Programming Forums > Application Development > Java

DaWei · Sep 21st, 2007, 9:05 AM

That's a lot of code to try to make sense of, but this:

   public static GridNode copy(GridNode a)
   {
      GridNode b = a;
      return b;
   } // end copy

will be a shallow copy. In other words, b and a will refer to the same thing. Similarly, here,

(Toggle Plain Text)

   // set leftChild
   public void setLeftChild(GridNode newLeftChild)
   {
      leftChild = newLeftChild;
   } // end setLeftChild

It's difficult to tell, in the second case, whether or not the reference is okay, or a Bad Thang.

When you clone a new GridNode, you need to at least copy the state, not just refer to the source's state.

I also don't know why you're using getters and setters inside the class, where you have access. To me, that just obfuscates things.

cwl157 · Oct 8th, 2007, 12:56 AM

its been a while, i know and this and the project's over but just for future reference, this whole shallow copy stuff, i think thats what tripped me up the most, how would i take those 2 methods daWei mentioned and make them make a deep copy rather then a shallow copy? Thanks.

titaniumdecoy · Oct 8th, 2007, 1:19 AM

Quote:

Originally Posted by cwl157

its been a while, i know and this and the project's over but just for future reference, this whole shallow copy stuff, i think thats what tripped me up the most, how would i take those 2 methods daWei mentioned and make them make a deep copy rather then a shallow copy? Thanks.

I practically wrote you an essay on the topic of shallow vs. deep copy in this thread. As such, I am not particularly inclined to answer the same question again. All that aside, a simple search for "shallow deep copy java" should more than suffice.

cwl157 · Oct 8th, 2007, 5:58 PM

so i could use the clone() method and instead of saying

(Toggle Plain Text)

 // set leftChild
   public void setLeftChild(GridNode newLeftChild)
   {
      leftChild = newLeftChild;
   } // end setLeftChild

i would say something like this (and i havent compile or tried this, i dont have time right now but i just want to get the idea)

(Toggle Plain Text)

 // set leftChild
   public void setLeftChild(GridNode newLeftChild)
   {
      leftChild = (GridNode) newLeftChild.clone();
   } // end setLeftChild

and that will make a seperate exact deep copy, correct? And then whatever i wanted to change in the copy i could and it will only change that one because it is not just a pointer to it, correct?

DaWei · Oct 8th, 2007, 6:25 PM

Java is based heavily on references. You are going to have to come to terms with the implications of that, or you are going to be tilting at windmills for a long time to come.

You still need to be careful with clone. While clone will make a new instance of the object, and copy it, there may be data stuctures that are part of the object that get copied structurally, but their contents may not. In fact, the action of clone varies for differing objects.

cwl157 · Oct 8th, 2007, 7:33 PM

wow,this isnt as straight forward as i realized. I thought java was supposed to make it easy to create multiple instances of objects and use them, but i guess not. So your saying that by calling clone like i do that the array could still be a reference and not a new array so i would have to call that method as well to make it its own separate array?

DaWei · Oct 8th, 2007, 10:51 PM

I'm saying you need to investigate the clone method for anything you use it for. See here for Object.clone ().

Read a line like this:

(Toggle Plain Text)

 
ArrayList myArray = new ArrayList ();

as

(Toggle Plain Text)

SomeObject myObjectReference = new SomeObject ();

Obviously, this:

(Toggle Plain Text)

SomeObject newObjectReference;
newObjectReference = myObjectReference;

merely gives you a new reference to the same object referred to by myObjectReference.

Note that if you instantiate a new object for newObjectReference, and then assign myObjectReference to newObjectReference, you have not made a copy. You have just disconnected newObjectReference from the new object and pointed it at the original object.

