COMP201 Java Programming - Google Sites

COMP201 Java Programming

Topic 2: Java Basics

COMP201 Topic 2 / Slide 2

Objective and Outline 

Objective 



Show basic programming concepts

Outline  

What do java programs look like? Basic ingredients – Java primitive types – Variables and constants – Operators and control flow



Simple commonly used built-ins. – Simple input/output – Arrays and Strings

COMP201 Topic 2 / Slide 3

What do java program look like? public class MyProgram { public static void main(String args[]) { System.out.println(“Hello world!”); } } //File: MyProgram.java 

public: Access modifier

class: everything is inside a class  MyProgram: class name. 



matches file name.



Case sensitive

main: method of the wrapping class 



Compilation and run: javac MyProgram.java => MyProgram.class java MyProgram

COMP201 Topic 2 / Slide 4

Objective and Outline 

Outline  

What do java programs look like? Basic ingredients – Java primitive types – Variables and constants – Operators and control flow



Simple commonly used built-ins. – Simple input/output – Arrays and Strings

COMP201 Topic 2 / Slide 5

Basic Ingredients/Primitive Types 



  

Integers  byte (1 byte, -128 to 127)  short (2 bytes) For really large numbers use  int (4 bytes) BigInteger and BigDecimal  long (8 bytes) classes Floating-point types  float (4 bytes, 6-7 significant decimal digits)  double (8 bytes, 15 significant decimal digits) char (2 byte, Unicode) (ASCII 1 byte) boolean (true or false) Those are all the primitive types in Java. Everything else is an object.

COMP201 Topic 2 / Slide 6

Basic Ingredients/Primitive Types 

Legal conversions between numeric types char bytes

short

int

float 



double

Arrows indicate direction of legal and automatic conversion – double x = 123; – long x = 123456789; float y = x;



long

Solid arrow: no loss of precision Dotted arrow: might lose precision – z=1.234567E8

COMP201 Topic 2 / Slide 7

Basic Ingredients/Primitive Types char bytes

short

int

float 

double

Conversion in the opposite direction required explicit cast.  





long

double x = 9.997; int num = (int) x; int num = x; // does not compile

Can easily lead to the loss of precision (round-up errors) Cannot convert between boolean and numerical values.

COMP201 Topic 2 / Slide 8

Objective and Outline 

Outline  

What do java programs look like? Basic ingredients – Java primitive types – Variables and constants – Operators and control flow



Simple commonly used built-ins. – Simple input/output – Arrays and Strings

COMP201 Topic 2 / Slide 9

Basic Ingredients/Variables and Constants 

Variables can be declared anywhere for (int i = 0; i < 20; i++) { System.out.println(“Hi”); char ch = ‘A’; } double pi = 3.14159;



Java compilers require initialization of local variables before use public void someMethod() { … int x; // does not compile System.out.println(“The value of x is “ + x); }



Instance variables of class automatically initialized.

COMP201 Topic 2 / Slide 10

Basic Ingredients/Variables and Constants 

final marks a variable “read-only”  Variable is assigned once and cannot be changed public void someMethod() { final double pi

=

3.14159;

.. .. ..

pi = 3.14; // illegal

} 

Use static final to define constants which are available to multiple objects of the same class public class Time { static final int MinHour = 0; static final int MaxHour = 23; private int hour, minute; // these properties are set to 0 // unless overwritten by constructor … }

COMP201 Topic 2 / Slide 11

Objective and Outline 

Outline  

What do java programs look like? Basic ingredients – Java primitive types – Variables and constants – Operators and control flow



Simple commonly used built-ins. – Simple input/output – Arrays and Strings

COMP201 Topic 2 / Slide 12

Basic Ingredients/Operators 

Basically the same as C++: +, -, *, /, %, ++, --, =, ==, !=, !, &&, || =, +=, -=, *=, /=, %=,



User cannot “overload” operators; although + is overloaded to do string concatenation 

In C++: – Int x=0; x += 1; – String& String::operator+=( const String &s)..



Note that methods can be overloaded

COMP201 Topic 2 / Slide 13

Basic Ingredients /Operators No Pointers! 

No explicit pointer types (although objects are implemented as references)



No & or * operators 

In C++:   

 

int *i = (int *) malloc( 3 * sizeof(int)); *(i+1) = 2; int& y = i;

No pointer arithmetic No function pointers

COMP201 Topic 2 / Slide 14

Basic Ingredients/Control flow Basically the same as C++: if (boolean-expr) statement; (has optional else) if ( x = 0 ) //leads to compiling error

for (expr; boolean-expr; expr) statement; while (boolean-expr) statement; (do-while variant also) switch (integer-expr) case constant: statement; break;

COMP201 Topic 2 / Slide 15

Basic Ingredients/Control flow Recursion Basically the same as C++: public class Factorial { public static int factorial( int n ) { if ( n