https://www.javatpoint.com/how-to-read-xml-file-in-java
Java SAX Parser
Java SAX parser stands for Simple API for XML. SAX parser parses an XML file line by line. 
It triggers events when it encounters the opening tag, closing tag, and character data in an xml file. 
SAX parser is also called the event-based parser.
SAX parser does not load any XML file into memory. It does not create any object representation of the XML document. 
SAX parser uses call back function to inform clients of the XML document structure. It is faster and uses less memory than DOM parser.

SAX is a streaming interface for XML, which means that XML file parses in sequential order starting at the top of the document, 
and ending with the closing of the root element.

Example of reading XML file using SAX parser

import javax.xml.parsers.SAXParser;  
import javax.xml.parsers.SAXParserFactory;  
import org.xml.sax.Attributes;  
import org.xml.sax.SAXException;  
import org.xml.sax.helpers.DefaultHandler;  
public class ReadXMLFileExample3  
{  
public static void main(String args[])   
{  
try   
{  
SAXParserFactory factory = SAXParserFactory.newInstance();  
SAXParser saxParser = factory.newSAXParser();  
DefaultHandler handler = new DefaultHandler()   
{  
boolean id = false;  
boolean firstname = false;  
boolean lastname = false;  
boolean subject = false;  
boolean marks = false;  
//parser starts parsing a specific element inside the document    
public void startElement(String uri, String localName, String qName, Attributes attributes) throws SAXException   
{  
System.out.println("Start Element :" + qName);  
if(qName.equalsIgnoreCase("Id"))  
{  
id=true;  
}  
if (qName.equalsIgnoreCase("FIRSTNAME"))   
{  
firstname = true;  
}  
if (qName.equalsIgnoreCase("LASTNAME"))   
{  
lastname = true;  
}  
if (qName.equalsIgnoreCase("SUBJECT"))   
{  
subject = true;  
}  
if (qName.equalsIgnoreCase("MARKS"))   
{  
marks = true;  
}  
}  
//parser ends parsing the specific element inside the document  
public void endElement(String uri, String localName, String qName) throws SAXException   
{  
System.out.println("End Element:" + qName);  
}  
//reads the text value of the currently parsed element  
public void characters(char ch[], int start, int length) throws SAXException   
{  
if (id)   
{  
System.out.println("ID : " + new String(ch, start, length));  
id = false;  
}  
if (firstname)   
{  
System.out.println("First Name: " + new String(ch, start, length));  
firstname = false;  
}  
if (lastname)   
{  
System.out.println("Last Name: " + new String(ch, start, length));  
lastname = false;  
}  
if (subject)  
{  
System.out.println("Subject: " + new String(ch, start, length));  
subject = false;  
}  
if (marks)   
{  
System.out.println("Marks : " + new String(ch, start, length));  
marks = false;  
}  
}  
};  
saxParser.parse("F:\\XMLFile.xml", handler);  
}   
catch (Exception e)   
{  
e.printStackTrace();  
}  
}  
}  
Output:

Start Element: class
Start Element: student
Start Element: id
ID: 101
End Element: id
Start Element: firstname
First Name: Naman
End Element: firstname
Start Element: lastname
Last Name: Kumar
End Element: lastname
Start Element: subject
Subject: Math
End Element: subject
Start Element: marks
Marks: 83
End Element: marks
End Element: student
Start Element: student
Start Element: id
ID: 102
End Element: id
Start Element: firstname
First Name: Kapil
End Element: firstname
Start Element: lastname
Last Name: Kumar
End Element: lastname
Start Element: subject
Subject: Chemistry
End Element: subject
Start Element: marks
Marks: 60
End Element: marks
End Element: student
Start Element: student
Start Element: id
ID: 103
End Element: id
Start Element: firstname
First Name: Harsh
End Element: firstname
Start Element: lastname
Last Name: Singh
End Element: lastname
Start Element: subject
Subject: English
End Element: subject
Start Element: marks
Marks: 70
End Element: marks
End Element: student
Start Element: student
Start Element: id
ID: 104
End Element: id
Start Element: firstname
First Name: Jitesh
End Element: firstname
Start Element: lastname
Last Name: Singh
End Element: lastname
Start Element: subject
Subject: Physics
End Element: subject
Start Element: marks
Marks: 76
End Element: marks
End Element: student
End Element: class