If you want a new instance of the object, make one. Either clone the original (check the docs for it) or instantiate a new one and copy the contents of the old one over piecemeal. Refer back to my example.

cwl157 · Oct 8th, 2007, 11:06 PM

so to copy i should just say

(Toggle Plain Text)

GrideNode node = new GridNode();
GridNode newNode = new GridNode();

// then here i would copy the state from node to new node?

titaniumdecoy · Oct 9th, 2007, 1:06 AM

The way you copy an object may depend on the way its author implemented it. An object's clone method may perform a deep copy or a shallow copy or anything in between. You would have to look at the source or documentation for that object to see how its clone method behaves.

Alternatively, if want to create a deep copy of an object without using its clone method, you would create a new object and set each of its non-basic member variables to reference copies of the other non-basic members of the object you are copying, presumably created in the same way. That is to say, you recurse until there are only basic members left to copy for each object.

cwl157 · Oct 27th, 2007, 12:35 PM

alright this is probably stupid, but to make it unique is all i have to do is say this. in the constructor so the constructor would look like this?

(Toggle Plain Text)

// set state to newState and firstChild to newFirstChild and nextSibling to newNextSibling
   public GridNode(int[][] newState, GridNode newLeftChild, GridNode newUpChild, GridNode newRightChild, GridNode newDownChild)
   {
      this.state = newState;
      this.leftChild = newLeftChild;
      this.upChild = newUpChild;
      this.rightChild = newRightChild;
      this. downChild = newDownChild;
   } // end GridNode constructor

Sep 21st, 2007, 9:05 AM	#11
DaWei Resident Grouch Join Date: Jun 2005 Posts: 6,453 Rep Power: 10	That's a lot of code to try to make sense of, but this: (Toggle Plain Text) public static GridNode copy(GridNode a) { GridNode b = a; return b; } // end copy public static GridNode copy(GridNode a) { GridNode b = a; return b; } // end copy will be a shallow copy. In other words, b and a will refer to the same thing. Similarly, here, (Toggle Plain Text) // set leftChild public void setLeftChild(GridNode newLeftChild) { leftChild = newLeftChild; } // end setLeftChild // set leftChild public void setLeftChild(GridNode newLeftChild) { leftChild = newLeftChild; } // end setLeftChild It's difficult to tell, in the second case, whether or not the reference is okay, or a Bad Thang. When you clone a new GridNode, you need to at least copy the state, not just refer to the source's state. I also don't know why you're using getters and setters inside the class, where you have access. To me, that just obfuscates things. __________________ Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

Oct 8th, 2007, 5:58 PM	#14
cwl157 Professional Programmer Join Date: Feb 2005 Posts: 327 Rep Power: 4	so i could use the clone() method and instead of saying (Toggle Plain Text) // set leftChild public void setLeftChild(GridNode newLeftChild) { leftChild = newLeftChild; } // end setLeftChild // set leftChild public void setLeftChild(GridNode newLeftChild) { leftChild = newLeftChild; } // end setLeftChild i would say something like this (and i havent compile or tried this, i dont have time right now but i just want to get the idea) (Toggle Plain Text) // set leftChild public void setLeftChild(GridNode newLeftChild) { leftChild = (GridNode) newLeftChild.clone(); } // end setLeftChild // set leftChild public void setLeftChild(GridNode newLeftChild) { leftChild = (GridNode) newLeftChild.clone(); } // end setLeftChild and that will make a seperate exact deep copy, correct? And then whatever i wanted to change in the copy i could and it will only change that one because it is not just a pointer to it, correct?

Oct 8th, 2007, 6:25 PM	#15
DaWei Resident Grouch Join Date: Jun 2005 Posts: 6,453 Rep Power: 10	Java is based heavily on references. You are going to have to come to terms with the implications of that, or you are going to be tilting at windmills for a long time to come. You still need to be careful with clone. While clone will make a new instance of the object, and copy it, there may be data stuctures that are part of the object that get copied structurally, but their contents may not. In fact, the action of clone varies for differing objects. __________________ Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

