http://proglit.com/
the java
language
SA
BY
introduced in 1995 by Sun Microsystems
• imperative, object-oriented
• C-style syntax
• statically-typed (mostly)
• no pointers
• JVM (Java Virtual Machine)
• garbage collection
• exceptions
• SE (standard edition)
• EE (enterprise edition)
• ME (micro edition)
Java 6 (2006)
Java 7 (2010?)
class (data type definition)
• field (data member)
• method (function member)
object / instance (piece of data)
class Moose {
Rat r;
Hamster h;
void foo()
{…}
}
encapsulation
(methods act as “interface” to object’s fields)
object.field
object.method(args)
apple.banana
apple.banana.orange
nadine.ed()
nadine.ed().laura
nadine.ed().laura.dale()
object.field
object.method(args)
apple.banana
(apple.banana).orange
nadine.ed()
(nadine.ed()).laura
((nadine.ed()).laura).dale
()
type name;
new type(args)
Moose m;
// null
m = new Moose();
Moose m = new Moose();
class Moose {
Rat r;
Hamster h;
Moose() {…}
constructor
void foo() {…}
}
//
Moose() {
this.r = new Rat();
this.h = new Hamster();
}
Moose(Hamster h, Rat r) {
this.r = r;
this.h = h;
}
new Moose(new Hamster(), new
Rat())
inheritance
Animal
A
B
Mammal
D
E
Cat
F
G
C
A
D
B
E
C
A
B
C
Ted
Milton
Kate
illegal
Object
Jack
Ted
Kate
Samantha
Milton
Brad
Oliver
Lisa
class Terry {…}
Object
// extends
class John extends Ben {…}
Jack
Ted
Milton
Kate
illegal
is-a
vs.
has-a
Think before
using inheritance.
Object
Jack
(probably)
Ted
bad
Lisa
Kate
Mike
Hugh
Olivia
Uri
Amber
Hamster h = new Hamster();
Mammal m = h;
h.runOnWheel();
m.runOnWheel();
illegal
// OK
//
void bar(Mammal m) {…}
x.bar(new Mammal())
x.bar(new Hamster())
Mammal foo() {
if (…) {
return new
Mammal();
} else {
return new
Hamster();
}
}
(type) expression
Mammal m = (Mammal) new Hamster(); //
upcast
(type) expression
Mammal m = new Hamster(); // implicit
upcast
Hamster h = (Hamster) m;
Hamster h = m;
// downcast
// illegal
Lamp l = (Lamp) new Duck (); //
illegal
Object
Jack
overriding
(redefining an
inherited method)
void
foo()
Ted t;
…
t.foo();
Ted
Lisa
Kate
Mike
void
foo()
Kate k;
…
k.foo();
? x;
if (…) {
x = new
Nick();
} else {
x = new
Diane();
}
x.foo();
Object
Nick
void
foo()
Diane
void
foo()
Object x;
if (…) {
x = new
Nick();
} else {
x = new
Diane();
}
x.foo();
//
illegal
Object
Nick
Diane
void
foo()
void
foo()
interface Philip {
void foo();
}
class Diane implements
Philip {…}
class Nick implements Philip
{…}
Philip x;
if (…) {
x = new
Nick();
} else {
x = new
Diane();
}
x.foo();
//
legal
Object
Nick
Diane
void
foo()
void
foo()
Philip x = new
Ian();
Object
Ian
Nick
Diane
void
foo()
void
foo()
1. Compiler checks compile time type.
2. Method invoked depends upon runtime type…
3. …except this always invokes version of own class.
Mary
Leo
void
foo()
void
bar()
void
foo()
void bar() {
this.foo();
}
Leo l = new Leo();
l.bar();
1.
2.
3.
4.
Compiler checks compile time type.
Method invoked depends upon runtime type…
…except this always invokes own class’s version…
…and super always invokes inherited version.
