Java

What are Marker or Tag Interfaces?

Mallik1 Comment

An interface is called a marker interface when it is provided as a handle by java interpreter to mark a class so that it can provide special behaviour to it at runtime and they do not have any method declarations.

Java Marker Interface Examples

  • java.lang.Cloneable
  • java.io.Serializable
  • java.util.EventListener

Why we need Marker Interface?

e.g.  Suppose you want to persist (save) the state of an object then you have to implement the Serializable interface otherwise the compiler will throw an error. To make more clearly understand the concept of marker interface you should go through one more example.

Suppose the interface Clonable is neither implemented by a class named Myclass nor it’s any super class, then a call to the method clone() on Myclass’s object will give an error. This means, to add this functionality one should implement the Clonable interface. While the Clonable is an empty interface but it provides an important functionality.

equals() and hashCode() methods of Object Class

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HashTable, HashMap and HashSet are the Collection classes in java.util package that make use of hashing algorithm to store objects. In all these Collection classes except HashSet, objects are stored as key-value pairs. For the storage and the retrieval of any user-defined objects it is a good practice to override the following methods which is mentioned below,

  • hashCode()
  • equals()

These methods are available in the Object class and hence available to all java classes.Using these two methods, an object can be stored or retrieved from a Hashtable, HashMap or HashSet.

hashCode() method

This method returns a hashcode value as an int for the object. Default implementation for hashcode() should be overridden in order to make searching of data faster. The implementation of hashCode() method for an user-defined object should be calculated based on the properties of the class which we wish to consider.

equals() method

This method returns a boolean which specifies whether two objects are equal or not. The default implementation of equals() method given by the Object Class uses the ‘==’ operator to compare two object references, and returns true only if they refer to the same object. But, we can meaningfully re-define this equals() method to have en equality check based on our own criterias.

Consider the following code, which defines two user defined classes Employee and EmployeeId which are supposed to be stored in a Map.

Employee.java

 

public class Employee {

private String name;

public Employee(String name){

this.name = name;

}

public String toString(){

return name;

}

}

EmployeeId.java

 

public class EmployeeId {

private String id;

public EmployeeId(String id){

this.id = id;

}

public String toString(){

return id;

}

}

The following class makes use of the above classes by storing it in a Map for later retrieval. We are adding Employee objects into the Map keyed with Employee Id.

HashCodeTest.java

 

public class HashCodeTest {

public static void main(String[] args) {

Map<EmployeeId, Employee> employees = new HashMap<EmployeeId, Employee>();

employees.put(new EmployeeId(“111”), new Employee(“Johny”));

employees.put(new EmployeeId(“222”), new Employee(“Jeny”)); // Line A

employees.put(new EmployeeId(“333”), new Employee(“Jessie”));

Employee emp =  employees.get(new EmployeeId(“222”)); // Line B

System.out.println(emp); // Line C

}

}

In Line B, we try to retrieve the Employee object who has Employee Id with a value of 222. We expect the output to be ‘Jeny’, because the Employee with Employee Id (222) was already there in the Collection, but surprisingly, the output of the above code is null. The reason is that we did not override the equals() method for EmployeeId and Employee classes because the default implementation of equals() in the Object class considers the new EmployeeId(“222”) in the put statement and new EmployeeId(“222”) in the get statement as two different instances, and hence the call to get() in Line B returns null.

Let us look at how the same code works when we provide our desired implementation for hashcode() and equals() methods. We basically override hashcode() here just to make the object to be searched fast.

Employee.java

 

public class Employee {

private String name;

public Employee(String name){

this.name = name;

}

public String toString(){

return name;

}

 

@Override

public boolean equals(Object obj){

if(obj == null) {

return false;

}

if(obj.getClass() != getClass()){

return false;

}

Employee emp = (Employee)obj;

if(this.name == emp.name){

return true;

}

return false;

}

@Override

public int hashCode(){

return name.hashCode();

}

}

EmployeeId.java

 

public class EmployeeId {

private String id;

public EmployeeId(String id){

this.id = id;

}

public String toString(){

return id;

}

 

public boolean equals(Object obj){

if(obj == null)

return false;

if(obj.getClass() != getClass()){

return false;

}

 

EmployeeId empId = (EmployeeId)obj;

if(this.id == empId.id){

return true;

}

return false;

}

@Override

public int hashCode(){

return id.hashCode();

}

}

Now, we get the desired output ‘Jeny’, because as per our implementation for the equals() method, the new EmployeeId(“222”) in the put statement and new EmployeeId(“222”) in the get statement are considered one and the same.

 

Sorting of int values in an array

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package in.javatutorials.test;

public class ManualSorting {
int[] arr = { 12, 1, 3, 22, 222, -9 };
public void ascendingOrder() {
for (int i = 0; i < arr.length; i++) {
for (int j = i + 1; j < arr.length; j++) {
int temp = 0;
if (arr[i] < arr[j]) {
temp = arr[j];
arr[j] = arr[i];
arr[i] = temp;
}
}
System.out.print(arr[i] + " ,");
}
System.out.println();
}
public void descendingOrder() {
for (int i = 0; i < arr.length; i++) {
for (int j = i + 1; j < arr.length; j++) {
int temp = 0;
if (arr[i] > arr[j]) {
temp = arr[j];
arr[j] = arr[i];
arr[i] = temp;
}
}
System.out.print(arr[i] + " ,");
}
System.out.println();
}
public static void main(String arr[]) {
ManualSorting ms = new ManualSorting();
ms.ascendingOrder();
ms.descendingOrder();
}
}

