Java - Trang ď Chap02

Programming Java

Classes & Methods

Incheon Paik

Java

1

Computer Industry lab.


Contents
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Java

The Structure of Method
Static Methods & Static Variables
Instance Methods & Variables

The Integer Class
The New Operator
Garbage Collection
Other Wrapper Classes
The StringBuffer Class
Arrays of Objects
Command-Line Arguments
The System Class

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Computer Industry lab.


The Structure of Method
Method
retType mthName(paramList) {
}

void displayInts (int array[])
{
}

Exception Definition of Method
retType mthName(paramList) throws exceptionList {
}

int parse(String s) throws Exc
eptionX {
}

Method Invocation
mthName(args);

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Static Methods & Variables
Invocation of Static Method
Clsname.mthName(args);

double x = Math.sin(3.12);

Access to Static Variable
clsName.varName

Class Aclass {

Object x

static int a;
int b;
… … ..

Java

Object y

a

same

a

b

different

b

}

x.a = 10; x.b = 20;

Class Aclass x = new Aclass();

y.a = 30; y.b = 40;

Class Aclass y = new Aclass();

Now, x.a = ?
4

x.b = ?
Computer Industry lab.

Static Methods & Variables
class AreaCircumference {
public static void main(String args[]) {
System.out.println(“Max of –8 and –4 is “ + Math.max(-8, -4));
System.out.println(“Min of –8 and –4 is “ + Math.min(-8, -4));
double radius = 5;
double area = Math.PI * radius * radius;
double circumference = 2 * Math.PI * radius;
System.out.println(“Radius is “ + radius);
System.out.println(“Area is “ + area);
System.out.println(“Circumference is “ + circumference);
}
}

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Instance Methods & Variables
Invocation of Instance Method
double x = myObjA.somemth();

objRef.mthName.mthName(args);
class DollarAmount {
public static void main(String args[]) {
String s1 = “The total cost is $45.67”;

int i1 = s1.indexOf(‘$’);
String s2 = s1.substring(i1);

Find out this position
Get substring after ‘$’ character

System.out.println(s2);
} // end of main
} // end of class

Java

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The Integer Wrapper Class
Wrapper Class : Corresponding class to the basic type: Number, Integer,
Float, Double,Boolean, Character, Byte, Short, Long
class StringToInt {
public static void main(String args[]) {
String s = “125”;
Integer obj = Integer.valueOf(s);
int i = obj.intValue();
i += 10;
System.out.println(i);
} // end of main

135

} // end of class

Java

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Computer Industry lab.


The New Operator
new Operator
clsName objRef = new clsName(args);

MyClass x = new MyClass();

class IntegerTest {
public static void main(String args[]) {
Integer iobj1 = new Integer(5);
int i1 = iobj1.intValue();
i1 = 5

System.out.println(“i1 = “ + i1);
System.out.println(iobj1);

5

} // end of main
} // end of class

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Computer Industry lab.


Garbage Collection
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Garbage Collector
? Automatic Memory Management

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finalize Method
? Call the finalize method before the garbage collector reclaim
the memory
Provide the method to release the resources
? Programmer can remove the resources(ex:open files)
directly using finalize method which garbage collector
cannot reclaim.

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Garbage Collection
protected
protectedvoid
voidfinalize()
finalize()throws
throwsThrowable
Throwable{{
////......
}}
public class GcTest{
static {
System.out.println("Initialize");
}
protected void finalize() throws Throwable {
System.out.println("Clean Up");
}

How can we display Clean Up
public static void main(String args[]) {
message? => In Exercise
System.out.println("In main .... ");
GcTest x = new GcTest();
x = null;
System.gc(); // We need to call this point to see the "Clean Up"
}
}
Java

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Computer Industry lab.


The Double Wrapper Class
class DoubleDemo {
public static void main(String args[]) {
// variable initialize
double d1 = 0;
double d2 = 0;
double d3 = d1 / d2;

Result :

System.out.println(d3);

NaN

System.out.println(Double.isInfinite(d3));

False

System.out.println(Double.isNaN(d3));

True

System.out.println(Double.MAX_VALUE);

1.797E308

System.out.println(Double.MIN_VALUE);

4.9E-324

System.out.println(Double.POSITIVE_INFINITY);
System.out.println(Double.NEGATIVE_INFINITY);

Infinity
-Infinity

}
}

Java

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The StringBuffer Class
StringBuffer Constructor
StringBuffer()
StringBuffer(int size)
StringBuffer(String s)
class StringBufferDemo {
public static void main(String args[]) {
StringBuffer sb1 = new StringBuffer();
StringBuffer sb2 = new StringBuffer(30);

StringBuffer sb3 = new StringBuffer(“abcde”);
System.out.println(“sb1.capacity = “ + sb1.capacity());

16

System.out.println(“sb1.length = “ + sb1. length());

0

System.out.println(“sb2.capacity = “ + sb1.capacity());

30

System.out.println(“sb2.length = “ + sb1. length());

0

System.out.println(“sb3.capacity = “ + sb1.capacity());
System.out.println(“sb3.length = “ + sb1. length());

21

What will be append()
and reverse() ?

5

} // end of main
} // end of class

Java

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Arrays of Objects
Declaration of Array
Make the Space for Array Element
class StringArray {
public static void main(String args[]) {
String array[] = new String[5];
array[0] = “String 0”;
array[1] = “String 1”;
System.out.println(array.length);
System.out.println(array[0]);
System.out.println(array[1]);
} // end of main
} // end of class

Java

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Command-Line Argument
Declaration of Array

Make the Space for Array Element
class CommandLineArguments {

java CommandLineArguments 1 2 abcde

public static void main(String args[]) {
System.out.println(args[0]);
System.out.println(args[1]);
System.out.println(args[2]);
} // end of main
} // end of class

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The System Class
Runtime Environment

System.out.println( ****)
Static variable of PrintStream

Exit() Method
void exit(int code)

currentTimeMillis() Method
long currentTimeMillis()

Method Invocation
Void arraycopy(Object source, int sourceIndex,
Object destination, int destinationIndex, int size)
Java

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Exercise
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Step 1 (Read Input)
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C Language
scanf(“%s”, name);

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Java
BufferedReader charStream = New BufferedReader(new
InputStreamReader(System.in));
String data = charStream.readLine();

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Step 2 (Read Input Double)

import java.io.*; ? You need to write this statement at top of your program
double d = Double.valueOf(charStream.readLine().trim()).doubleValue();

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Step 3 Using System.in.read() and StringBuffer
StringBuffer buf = new StringBuffer();
String
while ((c = (char) System.in.read()) != '\r')
buf.append(c);
RandomAccessFile file = new RandomAccessFile(buf.toString(), "rw");

Java

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Exercise
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Step 4 (OptionPane of Swing)
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Step 5 (Garbage Collection)
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Java

Show Dialog and Get Input from GUI

Refer to this slide #9,10

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Computer Industry lab.