Ryan
Heather
void
foo()
void
foo()
void
ack()
void ack() {
super.foo();
}
Heather h = new
Heather();
h.ack();
Ryan
Heather
void
foo()
void
foo()
void
ack()
void foo() {
super.foo();
…
}
void foo(Rat r) {
r.bar(this);
}
void foo(Rat r) {
r.bar(super);
illegal
}
//
primitive types
byte
short
int
long
1-byte signed integer
2-byte signed integer
4-byte signed integer
8-byte signed integer
char
2-byte unsigned integer
boolean
float
double
floating-point
true and false
single-precision floating-point
double-precision
BigInteger
BigDecimal
(arbitrary-precision integer)
(arbitrary-precision decimal)
reference types:
• variable holds a reference
• == tests identity
• cast appeases compiler
primitive types:
• variable holds a value
• == tests equality
• cast produces a new value
int i = 60;
float f = 5.4;
i = (int)f;
f = (float)i;
int i = 5;
byte b = 3;
b = (byte)i;
i = b;
// implicit
cast
static member
(a member which is not a member)
class Ian {
static Fran f;
Ian.f
static void foo() {…}
Ian.foo()
…
//
//
class Ian {
static Fran f;
//
Ian.f
static void foo() {…} //
Ian.foo()
…
Ian i = new Ian();
}
i.foo();
i.f = new Fran();
1.
2.
3.
4.
5.
Compiler checks compile-time type.
Method invoked depends upon runtime type…
…except this always invokes own class’s version…
…and super always invokes inherited version…
…and static determined solely by compile-time type.
package name;
package shark;
package pig.tiger;
package pig.tiger;
shark.Ant h = new shark.Ant();
package shark;
Ant h = new Ant();
pig.tiger.Cow h = new
pig.tiger.Cow();
package shark;
import pig.tiger.Cow;
import pig.tiger.Lemur;
Lemur l = new Lemur();
Cow c = new Cow();
java.lang
java.lang.Object
java.lang.String
String s = “hello”;
int l = s.length();
s = s.toUpperCase();
“Hello, ” + “Ron”
Ron”
5 + “ golden rings”
// 5
// “HELLO”

“Hello,

“5 golden
visibility
public
protected *
subclasses)
default
private *
(everywhere)
(same package +
(same package)
(same class)
*
applicable only to members
overloading
(same name, different method)
Sean
void
void
byte
void
foo()
foo(Mammal m, Lamp l)
foo(int a)
foo(Lamp l, Mammal m)
Sean s = new Sean();
s.foo(new Lamp(), new
Mammal());
s.foo(35);
overloading
(same name, different method)
Sean
void foo()
void foo(Mammal m, Lamp
byte foo(int a)
//
void foo(Lamp l, Mammal
float foo(int b)
//
l)
conflict
m)
conflict
Sean s = new Sean();
s.foo(35); // ambiguous
overloading
(same name, different method)
Sean
void
void
byte
void
char
foo()
foo(Mammal m, Lamp l)
foo(int a)
foo(Lamp l, Mammal m)
foo(Lamp l, Animal m)
Sean s = new Sean();
s.foo(new Lamp(), new
Hamster());
overloading
(same name, different method)
Sean
void
void
byte
void
char
foo()
foo(Mammal m, Lamp l)
foo(int a)
foo(Lamp l, Mammal m)
foo(Lamp l, Animal m)
Sean s = new Sean();
s.foo(new Lamp(), (Animal) new
Hamster());
1.
2.
3.
4.
5.
Compiler checks compile-time type and picks overload.
Method invoked depends upon runtime type…
…except this always invokes own class’s version…
…and super always invokes inherited version…
…and static determined solely by compile-time type.
Jill
Donald
void foo()
void foo(int a)
(overload)
void foo()
(override)
Jill
Donald
void foo()
void foo(int a)
(overload)
char foo()
(illegal)
Moose(Hamster h, Rat r) {
this.r = r;
this.h = h;
}
Moose() {
this.r = new Rat();
this.h = new Hamster();
}
new Moose(new Hamster(), new Rat())
Moose(Hamster h, Rat r) {
this.r = r;
this.h = h;
}
Moose() {
this(new Hamster(), new Rat());
}
Moose(Hamster h, Rat r) {
this.r = r;
this.h = h;
}
Moose() {
// not what we want
new Moose(new Hamster(), new Rat());
}
Moose(Hamster h, Rat r) {
this.r = r;
this.h = h;
}
Moose() {
// sensical, but not legal Java
Moose(new Hamster(), new Rat());
}
Moose(Hamster h, Rat r) {
this.r = r;
this.h = h;
}
Moose() {
this(new Hamster(), new Rat());
}
Moose(Hamster h, Rat r) {
super(); // invoke constructor of
Moose’s parent
this.r = r;
this.h = h;
}
Moose() {
this(new Hamster(), new Rat());
}
Object
Jack
Ted
Kate
new Kate()
constructor rules
1. Constructors invoked only via new, this(), or
super().