Oct 8th, 2007, 10:51 PM	#17
DaWei Resident Grouch Join Date: Jun 2005 Posts: 6,453 Rep Power: 10	I'm saying you need to investigate the clone method for anything you use it for. See here for Object.clone (). Read a line like this: (Toggle Plain Text) ArrayList myArray = new ArrayList (); ArrayList myArray = new ArrayList (); as (Toggle Plain Text) SomeObject myObjectReference = new SomeObject (); SomeObject myObjectReference = new SomeObject (); Obviously, this: (Toggle Plain Text) SomeObject newObjectReference; newObjectReference = myObjectReference; SomeObject newObjectReference; newObjectReference = myObjectReference; merely gives you a new reference to the same object referred to by myObjectReference. Note that if you instantiate a new object for newObjectReference, and then assign myObjectReference to newObjectReference, you have not made a copy. You have just disconnected newObjectReference from the new object and pointed it at the original object. If you want a new instance of the object, make one. Either clone the original (check the docs for it) or instantiate a new one and copy the contents of the old one over piecemeal. Refer back to my example. __________________ Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

Oct 8th, 2007, 11:06 PM	#18
cwl157 Professional Programmer Join Date: Feb 2005 Posts: 327 Rep Power: 4	so to copy i should just say (Toggle Plain Text) GrideNode node = new GridNode(); GridNode newNode = new GridNode(); // then here i would copy the state from node to new node? GrideNode node = new GridNode(); GridNode newNode = new GridNode(); // then here i would copy the state from node to new node?

Oct 8th, 2007, 12:56 AM	#12
cwl157 Professional Programmer Join Date: Feb 2005 Posts: 327 Rep Power: 4	its been a while, i know and this and the project's over but just for future reference, this whole shallow copy stuff, i think thats what tripped me up the most, how would i take those 2 methods daWei mentioned and make them make a deep copy rather then a shallow copy? Thanks.

Oct 8th, 2007, 7:33 PM	#16
cwl157 Professional Programmer Join Date: Feb 2005 Posts: 327 Rep Power: 4	wow,this isnt as straight forward as i realized. I thought java was supposed to make it easy to create multiple instances of objects and use them, but i guess not. So your saying that by calling clone like i do that the array could still be a reference and not a new array so i would have to call that method as well to make it its own separate array?

Oct 9th, 2007, 1:06 AM	#19
titaniumdecoy Expert Programmer Join Date: Nov 2005 Posts: 837 Rep Power: 3	The way you copy an object may depend on the way its author implemented it. An object's clone method may perform a deep copy or a shallow copy or anything in between. You would have to look at the source or documentation for that object to see how its clone method behaves. Alternatively, if want to create a deep copy of an object without using its clone method, you would create a new object and set each of its non-basic member variables to reference copies of the other non-basic members of the object you are copying, presumably created in the same way. That is to say, you recurse until there are only basic members left to copy for each object.

Oct 27th, 2007, 12:35 PM	#20
cwl157 Professional Programmer Join Date: Feb 2005 Posts: 327 Rep Power: 4	Re: can someone please help me with this tree alright this is probably stupid, but to make it unique is all i have to do is say this. in the constructor so the constructor would look like this? (Toggle Plain Text) // set state to newState and firstChild to newFirstChild and nextSibling to newNextSibling public GridNode(int[][] newState, GridNode newLeftChild, GridNode newUpChild, GridNode newRightChild, GridNode newDownChild) { this.state = newState; this.leftChild = newLeftChild; this.upChild = newUpChild; this.rightChild = newRightChild; this. downChild = newDownChild; } // end GridNode constructor // set state to newState and firstChild to newFirstChild and nextSibling to newNextSibling public GridNode(int[][] newState, GridNode newLeftChild, GridNode newUpChild, GridNode newRightChild, GridNode newDownChild) { this.state = newState; this.leftChild = newLeftChild; this.upChild = newUpChild; this.rightChild = newRightChild; this. downChild = newDownChild; } // end GridNode constructor

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Programming Forums > Application Development > Java

can someone please help me with this tree