Reference Details copying comparing References You were introduced

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, data. In iot in Some. Big. Rec. Type). . . Function get returnsa Some. Big. Rec. Type (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. data Big record next Big record data 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; wid public Box(int len, int wid) { set. Len(len); set. Wid(wid); } public void set. Len(int len) { this. len = len; } public void set. Wid(int wid) { this. wid = wid; } public int get. Len() { return len; } public int get. Wid() { return wid; } public int get. Area() { return get. Len()*get. Wid(); } public String to. String() { return ("Rec: l= "+get. Len()+", w= "+get. Wid()); } } // Box OK?

Confusion? Box b 1 = new Box(3, 4); Box b 2 = new Box(3, 4); if(b 1 == b 2) System. out. println("The boxes are obviously equal!"); else System. out. println("I'm sooooo confused!"); Your final answer? b 1 box(3, 4) b 2 box(3, 4)

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

Solution Box b 1 = new Box(3, 4); Box b 2 = new Box(3, 4); if(b 1. equals(b 2)) System. out. println("The boxes are obviously equal!"); else System. out. println("I’m sooooo confused!"); Your final answer? b 1 box(3, 4) b 2 box(3, 4)

Copying? Box b 1 = new Box(3, 4); Box b 2 = new Box(5, 7); b 2 = b 1; b 1 box(3, 4) b 2 box(5, 7) b 1 box(3, 4) b 2 Where did the other box go?

Duplicating*? What if I want to make another identical box? One way: Box b 1 = new Box(3, 4); Box b 2 = new Box(b 1. get. Len(), b 1. get. Wid()); Another way. . . add a duplicate method into the Box class! public Box duplicate() { return new Box(get. Len(), get. Wid()); } Now we can say: Box b 2 = b 1. duplicate(); *cloning?

Copying? Box b 1 = new Box(3, 4); Box b 2 = b 1. duplicate(); Box b 3 = b 2; b 2. set. Wid(8); System. out. println(b 3. get. Wid()); b 2 = null; b 1 box(3, 4) b 2 box(3, 8) b 3

Reference Details: int x; Creates an int primitive 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 q 2 = new Queue(); The q 2 reference can be created and initialized in one step. It is now pointing at a queue object.

Complete Box Class class Box { private int len; private int wid; public Box(int len, int wid) { set. Len(len); set. Wid(wid); } public void set. Len(int len) { this. len = len; } public void set. Wid(int wid) { this. wid = wid; } public int get. Len() { return len; } public int get. Wid() { return wid; } public int get. Area() { return get. Len()*get. Wid(); } public String to. String() { return "(Rec: l= "+get. Len()+", w= "+get. Wid(); } public boolean equals(Box b) { if(get. Len() == b. get. Len() && get. Wid() == b. get. Wid()) return true; else return false; } public Box duplicate() { return new Box(get. Len(), get. Wid()); } public static void main(String args[]) { Box b 1 = new Box(3, 4); Box b 2 = new Box(3, 4); if(b 1 == b 2) System. out. println("The boxes are obviously equal!"); else System. out. println("I'm sooooo confused!"); if(b 1. equals(b 2)) System. out. println("The boxes are obviously equal!"); else System. out. println("I'm sooooo confused!"); Box b 3 = b 1. duplicate(); if(b 1. equals(b 3)) System. out. println("The boxes are obviously equal!"); else System. out. println("I'm sooooo confused!"); } } // Box