unmigratedcomposition-wiki-markupsetup

Below

...

is

...

a

...

short

...

tutorial

...

designed

...

for

...

someone

...

who

...

is

...

familiar

...

with

...

programming

...

(potentially

...

only

...

at

...

the

...

level

...

of

...

MATLAB),

...

and

...

has

...

never

...

learned

...

or

...

long

...

forgotten

...

Java.

...

After

...

explaining

...

the

...

very

...

basics

...

of

...

classes

...

and

...

objects,

...

we

...

skip

...

all

...

of

...

the

...

syntax

...

and

...

control

...

flow

...

and

...

go

...

straight

...

to

...

what

...

you

...

will

...

need

...

to

...

complete

...

the

...

workshop.

...

If

...

your

...

Java

...

is

...

not

...

strong,

...

please

...

take

...

the

...

time

...

to

...

understand

...

this

...

tutorial,

...

otherwise

...

you

...

will

...

most

...

likely

...

be

...

lost

...

during

...

the

...

workshop.

...

It

...

should

...

take

...

about

...

30 minutes.

Java Basics: Classes, Objects, Fields, Methods, and the Keyword Static

In Java, all code is associated with a class. Classes have fields and methods. Instances of classes are created with Constructors and called Objects. Once an Object has been constructed, its methods can be called and its fields can be accessed. Here is a concrete example with two simple classes:

Section

border	false

Column

minutes. h1. What are classes and objects? In Java, all code is associated with a class. Classes have _fields_ and _methods_. _Instances_ of classes are created with _Constructors_ and called _Objects_. Once an _Object_ has been constructed, its methods can be called and its fields can be accessed. Here is a concrete example with two simple classes: {section:border=false} {column}{column} {column:width=300px} {code:title=Cat.java}

Column

width	300px

Code Block

title	Cat.java


public class Cat {
  private int age;
  private String name;
  private Cat friend;

  public Cat(String name, int age) {
    this.name = name;
    this.age = age;
    friend = null;
  }

  public String getName() {
    return name;
  }

  public Cat getFriend() {
    return friend;
  }

  public void setFriend(Cat friend) {
    this.friend = friend;
  }

  public String sayHello(String aboutMe) {
    return "Hello World!  My name is " + 
      name + " and I am " + age + 
      " years old." + " I am " + aboutMe + ".";
  }
}

{code} {column:width=10px}{column} {column:width=500px} {code:title=CatsTheMusical.java}

Column

width	10px

Column

width	500px

Code Block

title	CatsTheMusical.java


public class CatsTheMusical {

  public static void main(String[] args) {
    Cat mist = new Cat("Mr. Mistoffelees", 8);
    System.out.println(mist.sayHello("magical"));
    Cat garf = new Cat("Garfield", 12);
    System.out.println(garf.sayHello("tired/hungry"));
    mist.setFriend(garf);
    System.out.println(mist.getName() + " and " + garf.getName()
        + " are friends? " + CatsTheMusical.areFriends(mist, garf));
    garf.setFriend(mist);
    System.out.println(mist.getName() + " and " + garf.getName()
        + " are friends? " + CatsTheMusical.areFriends(mist, garf));
  }

  public static boolean areFriends(Cat cat1, Cat cat2) {
    if (cat1.getFriend() != null && cat2.getFriend() != null) {
      return cat1.getFriend() == cat2 && cat2.getFriend() == cat1;
    }
    return false;
  }
}

Column

If you understand this code, particularly how the keywords this and static work, and you can predict the output as shown below,

Toggle Cloak

id	Cat Code Output

Cat Code Output

Cloak

visible	false
id	Cat Code Output

then you probably don't need to expand the section below and can continue.

Toggle Cloak

id	Cat Code Explained

Cat Code Explained

Cloak

visible	false
id	Cat Code Explained

First, lets talk about the contents of the files Cat.java.

{code} {column} {column}{column} {section} Lets make sure we understand how this works. First, lets talk about the contents of the files {{Cat.java}}. * _Fields_: The class {{Cat}} has three fields, {{name}}, {{age}} and {{friend}}. Each field has a _type_, which ensures that we can only store a {{String}} in {{name}}, an {{int}} in {{age}} and another {{Cat}} in {{friend}}. The word {{private}} before each field indicates that the field can only be accessed from within the class {{Cat}}. * _Constructors_: The class {{Cat}} has one constructor {{public Cat(String name, int age){...\}}}. In the constructor, the fields {{name}} and {{age}} are initialized with the values given in the arguments, while {{friend}} is simply set to {{null}}. Because the constructor is public, it can be invoked from other classes. An example of the invoking the constructor can be seen in {{CatsTheMusical.java}}. * _Methods_: The class {{Cat}} has four methods, {{

Fields: The class Cat has three fields, name, age and friend. Each field has a type, which ensures that we can only store a String in name, an int in age and another Cat in friend. The word private before each field indicates that the field can only be accessed from within the class Cat.
Constructors: The class Cat has one constructor public Cat(String name, int age){...}. In the constructor, the fields name and age are initialized with the values given in the arguments, while friend is simply set to null. Because the constructor is public, it can be invoked from other classes. An example of the invoking the constructor can be seen in CatsTheMusical.java.
Methods: The class Cat has four methods,

getName(){...

