Oop lecture9 11

Lecture 11
File IO operations in Java

Object Oriented Programming
Eastern University, Dhaka
Md. Raihan Kibria

List all files/folders under a
directory
import java.io.File;

public class DirDemo {

static final String DIR_NAME = "/home/user";

public static void main(String[] args) {
File file = new File(DIR_NAME);
String[] files = file.list();
for (String f : files){
System.out.println(f);
}
}
}

java.io.File is the class of interest

Output
oop_lecture1.ppt
oop_lecture7.ppt
oop_lecture9_10.ppt
oop_lecture9.ppt
oop_lecture3.ppt
com
oop_lecture9_11.ppt
oop_lecture4.ppt
oop_lecture6.ppt
oop_lecture5.ppt
oop_lecture8.ppt

How many are folders and how
many are files under a directory
public class FileDirCheckDemo {

static final String DIR_NAME = "/home/user";

File file = new File(DIR_NAME);
String[] files = file.list();
int countOfDir = 0;
int countOfFile = 0;
for (String f : files){
File fileReal = new File(DIR_NAME + File.separator + f);
if (fileReal.isDirectory())
countOfDir++;
else
countOfFile++;
}
System.out.println("No of directories: " + countOfDir + "; no of
files: " + countOfFile);
}
} Output:

No of directories: 2; no of files: 18

Other useful methods on File

Create a new File:
File f = new File("D:tempnewfile.txt");
f.createNewFile();

Delete a File:
f.delete();

Rename a File:
File renF = new File("D:tempnewfile.txt");
f.renameTo(renF);

Copy a file to another
public class FileDemo {

static final String FIRST_FILE_NAME= "/home/user/raihan_fb.png";
static final String SECOND_FILE_NAME = "/home/user/out.png";

try{
File file1 = new File(FIRST_FILE_NAME);
File file2 = new File(SECOND_FILE_NAME);

byte[] b = new byte[1024];
FileInputStream fis = new FileInputStream(file1);
FileOutputStream fos = new FileOutputStream(file2);
int len = 0;
while ((len = fis.read(b)) != -1){
fos.write(b, 0, len);
}
fis.close();
fos.close();
}catch(Exception e){

}
}
}

Reading a text file and output to
console
public class ReadTextDemo {

static final String FILE_NAME = "/home/user/test.txt";

File file = new File(FILE_NAME);
try{
BufferedReader br = new BufferedReader(new FileReader(file));
String s = null;
while ( (s = br.readLine()) != null){
System.out.println(s);
}
br.close();
e.printStackTrace();
}
}
}

Output

text.txt: Console output:

Apples are red Apples are red
Coconuts are green
Coconuts are green Grapes are sour
Grapes are sour

Interface revisited
Two utility methods

public class InterfaceDemo {

static final String FILE_NAME = "/home/user/data.txt";

public static int getTotalVolume(List<Volume>items){
int total = 0;
for (Volume v : items){
total += v.getVolume();
}
return total;
}

public static int getTotalWeight(List<Weight>items){
int total = 0;
for (Weight w : items){
total += w.getWeight();
}
return total;
}

List<Volume>vols = new ArrayList<Volume>();
List<Weight>wts = new ArrayList<Weight>();
File file = new File(FILE_NAME);
try{
BufferedReader br = new BufferedReader(new FileReader(file));
String s = null;
while ( (s = br.readLine()) != null){
String[] split = s.split(",");
if (split[0].equals("Book")){
Book book = new Book();
book.width = Integer.parseInt(split[1]);
book.height = Integer.parseInt(split[2]);
book.depth = Integer.parseInt(split[3]);
vols.add(book);
wts.add(book);
}else {
Container c = new Container();
c.radius = Integer.parseInt(split[1]);
c.height = Integer.parseInt(split[2]);
vols.add(c);
wts.add(c);
}
}
br.close();

System.out.println("Total volume: " + getTotalVolume(vols));
System.out.println("Total weight: " + getTotalWeight(wts));
e.printStackTrace();
}
}
}

Two interfaces
interface Volume{
int getVolume();
}

interface Weight{
int getWeight();
}

data.txt:

Book,10,20,3
Book,20,40,2
Container,20,40
Container,30,50

class Book implements Volume, Weight{
int id;
int width;
int height;
int depth;

public int getVolume() {
return width * height * depth;
}

public int getWeight() {
return getVolume() * 2;
}
}

class Container implements Volume, Weight{
int id;
int radius;
int height;

public int getVolume() {
return (int)(Math.PI * radius * radius * height);
}

public int getWeight() {
return getVolume() * 5;
}
}

Output
Total volume: 193836
Total weight: 962580

Lessons learned

Polymorphism: an object can take more than
one forms. For example, getTotalVolume()
sees the items as of type Volume;
getTotalWeight() sees the items as of type
Weight

Oop lecture9 11

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