Text handling in java

Text handling in java

• Introduction to Java
• The complete History of Java Programming Language
• C++ vs Java vs Python
• How to Download and Install Java for 64 bit machine?
• Setting up the environment in Java
• How to Download and Install Eclipse on Windows?
• JDK in Java
• How JVM Works – JVM Architecture?
• Differences between JDK, JRE and JVM
• Just In Time Compiler
• Difference between JIT and JVM in Java
• Difference between Byte Code and Machine Code
• How is Java platform independent?

• Decision Making in Java (if, if-else, switch, break, continue, jump)
• Java if statement with Examples
• Java if-else
• Java if-else-if ladder with Examples
• Loops in Java
• For Loop in Java
• Java while loop with Examples
• Java do-while loop with Examples
• For-each loop in Java
• Continue Statement in Java
• Break statement in Java
• Usage of Break keyword in Java
• return keyword in Java

• Object Oriented Programming (OOPs) Concept in Java
• Why Java is not a purely Object-Oriented Language?
• Classes and Objects in Java
• Naming Conventions in Java
• Java Methods
• Access Modifiers in Java
• Java Constructors
• Four Main Object Oriented Programming Concepts of Java
• Inheritance in Java
• Abstraction in Java
• Encapsulation in Java
• Polymorphism in Java
• Interfaces in Java
• ‘this’ reference in Java

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String Handling in Java

String Handling is the basic concept of Object-Oriented Programming which revolves around real-life entities.

Table of Contents

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String Handling in Java

The string is a sequence of characters. We can call an array of characters a string.

public class Main< public static void main(String args[])< char[] c=; String s1=new String(c); String s2="java"; System.out.println(s1); System.out.println(s2); > >

Like the above example, character array c and string s2 are the same only. As we have already seen, we can define string using literal or using the new operator. String objects are immutable objects. You can perform operations over it without storing those values.

Now, let us explore how Java handles string with different examples.

String Length

String length is the number of characters in it. The string object has a predefined method available called length() to obtain no. of characters in it.

String Modification

String Concatenation

Java has a “+” operator to attach more than one string or string with some other data type value. We can have multiple chains of operations by using the + operator.

As we can see in the below example, we can join a string with another string literal, another string variable, or another data type variable here, in this case, an integer.

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We can also use concat() method to join strings same as above example.

Here you can see, even after concatenation operation, s2 object’s value is still the same. That is why we have termed that string objects are immutable. When we perform a concatenation operation, it will be a new object but the older object’s value won’t be changed.

Extraction

Java can extract substrings using the substring() method. Its general form is as below:

String substring(int startIndex, int endIndex)

Here, if we have omitted endIndex then, it will extract the substring from startIndex till the end of the string.

Like 1st statement, if we omit endIndex, it will fetch from startIndex to the end of the string.

Replace

Java has a method called replace() to replace one character with another. The general form of replace() method is shown below:

String replace(char original, char replace)

Here, instead of one character we can also pass a string and replace it with another string. Below is the

example for the same:

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Trim

Java has a trim() method to remove extra white spaces from strings. It is very useful while processing user inputs.

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Case Conversion

Java has methods to convert the case of the strings.

String toLowerCase( )
String toUpperCase( )

These methods will convert the case of the whole string. These are very useful for user inputs where there will not be any consistency in which case the user will input their values. Non Alphabetic characters are not getting affected by these methods.

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String Conversion

Java has a method called toString() to convert the object into a string object. So when we concat any objects, Java internally hit the toString() method to perform the operation and return the string object. Now, if we want to show an object as a string object, we can override the toString() object and return the information in whichever way we like.

