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KTH ROYAL INSTITUTEOF TECHNOLOGY

EH2745 – Lecture 5

Access, Exceptions & I/O

The lecture slides are somewhat based on the MIT OpenCourseware course 6.092 – Introduction to Java

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Outline

Access control & Class scopeExceptionsI/O

”Protecting” data and methods

When designing your program, you may want tto definecertain rules for how variables and methods should be accessed.

Like avoiding accessing an Array outside of its index.

For this matter, methods and variables can be marked as

private and public

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An example for consideration

Access control done properly

Will not compileVariable ”not visible”

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The right way to do it

Why Access control?

Keeping the interface of the Class separatefrom the implementation

Protecting the data of the class

Making sure others use your class the right way.

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Scope of variables – method scope

Scope of variable – Class scope

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The this keyword

this keyword refers to the object you areworking with

Clarifies the scope of the variable you arereferencing.

Updating the total ampsDisconnected

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Updating the object variable

Outline

Access control & Class scopeExceptionsI/O

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What to do when something goes wrong?

Like when accessing an Array outside of bounds?Or when dividing by zero?

The default solution is that Java stops executionand prints an error messahge to the console

Not very nice….....

What if we could catch this error, and try to fix the problem?

Throwing and catching Exceptions

An exception is an unexpected event during run-time.

For instance (again) accessing an array out of the index

This is the result of Java creating an Exception object and filling it with data.

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Warning Java about Exceptions

throws tells Java that get may throw the ArrayOutOfBoundsExceptionthrow actually throws the Exception

OK, but who should deal with it?

The code that calls the method (get in this case) needs to implement code that catches the exception.

try to run some code that may throw an exception

Tell Java what to do if it sees the exception catch

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So, in the code that calls get you need

If you do not want to fix the problem (catch the exception) just rethrow it!

A more complete example

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Outline

Access control & Class scopeExceptionsI/O

A computer (slightly simplified)

The CPU exectues machine codeinstructions stored in memory

The memory is used both to store the program and the data needed

Communication with the real world through I/O units (keyboard, monitor, speaker, harddrive, ethernet,…)

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Output example

Input – the full picture

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InputStream

InputStreamReader

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BufferedReader

Input – the full picture

System.in is the defaultinput (often keyboard)

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So what about input from the keyboard

Input from a file

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Code example for reading a file

That was again quite a lot

Make sure to get started programming!!!