picture("c:/cs123/mediasources/pictures/butterfly1.jpg")
   > Picture y = new Picture("c:/cs123/mediasources/pictures/turtle.jpg")
   > Picture z = x.pictureInAPictureVer2(300, 150, y)
The first memory map shows what memory looks like just before the pictureInAPictureVer2() method returns. Notice that the newly created object, referenced by p3, holds the merged data from the butterfly1.jpg and turtle.jpg Picture objects. That is, the pictureInAPictureVer2() method takes data from the object referenced by this and the object referenced by p2 and copies that data into the new object referenced by p3. This new object therefore holds what should be the result of this method.
The second memory map shows what memory looks like just after the pictureInAPictureVer2() method returned. Notice that, when the method returned, the value from the local reference variable p3 was copied into the interaction reference variable z. Therefore, the variable z references the object that holds the merged data from the two original Picture objects. And the object referenced by x was not in any way modified (or mutated) by the method call (as opposed to the pictureInAPicture.java example).
After the method returns, the pictureInAPictureVer2 frame (with all its local variables) goes away from memory. This is illustrated by changing the color of the pictureInAPictureVer2 frame to yellow.

Now let us consider the following example interaction. Below are two illustrations for this code. Be sure to compare this code and its illustrations to the above example and to the example from pictureInAPicture.java 

   > Picture x = new Picture("c:/cs123/mediasources/pictures/butterfly1.jpg")
   > Picture y = new Picture("c:/cs123/mediasources/pictures/turtle.jpg")
   > x = x.pictureInAPictureVer2(300, 150, y)
The first memory map shows what memory looks like just before the pictureInAPicture() method returns. Except for the lack of a interaction variable named z, this picture is the same as the corresponding picture above.
The second memory map shows what memory looks like just after the pictureInAPictureVer2() method returned. Notice that, when the method returned, the value from the local reference variable p3 was copied into the interaction reference variable x which over wrote the reference, that was in x, to the object with the butterfly1.jpg data. Therefore, the variable x now references the object that holds the merged data from the two original Picture objects. But the object with the butterfly1.jpg data no longer has any reference to it, so it becomes a garbage object, and this is illustrated by coloring the "object" yellow. It is important to know that we do not, in this case, say that the object referenced by x has mutated. There was no mutation of an object. The variable x has changed its reference between two distinct objects.


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