Universidad Nacional de Colombia

1
C
ertificación en
J
AVA

• Universidad Nacional de Colombia
• Facultad de Ingeniería
• Departamento de Sistemas

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1. LANGUAGE FUNDAMENTALS

• Source Files
• Keywords and Identifiers
• Primitive Data Types
• Literals
• Arrays
• Class Fundamentals
• Argument Passing
• Garbage collection

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Source Files

• .java extension
• At most one top-level public class definition
  (unlimited number of non-public class
  definitions)
• Class name unextended filename
• Three top-level elements (None of them required)

• Package declaration
• (package)
• import statements
• Class definitions

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Test.java

• // Package declaration
• package exam.prepguide
• // Imports
• import java.awt.Button // a class
• import java.util. // a package
• // Class Definition
• public class Test ...

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Keywords and Identifiers
Identifier word used by a programmer to name a
variable, method, class, or label

• Keywords and reserved words may not be used as
  identifiers!!
• An identifier must begin with a letter, a dollar
  sign (), or an underscore (_) subsequent
  characters may be letters, , _ , or digits

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Primitive Data Types

• Type Representation Minimum Maximum
• (bits)
• boolean 1
• char 16 0 216-1
• byte 8 -27 27-1
• short 16 -215 215-1
• int 32 -231 231-1
• long 64 -263 263-1
• float 32
• double 64

• Boolean variables true or false
• 4 signed integral data types
• byte, short, int and long

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Char 16-bits encoding Floating-point
types float and double IEEE 754 specification
Float.Nan Float.NEGATIVE_INFINITY Float.POSITIVE_I
NFINITY Double.Nan Double.NEGATIVE_INFINITY Double
.POSITIVE_INFINITY

• Nan Not a number

t
1. double d-10.0/0.0 2. if (dDouble.NEGATIVE_I
NFINITY) 3. System.out.println(d just
explodedd) 4.
t0
t1
t2
t3
t4
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Literals
A value that may be assigned to a primitive or
string variable or passed as an argument to a
method call
boolean boolean isBigtrue boolean
isLittletrue
char char cw char c \u4567
Unicode hexadecimal
t0
t1
t2
t3
t4
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SPECIAL CHARACTERS
\n new line \r return \t tab \b ba
ckspace \f formfeed \ single quote \
double quote \\ backslash
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Literals ...

• floating point
• Decimal point 1.414
• Letter E or e (scientific notation)
• 4.23E21
• Suffix f or F (float 32 bits)
• 1.828f
• Suffix D or d (double 64 bits)
• 1234d
• (default double 64 bits)

integral 28 034 (octal) 0x1c 0X1c 0x1C
String String s Characters in strings are 16
bits
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Arrays ...
An ordered collection of primitives, object
references, or other arrays
Homogeneous elements of the same type

• Declaration name and type of its elements
• Construction
• Initialization

t0
t1
t2
t3
t4
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Array declaration

• int ints
• double dubs
• Dimension dims
• float toDee

can come before or after the array name
myMethod(double dubs ) myMethod(double
dubs) double anotherMethod() double
anotherMethod()
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Arrays...
The declaration DOES NOT specify the size of an
array!! Array size is specified at runtime via
the new keyword

• int ints // Declaration to the compiler
• intsnew int25 // Run time construction

Size can be specified with a VARIABLE

• int size1152900
• int raster
• raternew intsize

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Declaration and construction in a single line

• int intsnew int25

automatic initialization when an array is
constructed
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Automatic Initialization
Element Initial Type Value
boolean false char \u0000 byte 0
short 0 int 0 long 0L float 0.0f
double 0.0d object reference null
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Initialization

• float diameters1.1f, 2.2f, 3.3f, 4.4f, 5.5f

• long squares
• squares new long6000
• for (int i0 ilt6000 i)
• squaresiii

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Class Fundamentals

• main() method
• Entry point for JAVA applications
• Application class that includes main()
• Execution java at the command line, followed by
  the name of the class

public static void main(String args)
Command line
java Mapper France Belgium
args1France args2Belgium
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Variables and initialization

• Member Variable
• When an instance is created
• Accessible from any method in the class
• Automatic (Local) Variable
• On entry to the method
• Exists only during execution of the method
• Only accessible within the method

• class HasVariables
• int x20
• static int y 30

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public int wrong() int i return
i5

• Error variable i may not have been initialized

public double fourthRoot(double d) double
result if (dgt0)
resultMath.sqrt(Math.sqrt(d))
return result
Error variable result may not have been
initialized
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Correct Initialization!!
public double fourthRoot(double d) double
result0.0 // initialize if (dgt0)
resultMath.sqrt(Math.sqrt(d))
return result
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Argument passing
A copy of the argument that gets passed
double radians1.2345 System.out.println(Sine
ofradians Math.sin(radians))

Changes to the argument value DO NOT affect the
original data
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Argument passing ...
public void bumper(int bumpMe)
bumpMe15
int xx12345 bumper(xx) System.out.println(Now
xx isxx)
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Object Reference
When an object is created, a value (bit pattern)
that identifies the object is returned
Button btn Btnnew Button(Ok)
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Button btn btn new Button(Good) replacer(btn
) System.out.println(btn.getLabel()) public
void replacer(Button replaceMe)
replaceMenew Button(Evil)
The string printed out is Good
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TextField tf tf new TextField(Yin) changer(t
f) System.out.println(tf.getText()) public
void changer(TextField changeMe)
changeMe.setText(Yang)
The string printed out is Yang, not Yin
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How to create a Reference to a primitive
public class PrimitiveReference public
static void main( String args ) int
myValue 1 modifyIt(myValue)
System.out.println(myValue contains
myValue0) public static void
modifyIt(int value) value 0

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Garbage Collection
Storage can remain allocated longer than the
lifetime of any method call

• releasing the storage when you have finished with
  it
• Corrupted data releasing storage too soon
• Memory Shortage forget to release it

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Automatic Garbage Collection The runtime system
keeps track of the memory that is allocated and
determines whether it is still usable
Garbage collector Low-priority thread
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public Object pop() return
storageindex--
public Object pop() Object
returnValuestorage index
storageindex-- null return
returnValue