Transcript Intro to Java

INTRO TO JAVA
WHAT THIS LECTURE IS & ISN'T
• It ISN'T
• A complete how-to of everything Java
• We'll see new features of Java all semester
• It IS
• Enough to get you through the first lab (hopefully)
• If I neglect to cover something…ask!
• Give you a taste of some major concepts, especially
• the OOP-nature of Java
• Basic I/O (including Files)
• Enough to show you the pace / style of lectures
• How to survive it
• Follow along
• If you're a fast typist, you can do it with me.
• Or just watch and download the project afterwards.
• Ask lots of questions
• Make observations
• Esp. comparisons to other languages you know (Python, MathPiper)
• Caffeine??
OUTLINE
1.
2.
3.
4.
5.
6.
Hello World
Variables
Console I/O
File I/O
Arrays
Intro to OOP
1. HELLO WORLD
• Do it on the computer (example01a_hello_world)
• Observations?
• Java is very "wordy"
• You have to create a class (Java is very OOP-based)
2. PRIMITIVES AND OBJECTS
• Java uses explicit declaration of variables.
• Unlike Python (and MathPiper?)
• In Java, most things are objects but there are a few primitives
• Primitives: long, int, char, byte, float, double and boolean
• Can assign literals to them (without new)
• More efficient internally
• There are Object-versions of each:
• Object-types: Long, Int, Char, Byte, Float, Double, Boolean
• Must allocate memory for them. The variable is a reference to that
memory.
• Objects all have methods.
• Objects are derived from the Object base class
• Comparison (==) doesn’t always work as you’d expect.
• Java normally auto-converts between the two.
• Strings are kind of a hybrid
• Has methods
• Internally a fixed-size char array.
• Can assign literals to it.
• Do example (example01b_variables)
• Observations?
3. CONSOLE I/O
• Do it on the computer (example01c_console_io)
• Observations?
• Input is based on the Iterator pattern
• We'll see this other places as well.
• Not always very intuitive.
• There are lots more ways to get input.
4. FILE I/O - INPUT
• Lots of variants – I'll show you two.
• [With a Scanner]
Scanner s = Scanner(new File("blarg.txt"));
// Now treat s just as before.
• [With a BufferedReader (and a tweaked try)]
try (BufferedReader br = new BufferedReader(new
FileReader("blarg.txt"))
{
String line;
line = br.readLine();
while (line != null)
{
// Process line
// Get next line (if there is one)
line = br.readLine();
}
}
4. FILE I/O - OUTPUT
• I like the BufferedWriter approach
try
{
File f= new File("new_blarg.txt");
if (!f.exists())
f.createNewFile();
BufferedWriter bw = new BufferedWriter(new FileWriter(f));
bw.write("This is the first line!\n");
bw.write("This is the second!\n");
bw.close();
// IMPORTANT – usually
}
catch (IOException e)
{
System.out.println("Error creating new_blarg.txt!");
}
5. ARRAYS IN JAVA
• Do an example on computer (example01d_arrays)
• Observations?
• Java only supports Homogeneous arrays
• We'll see with objects + inheritance you can side-step this limit.
• You have to declare size at creation time.
• No automatic support for dynamic arrays
• Linked Lists (our first data structure) have this.
• Creating / copy expanding lists is a costly operation.
6. INTRODUCING CLASSES
• Terminology
• A class is a blueprint for a new type
• An object is an instance (variable) of that type
• A class contains a description of the type:
• instance variables: each instance of the class has their own
copy of each of these.
• instance methods: each instance can call these methods (using
dot operator)
• If any instance variables are used, they belong to the calling
instance.
• class variables: The variable is associated with the class.
• There is only one copy.
• Identified with static
• class methods: Like an instance method, but can only access
class variables.
• Can be called through an instance or through the class.
6. EXAMPLE
• Do example an example (example01e_classes_intro)
• Observations?
• Access modifiers (public, private, protected, or [None])
• Limit who can access it.
• For now:
• Make classes public
• Make all callable methods public
• Make all instance / class variables protected (or private)
• Packages
• Static vs. Instances members
• Can be a bit confusing…