Sample Java Program for Implementing Singleton.

package com.test;

public class MainTest {

public static void main(String[] args) {

SingletonsTest test = SingletonsTest.getInstance();

SingletonsTest test2 = SingletonsTest.getInstance();

// SingletonTest tss = new SingletonTest();

}

}

class SingletonsTest{

private static SingletonsTest uniqTest;

private SingletonsTest(){

}

public static synchronized SingletonsTest getInstance(){

if(uniqTest == null){

System.out.println("New instances are created====");

uniqTest = new SingletonsTest();

}

return uniqTest;

}

}

Multiple row insert in sql server from java

We can insert the multiple rows into sql server from java using the statement.executeBatch()

function. The sample program for inserting the multiple rows are given below.

package com.test;

import java.sql.Connection;

import java.sql.DriverManager;

import java.sql.Statement;

public class MultipleRowUpdateTest {

public static void main(String[] args) {

try{

Class.forName("com.mysql.jdbc.Driver");

Connection conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/portal","root", "root");

Statement stmt = conn.createStatement();

stmt.addBatch("INSERT INTO role " +

"VALUES(5,'temp5','ROLE_ESWAR5')");

stmt.addBatch("INSERT INTO role " +

"VALUES(6, 'temp3','ROLE_ESWAR2')");

stmt.addBatch("INSERT INTO role " +

"VALUES(7, 'temp4','ROLE_ESWAR3')");

stmt.executeBatch();

}catch (Exception e) {

e.printStackTrace();

}

}

}

Sample Program for Comparable and Comparator

Comparator and Comparable in Java are two of fundamental interface of Java API which is very important to understand to implement sorting in Java. It’s often required to sort objects stored in any collection class or in Array and that time .

Comparable:

A comparable object is capable of comparing itself with another object. The class itself must implements the java.lang.Comparable interface in order to be able to compare its instances.

java.lang.Comparable: int compareTo(Object o1)

This method compares this object with o1 object. Returned int value has the following meanings.

1. positive – this object is greater than o1
2. negative – this object is less than o1
3. Zero – this object equals to o1

java.util.Collections.sort(List) and java.util.Arrays.sort(Object[]) methods can be used to sort using natural ordering of objects.

Sample Code:

package com.test;

import java.util.ArrayList;

import java.util.Collections;

import java.util.List;

public class SortingObjectList {

public static void main(String[] args) {

Employee emp1 = new Employee();

emp1.setAge(12);

emp1.setFirstName("Eswar");

emp1.setLastName("Palani");

emp1.setEmpId(13);

List<Employee> sortList = new ArrayList<Employee>();

sortList.add(emp1);

Employee emp2 = new Employee();

emp2.setAge(34);

emp2.setEmpId(15);

emp2.setFirstName("Karthi");

emp2.setLastName("Palani");

sortList.add(emp2);

Employee emp3 = new Employee();

emp3.setAge(50);

emp3.setEmpId(15);

emp3.setFirstName("Aru");

emp3.setLastName("Palani");

sortList.add(emp3);

Collections.sort(sortList);

System.out.println("The sorted lists are======>");

for (Employee employee : sortList) {

System.out.println(employee.getFirstName()+" "+employee.getLastName());

}

}

}

class Employee implements Comparable<Employee>{

private String firstName;

private String lastName;

private int age;

private int empId;

public int getEmpId() {

return empId;

}

public void setEmpId(int empId) {

this.empId = empId;

}

public String getFirstName() {

return firstName;

}

public void setFirstName(String firstName) {

this.firstName = firstName;

}

public String getLastName() {

return lastName;

}

public void setLastName(String lastName) {

this.lastName = lastName;

}

public int getAge() {

return age;

}

public void setAge(int age) {

this.age = age;

}

@Override

public int compareTo(Employee o) {

if (o.firstName.equals(this.firstName))

if (o.age-this.age == 0)

return o.empId - this.empId;

else

return o.age-this.age;

else

return o.firstName.compareTo(this.firstName);

}

}

Output:

The sorted lists are======>

Karthi Palani

Eswar Palani

Aru Palani

Comparator

A comparator object is capable of comparing two different objects. The class is not comparing its instances, but some other class’s instances. This comparator class must implement the java.util.Comparator interface.

java.util.Comparator: int compare(Object o1, Objecto2)

This method compares o1 and o2 objects. Returned int value has the following meanings.

1. positive – this object is greater than o1
2. negative – this object is less than o1
3. Zero – this object equals to o1

java.util.Collections.sort(List, Comparator) and java.util.Arrays.sort(Object[], Comparator) methods can be used if a Comparator is available for comparison.