2. Calls to this() and super() only can be first line.
3. The first line is always this() or super().
4. Default first line is super() with no arguments.
// infinite recursion
Moose(Hamster h, Rat r) {
this();
}
Moose() {
this(new Hamster(), new
Rat());
}
constructor rules
1. Constructors invoked only via new, this(), or
super().
2. The first line is always this() or super().
3. Calls to this() and super() only go in first line.
4. Default first line is super() with no arguments.
5. A this() call cannot be recursive.
6. To return, always just use return;
Object
exceptions
Throwable
Exception
RuntimeException
Error
Cat c;
…
c.meow(); // exception if c is
null
Cat c;
…
try {
c.meow(); // exception if c is
null
} catch (NullPointerException e) {
…
}
Object
Throwable
Exception
RuntimeException*
Error*
*unchecked
// illegal
void bar() {
if (…) {
throw new Leo();
exception
}
}
// checked
// OK
void bar() throws Leo {
if (…) {
throw new Leo(); // checked
exception
}
}
// OK
void bar() {
try {
if (…) {
throw new Leo(); // checked
exception
}
} catch (Leo e) {
…
}
}
void bar() throws FooException,
AckException {…}
void bar() throws Exception {…}
arrays
(fixed-sized, homogenous collection)
Apple[]
Apple ref
Apple object
Apple ref
Apple ref
Apple object
Apple ref
Fuji object
Mammal[] m;
m = new Mammal[3];
m[2] = new Mammal();
m[0] = new Cat();
int i = m.length;
// 3
Object
Object[]
Lisa
Lisa[]
Jerry
Wyatt
Jerry[]
Natalie
Natalie[]
Wyatt[]
Cat[] c = new Cat[3];
Mammal[] m = c;
Object[] o1 = c;
Object o2 = c;
Cat[] c = new Cat[3];
Mammal[] m = (Mammal [])
c;
Object[] o1 = (Object [])
c;
Object o2 = (Object) c;
c = (Cat[]) o2;
Mammal[] m = new Cat[4];
m[0] = new Mammal(); //
exception
Mammal[] m = new Cat[4];
m[0] = new Cat();
Cat mittens = m[0]; // compile
error
Mammal[] m = new Cat[4];
m[0] = new Cat();
Cat mittens = (Cat) m[0];
OK
//
Object
Object[][]
Lisa
Lisa[][]
Jerry
Wyatt
Jerry[][]
Natalie
Natalie[][]
Wyatt[][]
Apple[][]
Apple[] ref
Apple[] object
Apple[] ref
Apple[] ref
Apple[] ref
Fuji[] object
Cat[][] c = new Cat[3][];
c[1] = new Cat[5];
c[1][0] = new Cat();
Cat[][] c = new Cat[3][];
c[1] = new Cat[5];
(c[1])[0] = new Cat();
Object
Object[][][]
Lisa
Lisa[][][]
Jerry
Wyatt
Jerry[][][]
Natalie
Natalie[][][]
Wyatt[][][]
Apple[][][]
Apple[][] ref
Apple[][] object
Apple[][] ref
Apple[][] ref
Apple[][] ref
Fuji[][] object
Object
Object[]
Object[][]
Object[][][]
Object[][][][]
int[]
int
int
int
int
int[][]
int[] ref
int[] object
int[] ref
int[] ref
int[] ref
int[] object
Object
int[]
byte[][]
char[][][]
float[]
Hello, world!
public class HelloWorld {
public static void main(String[] args)
{
System.out.println(“Hello,
world!”);
}
}
•
•
•
•
•
•
•
•
inner classes
generics
abstract classes
wrapper classes
enumerations
annotations
assert
final
http://proglit.com/