Let us understand through the below example:

class employee < int empid; String ename; employee(int empid, String ename)< this.empid=empid; this.ename=ename; >public String toString() > public class Main < public static void main(String args[])< employee e1=new employee(10001,"ABC"); employee e2=new employee(10002,"XYZ"); System.out.println(e1);//compiler writes here s1.toString() System.out.println(e2);//compiler writes here s2.toString() >>

String Comparison

Java has the below methods to compare two strings or substrings. Let us understand their usage with examples.

equals() and equalsIgnoreCase()

These 2 methods returns true if both strings are same and false if not same. public class Main < public static void main(String []args)< String s1 = "Learn"; String s2 = "Java"; String s3 = "JAVASCRIPT"; System.out.println(s1.equals(s2)); System.out.println(s3.substring(0,4)); System.out.println(s2.equals(s3.substring(0,4))); System.out.println(s2.equalsIgnoreCase(s3.substring(0,4))); >>

Here in this example, we can see the usage of equals() and equalsIgnoreCase() methods. It always returns a Boolean value.

This works the same as equals() method and returns true in the case of similar strings. The only difference here is that == compares references, not values.

Here == operator compares 2 object references and see whether those two refer to the same instances. If the same instance then it will return true else it will return false even if both objects have the same values in it.

This method returns the integer value

0 – if both strings are same
>0 – if invoking string is greater than passed string

startsWith() and endsWith()

startsWith() method is used in Java to check whether a string starts with some specific string.
endsWith() method is used in Java to check whether a string ends with some specific string.
Both these methods return a Boolean value and their general form is as below:

boolean startsWith(String s)
boolean endsWith(String s)

If string matched the criteria, the statement returns true else it returns false.

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Text File Handling in Java

In this lesson, we will learn how to handle text file in Java.

What is Text File Handling

Text File Handling is a process in which we create a text file and store data permanently on a hard disk so that it can be retrieved from the memory later for use in a program.

In a text file, whatever data we store is treated as text. Even if we store numbers, it is treated as text.

Operation on Text File

There are different types of operations that we can perform on a text file, they are:

• Open a file.
• Write to a file.
• Read from a file.
• Copy a file from one location to another.
• Rename a file.
• Delete a file.

Open a file

We will use the File and FileWriter class to open a text file in Java. To use these classes, we must import the package java.io.* into our program. The FileWriter is also used for writing text content to a file. The syntax of the File class and FileWriter class is given below.

Syntax of File and FileWriter class

// Create a File object File f = new File("file_path"); // Create a FileWriter object using a File object in non-append mode FileWriter fw = new FileWriter(f); // Create a FileWriter object using a File object in append mode FileWriter fw = new FileWriter(f, true); // Create a FileWriter object using a file path in non-append mode FileWriter fw = new FileWriter("file_path"); // Create a FileWriter object using a file path in append mode FileWriter fw = new FileWriter("file_path", true);

While opening a file in non-append mode using the FileWriter class, if the file does not exist, it will be created by the FileWriter class. If the file exists, then it erases all the contents of the file.

Open a text file using File object passed to FileWriter class

import java.io.*; public class Example < public static void main(String args[]) < // create a File object File f=new File("d:/delta.txt"); // declare a FileWriter object FileWriter fw=null; try < // initialize the FileWriter object by passing the File object fw=new FileWriter(f, true); // close the file fw.close(); >catch(IOException e) < System.out.println(e.getMessage()); >> >

Open a text file using file path passed to FileWriter class

import java.io.*; public class Example < public static void main(String args[]) < // declare a FileWriter object FileWriter fw=null; try < // initialize the FileWriter object by passing the file path fw=new FileWriter("D:/Delta.txt", true); // close the file fw.close(); >catch(IOException e) < System.out.println(e.getMessage()); >> >

Write to a file

We will use the FileWriter class and Scanner class to write some text to a file. The program below demonstrates how we can write some text in a text file using the FileWriter and Scanner class.

