1

Tirgul no. 12Tirgul no. 12

Topics covered:

Binary Search Trees Exception Handling in java

5

Class BinarySearchTreeClass BinarySearchTree

public class BinarySearchTree {

protected Node root;

//some constructors

//methods (on next slides);

}

6

InorderTreeWalkInorderTreeWalk

public void inorderTreeWalk(){inorder(root);

}

//recursively traverse the tree:private void inorder(Node node)

if(node!=null) //stopping condition inorder(node.getLeft()); System.out.println(node.getData()); inorder(node.getRight());

}

7

SearchSearchpublic Node search(Comparable data){

return(treeSearch(root,data));}

//recursively search the tree:public Node treeSearch(Node node, Comparable data){

if( (node == null) || (node.getData().compareTo(data) == 0)

return(node);if(node.getData().compareTo(data) > 0) return(treeSearch(node.getLeft(),data);else return(treeSearch(node.getRight(),data);

}

8

InsertInsertpublic void insert(Comparable data){

Node currParent= null;Node node= root;while(node != null){

currParent= node;if(node.getData().compareTo(data) > 0 ) node= node.getLeft();else node= node.getRight();

}Node newNode= new Node(data);if( currParent == null) //tree is empty!

root= newNode;else if( currParent.getData().compareTo(data) > data)

currParent.setLeft(newNode)else

currParent.setRight(newNode);}

9

DeletionDeletionpublic Node delete(Comparable data){ Node delNode= search(data); if(delNode == null)

return; Node helper=null; Node temp= null;

//decide which node to splice: if( (delNode.getLeft() == null) ||

(delNode.getRight() == null) ) helper= delNode; else

helper= TreeSuccessor(delNode);//check for sons of splice node: if(helper.getLeft() != null) temp= helper.getLeft();

else temp= helper.getRight();

if(temp != null)temp.parent= helper.parent;

//check if successor is root nodeif(helper.getParent() == null)

root= temp;else if (helper == helper.getParent().getLeft() )

helper.getParent().setLeft(temp);else

helper.getParent().setRight(temp);if( helper != delNode)

//copy all fields of ydelNode.setData(helper.setData());

return(helper);}//end of method

10

• The way we deal with errors, exceptional/abnormal situations in the java programming language is by throwing exceptions.• Exceptions are instances of classes which are descendants of the class Exception.

Throwable

The Throwable family

Exception

descendants of Throwable

RunTimeExceptionGenerally work with this subtree

Exceptions

11

Exceptions: details

Many types of exceptions in java for examples look at java.lang api.

We can create our own types of exceptions by extending existing types.

Any Exception which is not a descendant of RuntimeException must be caught by a method or the method must be declared to throw the exception (more on this in a couple more slides).

12

Why do we use exceptions?

Make the code more readable - it separates the algorithm from error handling.

There are cases where returning a value to indicate errors is impossible or not reasonable - constructors and void methods.

Easier to indicate different types of errors.

Easy to propagate errors up the calling stack.

13

try {// guarded code.// exceptions A,B and C might be thrown here.

} catch (A a) {// what to do if A is thrown.

} catch (B b) {// handle B here.

} catch (C c) {// and here we have a chance to handle C.

} finally {// Thing that must happens after the rest of the code,// if an exception is thrown or not.

}

How to use Exceptions

14

We want to read a file into memory.

readFile (){ open the file; determine its size; allocate that much memory; read the file into memory; close the file;}

Example: Reading a file

15

int readFile() {openFile;if (file not opened) {

// handle the errorreturn -1;

}get the file length;if (cannot get the length) {

// handle this errorclose filereturn -2;

}allocate memory for the data;if (not enough memory) {

// another error to handle.close filereturn -3;

}

read data;if (read failed) {

// handle again.close file;return -4

}close file; // after read.reutrn 0; // success.

}

Reading file - without exceptions

16

void readFile() {try {

open file;get file size;allocate memory;read file;

} catch (FileNotOpened e1) {// handle this.

} catch (CannotGetFileLength e2) {// another handeling code.

} catch (MemoryAllocationFailed e3) {// yet another error.

} catch (ReadFailed e4) {// the last handle here.

} finally {if(file is open)

close file;}

}

This is the algorithm}

} Error handling

}code that happens in any case

Reading file - with exceptions

17

public class Example { public static void main(String args[]) { SimpleInput sin = new SimpleInput(System.in); int num;

boolean ok = false; while(!ok) { try { System.out.print("Enter an integer: "); num = sin.readlnInt(); ok = true; }catch(NumberFormatException nfe) { System.err.println("Read (" + nfe.getMessage() + ") not an integer."); }catch(IOException ioe) { System.err.println("Fatal IO error.\n"+ ioe + "Exiting program."); System.exit(1); } } }}

Recovering from errors

18

public class NimBoard { private int heaps[];

public NimBoard(int size) throws IllegalSizeException { if(size<1) throw new IllegalSizeException(size); heaps = new int[size]; }

// other methods of NimBoard}

Throwing exception from the constructor

19

public class IllegalSizeException extends IllegalArgumentException { public IllegalSizeException() {

super(); } public IllegalSizeException(String message) {

super(message); } public IllegalSizeException(int size) { super("Illegal nim board size ( " + size + ")"); }}

Defining a new exception type

20

Throwable

Exceptiondescendants of Throwable

RunTimeException

any class other than RunTimeException

descendants of RuntimeException

checkedunchecked

Checked versus Unchecked Exceptions