Sample Code:

package com.test;

import java.util.ArrayList;

import java.util.Collections;

import java.util.Comparator;

import java.util.List;

public class SortingObjectListComparator {

public static void main(String[] args) {

Staff emp1 = new Staff();

emp1.setAge(12);

emp1.setFirstName("Eswar");

emp1.setLastName("Palani");

emp1.setEmpId(13);

List<Staff> sortList = new ArrayList<Staff>();

sortList.add(emp1);

Staff emp2 = new Staff();

emp2.setAge(34);

emp2.setEmpId(15);

emp2.setFirstName("Karthi");

emp2.setLastName("Palani");

sortList.add(emp2);

Staff emp3 = new Staff();

emp3.setAge(50);

emp3.setEmpId(15);

emp3.setFirstName("Aru");

emp3.setLastName("Palani");

sortList.add(emp3);

Collections.sort(sortList, new Staff());

System.out.println("The sorted lists are===");

for (Staff staff : sortList) {

System.out.println(staff.getFirstName()+" "+staff.getLastName());

}

}

}

class Staff implements Comparator<Staff>{

private String firstName;

private String lastName;

private int age;

private int empId;

public int getEmpId() {

return empId;

}

public void setEmpId(int empId) {

this.empId = empId;

}

public String getFirstName() {

return firstName;

}

public void setFirstName(String firstName) {

this.firstName = firstName;

}

public String getLastName() {

return lastName;

}

public void setLastName(String lastName) {

this.lastName = lastName;

}

public int getAge() {

return age;

}

public void setAge(int age) {

this.age = age;

}

@Override

public int compare(Staff o1, Staff o2) {

return o1.getFirstName().compareTo(o2.getFirstName());

}

}

output:

The sorted lists are===

Aru Palani

Eswar Palani

Karthi Palani

Difference between Comparator and Comparable:

1) Comparator in Java is defined in java.util package while Comparable interface in Java is defined in java.lang package.

2) Comparator interface in Java has method public int compare (Object o1, Object o2) which returns a negative integer, zero, or a positive integer as the first argument is less than, equal to, or greater than the second. While Comparable interface has method public int compareTo(Object o) which returns a negative integer, zero, or a positive integer as this object is less than, equal to, or greater than the specified object.

3) If you see then logical difference between these two is Comparator in Java compare two objects provided to him, while Comparable interface compares "this" reference with the object specified.

4) Comparable in Java is used to implement natural ordering of object. In Java API String, Date and wrapper classes implement Comparable interface.

5) If any class implement Comparable interface in Java then collection of that object either List or Array can be sorted automatically by using  Collections.sort() or Arrays.sort() method and object will be sorted based on there natural order defined by CompareTo method.

6)Objects which implement Comparable in Java  can be used as keys in a sorted map or elements in a sorted set for example TreeSet, without specifying any Comparator.

Java Exception Handling

Java Exceptions:

Errors:

There are three types of errors.

1. Compile Time Errors:
         Syntactical error found in the program due to which program fails to compile.
   Ex: Unable to compile if semicolon is missing end of the line.
                 public static void main(String args[]) throws Excetpion{
                 System.out.println(“Executed”)
                }

2. Runtime Errors:

                   These errors represent inefficiency of computer system to execute a particular program.

  3. Logical Errors:

These errors depict flaws in the logic .Program might be using the wrong formula.

Exceptions:

        An exception is a runtime error. Compile time errors are not exceptions. These are all errors.

         All exception occurs at runtime but some exceptions are detected at compile time and some others at runtime. The Exceptions that are checked by the compiler at compilation time are called Checked Exceptions. The Exceptions are checked by JVM at the runtime are called unchecked Exceptions.

                           +-------+

                    | Object |
                    +--------+
                        |
                        |
                   +-----------+
                   | Throwable |
                   +-----------+
                    /         \
                   /           \
          +-------+          +-----------+
          | Error |          | Exception |
          +-------+          +-----------+
           /  |  \           / |        \
         \________/       \______/       \
                                        +------------------+
        unchecked        checked        | RuntimeException |
                                        +------------------+
                                          /   |    |      \
                                         \_________________/
                                           
                                           unchecked

        Unchecked exceptions and errors are considered as unrecoverable and the programmer cannot do anything whey they occur.

The Exceptions occur at runtime but in case of checked exceptions whether you are handling it or not it is detected at compilation time.

                  public static void main(String args[]){

                            calFileReader();

               }

             public void calFileReader() throws IOException{

                     BufferedReader in = new BufferedReader(new FileReader("file.xml"));

                }

Exception Handling: 

               1. We can handle the exceptions using try, catch and finally.

               2. Handle multiple exceptions in a method.

               3. Only one exceptions can handle at a same time.

Types of Exceptions:

         There are two types of exceptions.

              1. Built in exceptions

                           - NullpointerException

                           - ArithmaticException

                           -ArrayIndexoutboundException

                           - NumberFormatException

             2. User defined exceptions

                      - User defined exceptions are done by extending the Exception class since Exception is the super class for Runtime exceptions/Unchecked Exceptions.

                public class MyException extends Exception {

                   public MyException() {

                    super();

                   }

                 public MyException(final String msg) {

                  super(msg);

                 }

             }