java 1234319 coursewinter 2009/10. introduction object oriented, imperative programming language....

JavaJava

1234319 CourseWinter 2009/10

IntroductionIntroduction

• Object oriented, imperative programming language.• Developed: 1990.

• Inspired by C++ programming language.• Improvements:

– Syntax– General purpose libraries– Garbage collection– Platform independence with ‘Java Virtual Machine’

• Performance?

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OOP BasicsOOP Basics

• Java programs define classes, and objects that are instantiated from them.– All methods (functions) are class members. – Variables are class members, or local function variables.– No multiple inheritance

• But can implement multiple interfaces (learn in OOP course).

– Classes can be nested.

• These definitions allow us to hide away the technical details inherent to imperative programming.– set value to X, read value from Y, etc…


No Pointers in JavaNo Pointers in Java

• Another way to hide away the technical details– No pointers arithmetic, no pointers to functions, no

memory management, etc.

• Things you could do in C++ int a[10], b[10];

for (int i = 0; i < 20; i++) {

a[i] = 0;

}

• But also int a[10];

for (int i = 0; i <= 10; i++)

a[i] = some_function(i);


When will the problem be detected?

Java has a safe array accessJava has a safe array access• Yet another way to hide away the gory details• Every line like

a[i] = 7;

is compiled into:

if (i < 0 || i > 9) BOOM!

else a[i] = 7;

• BOOM is actually an IndexOutOfBoundsExceptioninstance being thrown.

• C/C++ compilers cannot add such checks to generated code.


Java performanceJava performance• Historically, Java's performance is considered slow.• Two reasons:

– Code runs on Virtual Machine– Garbage collector

• In reality, modern JVMs are very powerful, and can optimize code aggressively.

• Java programs often have slow start-up times, but after that, performance is often on-par with C++ code.


Saying Hello!Saying Hello!class Hello {

public static void main(String[] args) {

System.out.println(“Hello World”);

}

}





}

}

• The class keyword is used to define classes• By convention, class names begin with capital letter• All code and variables in Java reside in classes• If no parent was explicitly specified, the new class is

derived from the java.lang.Object class





}

}

• The method is public– Same meaning as in C++

• Unlike C++, in Java the access level is specified explicitly before each and every member variable and method





}

}

• The method is static– Same meaning as in C++– Can be accessed without a class instance

• static can be applied both to methods and member variables





}

}

• We are defining a void method– Meaning it does not return a value– Like void functions in C++, or procedures in Pascal





}

}

• The method is called “main”• Unlike in C++, there’s nothing special about the name

“main” by itself• It is only in combination with the other properties of this

method that it gets some special meaning– As the program’s starting point, of course





}

}

• Function accepts a single formal argument• The argument’s name is “args”• The argument’s type is an array of Strings





}

}

• System is the name of a class– From the java.lang package

• This class contains many basic system-related objects– Such as the console I/O objects





}

}

• The class System has a member field called out– of type PrintStream– It is static, so we can access it without an instance of theSystem class

– It is also public, otherwise we would have been unable to access it





}

}

• The out object, just like any instance of PrintStream, has a method called println

• Can you guess what this method does?


Remember simple problem?Remember simple problem?

• Given a range of positive numbers:– Summarize all numbers in range that divide by 3 or 5.– Print the result.


Version 1Version 1public class Sum {

private int sum;public Sum() {

sum = 0;}private void sumOfMatching(int begin, int end) {

for ( int i = begin ; i < end ; ++i )if ( i % 3 == 0 || i % 5 == 0 )

sum += i;}private int getSum() {

return sum;}public static void main(String[] args) {

Sum s = new Sum();s.sumOfMatching(1, 100);System.out.println( s.getSum() );

}}









}}

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Private field

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return sum;} public static void main(String[] args) {


}}

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Constructor

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}}

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Private methods

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}}

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No unsigned int

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}}

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Main function

Can ‘main’ be located in a different class?

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private int sum = 0;

private boolean isMatching(int i) {

return i % 3 == 0 || i % 5 == 0;

}

private void sumOfMatching(int begin, int end)

throws IllegalArgumentException {

if ( begin < 0 || end < begin )

throw new IllegalArgumentException();

for ( int i = begin ; i < end ; ++i )

if ( isMatching(i) )

sum += i;

}

private int getSum() { … }

public static void main(String[] args) { … }

}





return i % 3 == 0 || i % 5 == 0;

}







sum += i;

}



}

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Initializing field w/o constructor

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return i % 3 == 0 || i % 5 == 0;

}







sum += i;

}



}

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Extract method

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return i % 3 == 0 || i % 5 == 0;

}







sum += i;

}



}

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Exception declaration

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return i % 3 == 0 || i % 5 == 0;

}







sum += i;

}



}

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Throwing exception

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More improvementsMore improvements• Like in the Pascal implementation we can

change ‘3’ and ‘5’ to be given as parameters:– To sumOfMatching.– To class itself (via constructor).

• But let’s try something a bit more general and OO…– Define abstract class Matcher with abstract method isMatching.

– Subclass Matcher and implement isMatching the way we want to.


Version 3Version 3public abstract class Matcher {

public abstract boolean isMatching( int i );

}

public class TwoNumDivMatcher extends Matcher {

private int div1;

private int div2;

public TwoNumDivMatcher( int div1, int div2 ) {

this.div1 = div1;

this.div2 = div2;

}

public boolean isMatching( int i ) {

return i % div1 == 0 || i % div2 == 0;

}

}


Version 3Version 3public abstract class Matcher {

public abstract boolean isMatching( int i );

}

public class TwoNumDivMatcher extends Matcher {

private int div1;

private int div2;

public TwoNumDivMatcher( int div1, int div2 ) {

this.div1 = div1;

this.div2 = div2;

}

public boolean isMatching( int i ) {

return i % div1 == 0 || i % div2 == 0;

}

}

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We must override ‘isMatching’

(Why?)

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Version 3 – cont.Version 3 – cont.public class Sum {


private void sumOfMatching(int begin, int end, Matcher m)





if ( m.isMatching(i) )

sum += i;

}


Sum s = new Sum();

s.sumOfMatching( 1, 100, new TwoNumDivMatcher(3, 5) );

System.out.println( s.getSum() );

}

…


java 1234319 coursewinter 2009/10. introduction object oriented, imperative programming language....

Documents

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c int a10

oop course

c programming language

oop basics java programs

pointers arithmetic

class members

technical details inherent