Read Properties file in Java

Mallik1 Comment

package com.test;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.util.Enumeration;
import java.util.Locale;
import java.util.Properties;
import java.util.ResourceBundle;
public class ReadPropetiesFile {
public static void main(String a[]){
String fileName = "test.properties";
ReadPropetiesFile readProp = new ReadPropetiesFile();
readProp.readProperties(fileName);
}
public void readProperties(String fileName){
try {
ResourceBundle labels = ResourceBundle.getBundle(fileName, Locale.ENGLISH);
Enumeration bundleKeys = labels.getKeys();
while (bundleKeys.hasMoreElements()) {
    String key = (String)bundleKeys.nextElement();
    String value = labels.getString(key);
    System.out.println("key = " + key + ", " +   "value = " + value);
}
}catch (Exception e) {
e.printStackTrace();
}
}
}

SAX Parser Example in JAVA

Mallik2 Comments

package com.test;
import java.io.StringReader;
import java.util.Enumeration;
import java.util.Locale;
import java.util.ResourceBundle;
import javax.xml.parsers.SAXParser;
import javax.xml.parsers.SAXParserFactory;
import org.xml.sax.Attributes;
import org.xml.sax.InputSource;
import org.xml.sax.SAXException;
import org.xml.sax.helpers.DefaultHandler;


public class ParseXMLUsingSAX {
public static String xmlString = "<?xml version='1.0'?><EMPLOYEE><USER_ID>admin</USER_ID><PASSWORD>admin123</PASSWORD></EMPLOYEE>";
public static void main(String argv[]) {
String fileName = "D:\\test.xml";
parseXml(fileName);
}
public static void parseXml(String fileName){
try {
SAXParserFactory factory = SAXParserFactory.newInstance();
SAXParser saxParser = factory.newSAXParser();
InputSource is = new InputSource();
is.setCharacterStream(new StringReader(xmlString));
DefaultHandler handler = new DefaultHandler() {
boolean userID = false;
boolean password = false;
public void startElement(String uri, String localName, 
String qName, Attributes attributes) throws SAXException {
if (qName.equalsIgnoreCase("USER_ID")) {
userID = true;
}
if (qName.equalsIgnoreCase("PASSWORD")) {
password = true;
}
}
public void endElement(String uri, String localName,
String qName) throws SAXException {
}
public void characters(char ch[], int start, int length) throws SAXException {
if (userID) {
System.out.println("userID : " + new String(ch, start, length));
userID = false;
}
if (password) {
System.out.println("password : " + new String(ch, start, length));
password = false;
}
}
};
//saxParser.parse(fileName, handler);
saxParser.parse(is, handler);
} catch (Exception e) {
e.printStackTrace();
}
}
}

XML Parsing Example using DOM in Java

Mallik1 Comment

package com.test;


import java.io.StringReader;
import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import org.w3c.dom.CharacterData;
import org.w3c.dom.Document;
import org.w3c.dom.Element;
import org.w3c.dom.Node;
import org.w3c.dom.NodeList;
import org.xml.sax.InputSource;
public class XMLParsing {
  public static void main(String arg[]) throws Exception{

String xmlRecords = “<data><employee><name>A</name>”
+ “<title>Manager</title></employee></data>”;


    DocumentBuilder db = DocumentBuilderFactory.newInstance().
newDocumentBuilder();
    InputSource is = new InputSource();
    is.setCharacterStream(new StringReader(xmlRecords));


    Document doc = db.parse(is);
    NodeList nodes = doc.getElementsByTagName("employee");


    for (int i = 0; i < nodes.getLength(); i++) {
      Element element = (Element) nodes.item(i);


      NodeList name = element.getElementsByTagName("name");
      Element line = (Element) name.item(0);
      System.out.println("Name: " + getCharacterDataFromElement(line));


      NodeList title = element.getElementsByTagName("title");
      line = (Element) title.item(0);
      System.out.println("Title: " + getCharacterDataFromElement(line));
    }
  }
  public static String getCharacterDataFromElement(Element e) {
    Node child = e.getFirstChild();
    if (child instanceof CharacterData) {
      CharacterData cd = (CharacterData) child;
      return cd.getData();
    }
    return "";
  }
}

Interface in Java

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Interfaces can be used to implement the Inheritance relationship between the non-related classes that do not belongs to the same hierarchy, i.e. any Class and any where in hierarchy.  Using Interface, you can specify what a class must do but not how it does.

  1. A class can implement more than one Interface.
  2. An Interface can extend one or more interfaces, by using the keyword extends.
  3. All the data members in the interface are public, static and Final by default.
  4. An Interface method can have only Public, default and Abstract modifiers…. More

Replace String Example in Java

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/**
 * @author Mallikarjun G
 *
 */
public class JavaStringReplaceExample {
    /**
     * @param args
     */
    public static void main(String args[]) {      
        String str = “Replace String”;
        System.out.println(str.replace(‘R’, ‘A’));// Replaceing R with a in str
        System.out.println(str.replaceFirst(“Re”, “Ra”));// Replaceing First Re with Ra in str  
        System.out.println(str.replaceAll(“Re”, “Ra”)); // Replaceing All Re with Ra in str   

    }
}

Connecting SQL Server using Java/JDBC

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Below program consists of steps to connect SQL server using JDBC

package com.javatutorials;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;

public class SqlServerConnection {
public static Connection connection = null;
public static Connection getDbConnection() {
try {
Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");
connection = DriverManager
.getConnection("jdbc:odbc:mydsn;user=sa;password=sa12$");
} catch (ClassNotFoundException e) {
e.printStackTrace();
} catch (SQLException e) {
e.printStackTrace();
}
return connection;
}

public static void closeConnection() {
if (connection != null)
try {
connection.close();
} catch (SQLException e) {
e.printStackTrace();
}
}
}