Transcript Lecture 15 Slides

Programming for Beginners
Lecture 15: More-Advanced Concepts
Martin Nelson
Elizabeth FitzGerald
Session 15 – aims and objectives
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Review of the course to date.
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Brief introduction to some more advanced topics.
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Where to go next?
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Chance to wrap up anything that’s not been clear so far.
Recap of the course so far!
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We now know LOADS about Java!
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Most of the Java concepts we’ve talked about transfer
directly to other languages.
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We’ve seen different types of data:
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int, double, String, boolean, char, char[], objects,...
We know how to design and structure codes
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Pen & paper, pseudocode, testing, successive refinement.
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We now know the difference between procedural and
object oriented programming.
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We’ve seen a bit of procedural C++ and some scripting
languages.
Structure of a Java program
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We’ve seen this many times now...
class myprog
{
public static void main(String[] args)
{
System.out.println(“...”);
}
}
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But what does it all mean?
Structure of a Java program
Every Java program is a class.
class myprog
Here is the class’ name.
{
public static void main(String[] args)
{
System.out.println(“...”);
}
}
Every Java application must have a method called main,
which is run as soon as the application is started.
Structure of a Java program
Return type
(void by convention)
Name of the method
public static void main(String[] args)
main has to be public
i.e. Accessible outside
of class myprog
main is static so that
it can be used without
creating an instance of
class myprog
Argument list
Convention: main always receives
an array of Strings.
Using String[] args
class myprog
{
public static void main(String[] args)
{
for(int i=0; i<args.length; i++)
{
System.out.println(args[i]);
}
All arrays have a length
variable representing the
number of elements
}
}
To compile and run:
javac myprog.java
java myprog here is some text
Using String[] args
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What if you want to pass in something other than Strings?
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main() can only receive Strings!
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Need some way of converting Strings to, say, integers...
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Can do this using Wrapper classes.
Wrapper classes
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We have seen that classes (e.g. Strings) are really useful
as they have loads of associated methods which we can
use.
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Primative data types (e.g. int, char, double, etc.) don’t
have such methods.
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However, wrapper classes can be used to `wrap’ a
primative variable into an object.
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They provide more methods & greater flexibility.
Wrapper classes
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int can be wrapped as an Integer object.
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double can be wrapped as a Double object.
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char can be wrapped as a Character object.
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Include methods to convert the numeric value of a String
into an Integer, and then convert an Integer back to an
int.
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It is your responsibility to check that this conversion is
sensible.
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For more info, see the Core Class documentation, or come
to the next Java course!
Arrays vs. Vectors
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Arrays can store a number of variables of the same type
in a logical manner.
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We must know the maximum number of variables to store
in advance!
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What if we don’t know the maximum number of
variables?
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Could just declare a HUGE array, but this wastes memory and
risks causing a bug.
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Instead of arrays, we can use Vectors.
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Vectors can increase/decrease in size dynamically.
Vectors
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Import the java.util.Vector class:
import java.util.Vector;
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Instantiate a new Vector:
Vector numbers = new Vector();
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Add a new element:
numbers.addElement(3);
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Get an element (the 0th element in this case):
numbers.elementAt(0)
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Delete an element (the 7th element in this case):
numbers.remove(7);
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See the Core Class documentation for more details.
Other uses of Java
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Graphical programming
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External libraries java.awt and java.swing can be used to create
graphical programs.
The JVM makes program design independent of the operating
system.
Min/Max/Close buttons are automatic, for example!
Applets
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Small applications which are distributed over the web.
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Byte code is held remotely on a server.
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A JVM is built in to most modern web browsers.
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Must have a java enabled browser to work!
Memory Issues
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When designing code, we should always try to minimise
the amount of memory used.
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Not as much of a problem these days as memory is more
abundant.
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For long running, high intensity jobs, memory
management is more important.
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Numerical simulations and computations, Matlab codes...
Things to think about...
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Am I storing data that I don’t need?
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Am I looping un-necessarily? (Very expensive)
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What do I do with data after I am finished with it?
Memory Issues
In general, the computer’s memory is divided into three
segments:
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The Code Segment
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The Stack Segment
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Where the compiled code itself is stored.
Used for variables which are created as functions are started.
The Heap Segment
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Dynamic memory allocation.
This is a bit more complicated in Java!
The Stack
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The stack segment is used to store variables which are
created as a function runs.
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When a new function starts, a new block of memory is
reserved for that function’s data.
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Minimal control of how much memory is allocated.
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Memory is cleared in a last in – first out manner.
Can’t release memory of func1 until func2 ends
func2()
main()
func1()
func1()
func1()
main()
main()
main()
main()
The Heap
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Memory is allocated dynamically.
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Slower than the stack, as memory allocation is more
complicated.
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Much more memory can be made available, and memory
which is no longer needed can be released straight away.
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Heap data is accessed via its memory address using
pointers.
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Heap data survives after
function which created it
ends... until you delete it!
the
Data
Data
Data
Data
Data
Data
Pointer
Data
Pointers & Dynamic Memory
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Pointers are used a lot in languages such as C++.
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Some people find pointers very complicated.
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When using dynamic memory, you must always explicitly
delete data when it is finished with, or the memory
cannot be re-used.
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Failure to empty memory is known as a memory leak.
They will often cause your program to crash as memory
runs out!
Pointers & Dynamic Memory
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We haven’t seen pointers in Java. Why?
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Or, more accurately, everything in java is a pointer, so you never
have to worry about them.
In java, everything is stored on the heap.
Big advantage of Java: Memory management is
automatic.
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Java doesn’t have them!
Don’t have to worry about deleting variables when they are
finished with. Not the case with C++.
To find out more about pointers and dynamic memory
allocation, sign up for the forthcoming C++ programming
course...
Forthcoming Courses

C++ Programming
Ten Sessions: 12th April – 17th May 2011
Tutor: Dr. Robert Oates ([email protected])
Object orientation, pointers, efficient use of memory etc.

Introduction to Java Programming
Ten Sessions: 7th June – 7th July 2011
Tutor: Dr. Martin Nelson ([email protected])
Vectors, wrapper classes, graphics, applets

More info: www.nottingham.ac.uk/csc
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