Java Objects

Classes

What is a class?

Classes

• Collection of methods and attributes (information/values)
• Template, blueprint of cookie cutter to construct many objects

Modularity: Classes allow us to split the problem to be solved into distinct tasks

Scanner

The Scanner class (from java.util) is used to get different types of input

We use System.in to read keyboard input:

    Scanner myObj = new Scanner(System.in);  // Scanner object
    String userInput = myObj.nextLine();  // method to read user input
    System.out.println(userInput); 

For files, we need the File class (from java.io)

    File myFile = new File("myTextFile.txt");
    Scanner myReader = new Scanner(myFile);

However, we need to deal with FileNotFoundException

Scanner

import java.io.File;
import java.io.FileNotFoundException;
import java.util.Scanner;

public class FileReadExample {

    public static void main(String[] args) throws FileNotFoundException {
        File myFile = new File("myTextFile.txt");
        Scanner myReader = new Scanner(myFile);
    }

}

Scanner

We use the same Scanner method .nextLine()

String line = myReader.nextLine();

Task:

• Read a line in the file and and print it out
• Read all lines in a file and print each one out

Read file line – solution 1

import java.io.File;
import java.io.FileNotFoundException;
import java.util.Scanner;

public class FileReadExample {

    public static void main(String[] args) throws FileNotFoundException {
        File myFile = new File("myTextFile.txt");
        Scanner myReader = new Scanner(myFile);
        
        String line1 = myReader.nextLine();
        System.out.println(line1);
        
        String line2 = myReader.nextLine();
        System.out.println(line2);
        
        myReader.close();
    }

}

Read file line – solution 2

import java.io.File;
import java.io.FileNotFoundException;
import java.util.Scanner;

public class FileReadExample {

    public static void main(String[] args) throws FileNotFoundException {
        File myFile = new File("myTextFile.txt");
        Scanner myReader = new Scanner(myFile);
        
        System.out.println(myReader.nextLine());
        System.out.println(myReader.nextLine());
        
        myReader.close();
    }

}

While loop

What if we didn’t know how many lines there were in the file?

The method .hasNextLine() returns a boolean

Modify the previous solution to generalize it to any number of lines

While loop – solution

import java.io.File;
import java.io.FileNotFoundException;
import java.util.Scanner;

public class FileReadExample {

    public static void main(String[] args) throws FileNotFoundException {
        File myFile = new File("myTextFile.txt");
        Scanner myReader = new Scanner(myFile);
        
        while (myReader.hasNextLine()) {
            System.out.println(myReader.nextLine());
        }
        myReader.close();
    }

}

String

String is a class (title capitalization)

We can construct strings two ways:

String str = new String("with new");
String str2 = "Don't need new"

Some string methods:

.equals(String otherString)
.length() // len() function in python
.charAt(int index) // [] indexing in python
.substring(int beginIndex, int endIndex) // [:] indexing in python
.indexOf(String str) // .find() method in python

Try these methods out

String methods – solution

import java.io.File;
import java.io.FileNotFoundException;
import java.util.Scanner;

public class FileReadExample {

    public static void main(String[] args) throws FileNotFoundException {
        File myFile = new File("myTextFile.txt");
        Scanner myReader = new Scanner(myFile);
        
        while (myReader.hasNextLine()) {
            String line = myReader.nextLine();
            System.out.println(line.length());
            System.out.println(line.charAt(0));
            if (line.length() > 0) System.out.println(line.substring(0, 2));
            System.out.println(line.indexOf("line"));
            
        }
        myReader.close();
    }

}

Java type casting

Both Double and Integer classes have methods to cast a string into a double or int

Double.valueOf("10");
Integer.valueOf("10");