Universidad Nacional de Colombia Facultad de Ingeniería Departamento de Sistemas

1. C ertificación en J AVA Universidad Nacional de Colombia Facultad de Ingeniería Departamento de Sistemas

2. 1. LANGUAGE FUNDAMENTALS • Source Files • Keywords and Identifiers • Primitive Data Types • Literals • Arrays • Class Fundamentals • Argument Passing • Garbage collection

3. 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

4. Test.java // Package declaration package exam.prepguide; // Imports import java.awt.Button; // a class import java.util.*; // a package // Class Definition public class Test { ... }

5. 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

6. 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 • Booleanvariables: trueorfalse • 4 signed integral data types: byte, short, intandlong

7. Char:16-bits encoding Floating-point types: floatanddouble IEEE 754 specification Nan:Not a number Float.Nan Float.NEGATIVE_INFINITY Float.POSITIVE_INFINITY Double.Nan Double.NEGATIVE_INFINITY Double.POSITIVE_INFINITY t 1. double d=-10.0/0.0; 2. if (d==Double.NEGATIVE_INFINITY) { 3. System.out.println(“d just exploded”+d); 4. } t[0] t[1] t[2] t[3] t[4]

8. Literals A value that may be assigned to a primitive or string variable or passed as an argument to a method call boolean: boolean isBig=true; boolean isLittle=true; char: char c=´w´; char c= ´\u4567´; Unicode hexadecimal t[0] t[1] t[2] t[3] t[4]

9. SPECIAL CHARACTERS ´\n’ new line ´\r’ return ´\t’ tab ´\b’ backspace ´\f’ formfeed ´\’ ’ single quote ´\” ’ double quote ´\\ ’ backslash

10. Literals ... • floating point • Decimal point : 1.414 • Letter E or e (scientific notation): • 4.23E+21 • 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”;

11. 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 t[0] t[1] t[2] t[3] t[4]

12. 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() doubleanotherMethod() []

13. Arrays... The declarationDOES NOTspecify the size of an array!! Array size is specified atruntimevia thenewkeyword int ints[]; // Declaration to the compiler ints=new int[25]; // Run time construction Size can be specified with aVARIABLE int size=1152*900; int raster[]; rater=new int[size];

14. Declaration and construction in a single line int ints[]=new int[25]; automaticinitialization when an array is constructed

15. 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

16. Initialization float diameters[]={1.1f, 2.2f, 3.3f, 4.4f, 5.5f}; long squares[]; squares =new long[6000]; for (int i=0; i<6000; i++) squares[i]=i*i;

17. Class Fundamentals main() method • Entry point for JAVA applications • Application:class that includesmain() • 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 args[1]=“France” args[2]=“Belgium”

18. 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 x=20; • static int y =30;

19. public int wrong() { int i; return i+5; } Error:“variablei may not have been initialized” public double fourthRoot(double d) { double result; if (d>=0) { result=Math.sqrt(Math.sqrt(d)); } return result; } Error:“variableresultmay not have been initialized”

20. Correct Initialization!! public double fourthRoot(double d) { double result=0.0; // initialize if (d>=0) { result=Math.sqrt(Math.sqrt(d)); } return result; }

21. Argument passing Acopyof the argument that gets passed double radians=1.2345; System.out.println(“Sine of”+radians +”=“+Math.sin(radians)); Changes to the argument valueDO NOTaffect the original data

22. Argument passing ... public void bumper(int bumpMe) { bumpMe+=15; } int xx=12345; bumper(xx); System.out.println(“Now xx is”+xx);

23. Object Reference When an object is created, a value (bit pattern) that identifies the object is returned Button btn; Btn=new Button(“Ok”);

24. Button btn; btn = new Button(“Good”); replacer(btn); System.out.println(btn.getLabel()); public void replacer(Button replaceMe) { replaceMe=new Button(“Evil”); } The string printed out is”Good”

25. TextField tf; tf = new TextField(“Yin”); changer(tf); System.out.println(tf.getText()); public void changer(TextField changeMe) { changeMe.setText(“Yang”); } The string printed out is”Yang”,not”Yin”

26. 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 “+ • myValue[0]); • } • public static void modifyIt(int [] value) { • value [0]++; • } • }

27. 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 storagetoo soon • Memory Shortage:forgetto release it

28. 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

29. public Object pop() { • return storage[index--]; • } public Object pop() { Object returnValue=storage [index]; storage[index--] =null; return returnValue; }