Example

import java.io.*; import java.util.Scanner; public class Example < public static void main(String args[]) < File f=new File("D:/delta.txt"); FileWriter fw=null; String str; Scanner sc=new Scanner(System.in); try < fw=new FileWriter(f,true); System.out.println("Enter a few lines of text:"); str=sc.nextLine(); while(str.length()>0) < fw.write(str+"\r\n"); str=sc.nextLine(); >> catch(IOException e) < System.out.println(e.getMessage()); >finally < try < fw.close(); >catch(IOException e) <> > > >

Output

Enter a few lines of text: Hello I am learning Text File Handling in Java

In the above program, with help of while loop we are checking if the length of the str is greater than 0 then write all the contents of str into the file using the write() method of FileWriter class along with \r\n escape sequence character that represents a new line in the file.

The while loop stops working when we press the enter key from the keyboard twice without writing anything on the screen. In this case the length of str becomes 0 and the loop terminates.

Read from a file

We will use the FileReader class and Scanner class to read text from a file. The program below demonstrates how we can read from a text file using the FileReader and Scanner class.

Example

import java.io.*; import java.util.Scanner; public class Example < public static void main(String args[]) < String str; // create a File object File f=new File("d:/delta.txt"); // declare a Scanner object Scanner sc=null; try < // initialize the Scanner object by passing the File object sc=new Scanner(f); while(sc.hasNextLine()) < str=sc.nextLine(); System.out.println(str); >> catch(IOException e) < System.out.println(e.getMessage()); >// close the file sc.close(); > >

Output

Hello I am learning Text File Handling in Java

We can also read a file character by character using the read() method of the FileReader class. See the example given below.

Example

import java.io.*; public class Example < public static void main(String args[]) < int ch; // create a File object File f=new File("d:/delta.txt"); // declare a FileReader object FileReader fr=null; try < // initialize the FileReader object by passing the File object fr=new FileReader(f); ch=fr.read(); // read a character and return its integer value while(ch!=-1) < System.out.print((char)ch); // typecasting the integer into character ch=fr.read(); // read the next character >> catch(IOException e) < System.out.println(e.getMessage()); >finally < try < // close the file fr.close(); >catch(IOException e) <> > > >

Output

Hello I am learning Text File Handling in Java

The read() method reads a character from the file and returns its integer value. It returns -1 if it reaches the end of the file.

Copy a file from one location to another

The program given below demonstrates how to copy a file from one location to another.

Example

import java.io.*; public class Example < public static void main(String args[]) < File x=new File("D:/delta.txt"); File y=new File("E:/alpha.txt"); FileReader fr=null; FileWriter fw=null; int ch; try < fr=new FileReader(x); fw=new FileWriter(y); ch=fr.read(); while(ch!=-1) < fw.write(ch); ch=fr.read(); >System.out.println("File copied successfully"); > catch(IOException e) < System.out.print(e.getMessage()); >finally < try < // close the files fr.close(); fw.close(); >catch(IOException e) <> > > >

In the above program, we created two File objects, one for the source from where we will read the characters and another for the destination file where we will write all the characters. We created a FileReader object and a FileWriter object.

Using a while loop, we read each character from the source file using the read() method of the FileReader object and write the character into the destination file using the write() method of the FileWriter object.

Rename a file

The program given below demonstrates how to rename a file.

Example

import java.io.*; public class Example < public static void main(String[] args) < File oldName = new File("D:/delta.txt"); File newName = new File("D:/deltainfo.txt"); if(oldName.renameTo(newName)) < System.out.println("Renamed"); >else < System.out.println("Error Occured"); >> >

Note: Before renaming the file, make sure that the file is closed else it cannot be renamed.

Delete a file

The program given below demonstrates how to delete a file.

Example

import java.io.*; public class Example < public static void main(String[] args) throws IOException < File file = new File("D:/deltainfo.txt"); if(file.delete()) < System.out.println("File deleted successfully"); >else < System.out.println("Delete operation is failed."); >> >

Note: Before deleting the file, make sure that the file is closed else it cannot be deleted.

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