Chapter 1
Overview: The Mental
Landscape
TCC Slides for Java I by David H. Straayer is licensed under a Creative Commons
Attribution-ShareAlike 4.0 International License.
Based on a work at http://math.hws.edu/javanotes/.
1.1 The Fetch and Execute Cycle: Machine Language
• CPU - Central Processing Unit
• Program
• Machine Language
• Main memory
• Locations, addresses
• Fetch-and-execute cycle
• Program Counter
• Bits and bytes
1.2 Asynchronous Events: Polling Loops and Interrupts
• I/O devices
• Device drivers
• Busses
I/O (Input/Output) devices
• A hard disk or solid state drive for storing programs and data files
• A keyboard and mouse for user input.
• A monitor and printer which can be used to display the computer’s
output.
• An audio output device that allows the computer to play sounds.
• A network interface that allows the computer to communicate with
other computers that are connected to it on a network, either
wirelessly or by wire.
• A scanner that converts images into coded binary numbers that can
be stored and manipulated on the computer.
•…
How does the CPU handle data?
• Polling
• Keep checking for data (in a loop)
• Simple
• Inefficient
• Interrupts
• When an event happens, it sends a signal to the CPU.
• A special type of program is invoked: an interrupt handler
• First: make note of what we were doing
• Now, do what is needed to deal with this new data
• Finally, return to what we were doing
Interrupts and fetch-and-execute cycle
• Usually, the CPU processes instructions in a predetermined order –
synchronized.
• Interrupts alter this flow – this is referred to as asynchronous
processing.
• The idea is to avoid having to waste a lot of time waiting for slow I/O
devices
Analogy with human-human communication
• When you are talking with someone, in person or on the phone, you
can’t do anything else, you have to wait for their reply every time you
say something – your communication is synchronized.
• If you are texting or using email (or even snail mail), your
communication is asynchronous. While you’re waiting for a reply to
one message, you can be doing something else, or even sending
messages to other people.
• Call waiting on your phone is like an interrupt
Terms and concepts
• Asynchronous events
• Trigger interrupts, and alter the flow of processing
• Multitasking
• Makes it seem as though the computer is doing several things at once, even
though at any instant in time, it is doing only one thing.
• Timesharing
• Kind of an old term, referring to a computer dealing with many users at one
time
• Thread or process
• Each task that the CPU is working on is called a thread or process.
Threads continue to run until:
• The thread might voluntarily yield control.
• The thread might have to wait, perhaps for data from a drive, or for a
user to type something. While it is waiting, it is blocked, and other
threads may run. When the event occurs, an interrupt will “wake up”
the thread so that it can continue running.
• Preemptive multitasking: even though the thread is doing something,
it may use up an allotted slice of time.
Two tribes of programs and programmers
• Applications
• Most applications programmers ignore threads and multitasking most of the
time: they write the programs people use to get work done.
• Operating Systems
• Systems programmers work on the operating system, the “glue” software that
ties it all together.
1.3 The Java Virtual Machine
• High level programming languages
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Java
Fortran
Pascal
C
Basic
Cobol
Ada
• Machine Languages
• Assembler
Compilers, interpreters, and Virtual Machines
• Compiler: This is a program that reads source code (parses it), and
produces object code, a special version of machine language, that can
be linked and relocated to run on a particular type of machine.
• Interpreter: This is a program that reads source code, line-by-line, and
runs it. The fetch-and-execute cycle is at a “higher level”.
• Virtual Machine: This is what Java uses, and it is a hybrid of compiling
and interpreting. The Java compiler reads the source code and
produces a special file of “predigested” but computer-independent
bytecodes. Then a virtual machine (JVM- Java Virtual Machine) reads
and executes the bytecode very quickly.
Why the JVM approach?
• Speed: parsing takes time, and this allows it to be done once, the way
a compiler does.
• Machine-independence: The resulting programs can be run on many
different types of computers and operating systems.
• Source-code security: Source code is considered a valuable resource.
This allows companies to keep it secret, but still distribute their
programs to their customers.
• Network security: being able to run a program greatly extends what a
web page can do, for good or evil. A JVM with limited capabilities can
provide some security.
J-I-T and all that
• Just-In-Time compilers can be even faster than JVM interpreting
bytecode. Is this less of an issue today?
• Java and Java bytecode are not necessarily linked. Java could be
compiled into “native” machine code, but this loses platformindependence.
• Some other languages generate java bytecode.
1.4 Fundamental Building Blocks of Programs
• Variables: names for locations in memory.
• Types: each variable has a type. The type describes how the bits in
memory are to be interpreted, and what we can do with them.
• Primitive types are the built-in uses of memory
• Integers, doubles (real number), characters, strings of characters, etc.
• Objects: programmer-defined types that can have more flexibility in the
information that can be encoded, and what we can do with them.
• Control structures: altering the sequential flow of instructions
• Loops
• Branches
• Subroutines: Modular programming
It’s all 1’s and 0’s
• That’s all binary computers deal with: 1’s and 0’s.
• These are grouped into bytes, words, etc.
• But when we deal with the data in a computer, we give names to
chucks of memory: we call these chunks variables.
• Each variable has a type. The type describes:
• How much data is associated with this chunk (bits, bytes, etc.)
• How the 1’s and 0’s are to be interpreted (numbers, text, pictures, sounds…)
• What we can do with these chunks of memory. Things we can do are called
operations, and include things like add, copy, render, etc.
Types of types: primitives and objects
• Primitives are the built-in types anticipated by the designers of the
computer and language.
• They are fixed-size chunks of memory.
• They include counting numbers, measuring numbers, and characters (text)
• Their operations involve operators like +, -, *, /, %
• Objects are programmer-defined types. They are whatever you want
them to be.
• The data stored in the objects can include primitives and other objects
• Their operations are defined by the programmer and called subroutines or
methods.
1.5 Objects and Object-oriented
Programming
• History of software engineering
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Modular programming
Structured programming
Top-down programming
Bottom-up design
Information hiding
Object-oriented programming
Object-Oriented Programming (OOP)
• State (chunks of memory: the data in an object)
• Messages (subroutines, methods: the things you can do to objects)
• Polymorphism: that different objects can respond to the same message in
different ways
• Class: the Java language item
that lets you define
the template for an object
• Inheritance: Being able to
define a new type of object
based on an already-defined
type of object.
1.6 The Modern User Interface
• Batch processing
• Command-line interface
• Graphical User Interface (GUI)
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Graphical display
Graphical input device (mouse, touchscreen)
Windows
Standard GUI interface components (button, checkbox, …)
GUI in Java
• AWT and Swing and more…
• GUI programming is event-driven. Instead of the programmer being
in ultimate control, the user is. The programmer writes pieces of
code (methods) that respond
to events generated by the
user.
• This is sometimes referred to
as messages.
1.7 The Internet and Beyond
• Networks connect computers together.
• The Internet is a huge network of networks.
• Protocols are detailed specifications about how data is sent and
received
• Internet Protocol (IP)
• Transmission Control Protocol (TCP)
• Packets
• IP Addresses
• Domain names
Other internet items
• Mail: SMPT, POP, IMAP
• Web: pages, links, servers, browsers, HTTP, HTML, applets
Beyond
• Embedded programming: things like thermostats, set-top boxes,
video players, game systems, …
• Android (Uses Java, but special UI components)
• Beyond Java: C, C++, Python, …
• It is a lot easier to learn another programming language after you’ve
mastered one. Java is a fine starting point.