Reference Details (copying, comparing)

Reference Details(copying, comparing)

References • You were introduced to the concept of pointers earlier this term. • The pointer capabilities in pseudocode are relatively tame as compared to languages such as C which was originally developed for system programming • In fact, pointers (and their misuse) are considered by some experts to be a very dangerous aspect of programming. • The designers of Java decided to implement their language and eliminate many of the "dangerous" aspects of pointers. They even decided to call them something else: references

What’s all the fuss? • Pointers (and references) are used for a very simple reasons: efficiency • Pointers save time • Pointers save space • Example: Procedure add(cur iot in/out Ptr toa Node, dataIn iot in SomeBigRecType) ... Function get returnsa SomeBigRecType (cur iot in Ptr toa Node) ...

Efficient? • The add procedure asks us to pass in a big record of some type. This (as we have defined it) would require us to make a local copy inside the module. • Then with a line of code like: temp^.data <- datain • We would make another copy! • The get module would reverse the process. It might again copy the record one or more times. • All this space...all this time to copy!

The Real World • In the real world code would be much more likely to simply make some space for the data once and from then on just pass pointers to the data. • So the linked list Node wouldn’t really contain a data record and a next pointer. • It would contain a pointer to a data record AND a pointer to the next Node. Big record data Big record data next next

Java References • Java takes advantage of this concept by manipulating all objects (which are actually chunks of memory on the heap) using references. • This may cause some confusion! • Let’s make a small class to use as an illustration. • This class will hold info about boxes.

class Box len class Box { private int len; private int wid; public Box(int len, int wid) { setLen(len); setWid(wid); } public void setLen(int len) { this.len = len; } public void setWid(int wid) { this.wid = wid; } public int getLen() { return len; } public int getWid() { return wid; } public int getArea() { return getLen()*getWid(); } public String toString() { return ("Rec: l= "+getLen()+", w= "+getWid()); } } // Box wid OK?

box(3, 4) b1 box(3, 4) b2 Confusion? Box b1 = new Box(3, 4); Box b2 = new Box(3, 4); if(b1 == b2) System.out.println("The boxes are obviously equal!"); else System.out.println("I'm sooooo confused!"); Your final answer?

Solution We need a method that compares two boxes! (And it has to be in the Box class!) public boolean equals(Box b) { if(getLen() == b.getLen() && getWid() == b.getWid()) return true; else return false; } class Box { private int len; private int wid; public Box(int len, int wid) { setLen(len); setWid(wid); } public void setLen(int len) { this.len = len; } public void setWid(int wid) { this.wid = wid; } public int getLen() { return len; } public int getWid() { return wid; } public int getArea() { return getLen()*getWid(); } public String toString() { return "(Rec: l= "+getLen()+", w= "+getWid(); } }

box(3, 4) b1 box(3, 4) b2 Solution Box b1 = new Box(3, 4); Box b2 = new Box(3, 4); if(b1.equals(b2)) System.out.println("The boxes are obviously equal!"); else System.out.println("I’m sooooo confused!"); Your final answer?

Copying? box(3, 4) b1 Box b1 = new Box(3, 4); Box b2 = new Box(5, 7); b2 = b1; Where did the other box go? box(5, 7) b2 box(3, 4) b1 b2

Duplicating*? What if I want to make another identical box? One way: Box b1 = new Box(3, 4); Box b2 = new Box(b1.getLen(), b1.getWid()); Another way...add a duplicate method into the Box class! public Box duplicate() { return new Box(getLen(), getWid()); } Now we can say: Box b2 = b1.duplicate(); *cloning?

Copying? box(3, 4) b1 Box b1 = new Box(3, 4); Box b2 = b1.duplicate(); Box b3 = b2; b2.setWid(8); System.out.println(b3.getWid()); b2 = null; box(3, 4) b1 box(3, 8) b2 b3 box(3, 4) b1 box(3, 8) b2 b3

Creates an int primitive int x; Queue q; Creates a reference (pointer) to a queue object. Note: No object has been created q = new Queue(); q is now pointing to a Queue object or more correctly q now has a reference to a queue object. Queue q2 = new Queue(); The q2 reference can be created and initialized in one step. It is now pointing at a queue object. Reference Details:

Complete Box Class class Box { private int len; private int wid; public Box(int len, int wid) { setLen(len); setWid(wid); } public void setLen(int len) { this.len = len; } public void setWid(int wid) { this.wid = wid; } public int getLen() { return len; } public int getWid() { return wid; } public int getArea() { return getLen()*getWid(); } public String toString() { return "(Rec: l= "+getLen()+", w= "+getWid(); } public boolean equals(Box b) { if(getLen() == b.getLen() && getWid() == b.getWid()) return true; else return false; } public Box duplicate() { return new Box(getLen(), getWid()); } public static void main(String args[]) { Box b1 = new Box(3, 4); Box b2 = new Box(3, 4); if(b1 == b2) System.out.println("The boxes are obviously equal!"); else System.out.println("I'm sooooo confused!"); if(b1.equals(b2)) System.out.println("The boxes are obviously equal!"); else System.out.println("I'm sooooo confused!"); Box b3 = b1.duplicate(); if(b1.equals(b3)) System.out.println("The boxes are obviously equal!"); else System.out.println("I'm sooooo confused!"); } } // Box

Reference Details (copying, comparing)