\

}

}}

,

{{

getFriend(){...

\}}

},

{{

setFriend(

...

Cat friend){...

\}}

}

and

{{

sayHello(

...

String aboutMe){..

\}

.}

}

.

The

keyword

{{

public

}}

indicates

that

the

methods

can

be

called

outside

of

the

class

{{

Cat

}}

.

The

second

word

in

each

method

is

the

_

return

type

_

,

or

the

_

type

_

of

the

result

of

the

method.

Notice

that

{{

setFriend(...)

}}

has

a

return

type

of

{{

void

}}

,

indicating

that

nothing

is

returned.

The

first

two

methods

are

examples

of

_

getter

_

methods,

as

they

simply

retrieve

the

value

of

a

private

field.

The

third

method

is

a

_

setter

_

method,

which

allows

the

field

{{

friend

}}

to

be

changed

from

outside

the

class

{{

Cat

}}

.

Now

lets

talk

about

the

other

file

{{

CatsTheMusical.java

}}. * _Static Methods_: Methods that are {{static}}, while associated with the class they are written in, are not associated with any particular instance. They can be invoked by using{{

.

Static Methods: Methods that are static, while associated with the class they are written in, are not associated with any particular instance. They can be invoked by using [class-name].[method]

}}

.

For

example,

see

the

invocation

of

{{

areFriends(...)

}}

within

{

main(...)

}

.

* _

main(...)

_

:

All

Java

program

must

start

with

the

code

{{

public

static

void

main(String[]

args){...

\}}

}.

From

here,

we

will

follow

the

methods

and

constructors

into

other

class

files.

The

arguments,

{{

String[]

args

}}

can

be

accessed

by

calling

{{

args[0]

}}

etc.

and

can

be

set

by

going

to

the

top

menu

bar

and

then

selecting

_

Run

→

Run

Configurations...

_

,

then

selecting

the

_

Arguments

_

tab

in

the

center

and

filling

out

the

_

Program

Arguments

_

box.

Now

that

we

have

the

basic

terminology

down,

lets

examine

some

of

the

finer

points

which

may

be

confusing

to

non-Java

programmers:

*

Given

an

instance

of

a

class

(e.g.

for

{{

Cat

c

=

new

Cat(...)

}}

,

{{

c

}}

is

an

instance),

non-static

methods

from

the

class

can

be

called

by

\

[instance

name

\

].

\

[method

name

\

]

(e.g.

{{

c.getName()

}}

).

Similarly,

non-static

fields

can

be

accessed

by

calling

\

[instance

name

\

].

\

[field

name

\

],

(e.g.

{{

c.name

}}

).

However,

methods

or

fields

labeled

{{

private

}}

can

only

be

called/accessed

from

code

written

within

their

own

class

file.

*

Within

a

constructor,

we

can

directly

access

the

fields

of

a

class

by

name

without

reference

to

any

particular

instance,

as

it

is

implied

that

the

instance

is

the

one

that

is

being

built.

For

example,

in

the

{{

Cat

}}

constructor,

we

make

the

assignment

{{

friend

=

null

}}

.

However,

when

there

are

argument

variables

with

the

same

name

as

the

field

variables,

they

take

precedence

and

_

shadow

_

the

fields

variables.

To

reference

the

fields

once

they

have

been

shadowed,

you

need

to

use

the

keyword

{{

this

}}

.

In

fact,

you

can

use

the

keyword

{{

this

}}

whenever

you

want

to

refer

to

a

field

or

method

of

the

instance

of

the

class

being

built

by

the

constructor,

regardless

of

whether

there

is

shadowing.

Analogous rules apply for fields and methods inside other non-static methods. * In static methods, we cannot refer to any non-static fields or non static methods of the class without also providing an instance, as static methods are not associated with any instance. Similarly, the keyword this cannot be applied.

The same rules when you are inside a non-static method instead of inside a constructor.
In static methods, we cannot refer to any non-static fields or non static methods of the class without also providing an instance, as static methods are not associated with any instance. Similarly, the keyword this cannot be applied.

Now you are ready for some more advanced Java!

Types, Primitives, and Generics

TODO... but until then

Try here for generics, up to Generics, Inheritance, and Subtypes.
Autoboxing http://docs.oracle.com/javase/tutorial/java/data/autoboxing.html

Inheritance, Abstract Classes, Interfaces, and Inner Classes

TODO... but for now

Things in here are all pretty important http://docs.oracle.com/javase/tutorial/java/IandI/index.html

Data Structures

TODO... but for now

The Java Collections documentation http://docs.oracle.com/javase/tutorial/collections/index.html
- Interfaces: Sets, Lists, and Maps
- Implementations: HashSet, ArrayList, HashMap
Guava
- BiMap
- Immutable Collections

Child pages

Versions Compared

Old Version 22

New Version Current

Key

Java Basics: Classes, Objects, Fields, Methods, and the Keyword Static

Types, Primitives, and Generics

Inheritance, Abstract Classes, Interfaces, and Inner Classes

Data Structures

Child pages

Page History

Versions Compared

Old Version 22

New Version Current

Key

Java Basics: Classes, Objects, Fields, Methods, and the Keyword Static

Types, Primitives, and Generics

Inheritance, Abstract Classes, Interfaces, and Inner Classes

Data Structures