WHY NullPointerException occur and How to Handle NullPointerException in Java

How to Effectively Handle NullPointerException in Java

ans:-NullPointerException is an unchecked exception and extends RuntimeException.

      It doesn’t force you to use catch block to handle it

Q-Why NullPointerException occur in your code?

ans-"NullPointerException is a situation in code where you try to access/ modify an object which has not been initialized yet"

means that object reference variable is not pointing anywhere and refers to nothing or ‘null’.

 A simple example can be:

package com.piyush.npe; public class SampleNPE {     public static void main(String[] args) {         String s = null;         System.out.println(s.toString()); // s is un-initialized and is null     } } Note:-Common places where NullPointerException usually occur ans:-NullPointerException can occur anywhere in your code 1:-) Invoking methods on an object which is not initialized. 2:-) Incorrect configuration for frameworks like spring which works on dependency injection 3:-) Parameters passed in a method are null. 4:-) Calling toString() method on object which is null. 5:-) Using synchronized on an object which is null 6:-) Chained statements i.e. multiple method calls in a single statement. 7:-) Comparing object properties in if block without checking null equality. Q-How to avoid the NullPointerException? ans:-First ensure that all your objects are initialized properly. note:-If you are dealing with static variables or static method than you won’t get null pointer exception Check Each Object For Null Before Using I listed how to avoid NPE:- 1- Empty Collection 2- Use some Methods 3- assert Keyword 4- Assert Class 5- Exception Handling 6- Too many dot syntax 7- StringUtils Class 1:-) Prefer String.valueOf() method instead of toString(). example:- package com.piyush.npe; public class SampleNPE {     public static void main(String[] args) {         String s = null;         System.out.println(String.valueOf(s)); // using String.valueOf(s) we can avoid nullpointerException;     } } 2:-) Use annotations @NotNull and @Nullable. 3:-) Empty Collection Empty collection is collection which hasn’t any elements. Some developers return null value for Collection which has no elements but this is false, you should return Collections.EMPTY_LIST, Collections.EMPTY_SET andCollections.EMPTY_MAP. example:- public static List getEmployees() {  2   List list = Collections.EMPTY_LIST;  3   return list;  4 }  4:-)Use some Method Use some methods to assure that null value not exist like contains(), indexOf(), isEmpty(), containsKey(), containsValue() and hasNext(). example:- public class Example { 02      private static List<Integer> numbers = null; 03 04      public static List<Integer> getList() { 05           if (numbers == null) 06                return Collections.emptyList(); 07           else 08                return numbers; 09      } 10 }  example2:-  String myName = "Piyush Kumar  Bangalore";  List list = Collections.EMPTY_LIST; boolean exist = list.contains(myName); int index = list.indexOf(myName); boolean isEmpty =list.isEmpty(); Map map =Collections.EMPTY_MAP; exist=map.containsKey(myName); exist=map.containsValue(myName); isEmpty=map.isEmpty(); Set set=Collections.EMPTY_SET; exist=set.contains(myName); isEmpty=set.isEmpty(); Iterator iterator; exist = iterator.hasNext(); 5:-)Exception Handling:- example:-a- try catch statement I will change calling code to below code 1 String managerId = getManager("A015");  2 try {  3   System.out.println(managerId.toString());  4 } catch (NullPointerException npe) {  5   //write your code here  6 } b- Checking of Null values I will change calling code to below code view sourceprint? 1 String managerId = getManager("A015");  2 if (managerId != null) {  3   System.out.println(managerId.toString());  4 } else {  5   //write your code here  6 } 6:-) StringUtils Class StringUtils class is part of org.apache.commons.lang package, I can use it to avoid NPE specially all its methods are null safe For example StringUtils. IsEmpty(), StringUtils. IsBlank(), StringUtils.equals(), and much more. examle:-You can make use of the StringUtils.isNotEmpty, StringUtils.IsEmpty and StringUtils.equals methods, in order to avoid the NullPointerException. For example: 1 if (StringUtils.isNotEmpty(str)) { 2      System.out.println(str.toString()); 3 } 7:-)assert Keyword 8-:) Assert Class second way:- How to solve a NullPointerException in Java? It is simple, put a null check! Surround your object with if statement like Object mayBeNullObj = getTheObjectItMayReturnNull(); if (mayBeNullObj != null) { // to avoid NullPointerException mayBeNullObj.workOnIt(); }

How to read a text file and excel sheet in Java?

Q1:-How to read a text file in Java?

ans:-

Solution 1 - Reading File using Scanner:-

Once you imported this class, you can create object of Scanner by passing a FileInputStream to it, pointing to the file you want to read

Scanner provides a method called hasNextLine() which returns true if file has one more line to read.

example:-public class FileReaderDemo{

 public static void main(String args[]) throws IOException

 {

 final String FILE_NAME = "C://temp//xyz.txt"; 

// 1st way to read File in Java - Using 

 Scanner scnr = new Scanner(new FileInputStream(FILE_NAME));

 while (scnr.hasNextLine()) { 

System.out.println(scnr.nextLine()); 

} 

scnr.close();
  }

 }

}

Solution 2 - Reading File using BufferedReader:-

ans:-how to read a file using BufferedReader class. Both classes are defined in java.util package so you need to import them before using it

InputStreamReader actually acts as a bridge between streams and reader classes. Once you have an object of BufferedReader, you can call readLine() method to read the next line from file.

example:-public class FileReaderDemo{

 public static void main(String args[]) throws IOException

 {

 final String FILE_NAME = "C://temp//xyz.txt"; 

// 2nd way to read File in Java - Using BufferedReader BufferedReader buffReader = new BufferedReader(new InputStreamReader(new FileInputStream(FILE_NAME))); 

String line = buffReader.readLine(); 

while (line != null) {

 System.out.println(line);

 line = buffReader.readLine();

       }

   }

 }
Q-How to read XLS and XLSX file in Java?

ans:-


InputStream  inputWorkbook1 = Injecting.class.getClassLoader().getResourceAsStream("data/piyush.xls");
        BufferedReader br = new BufferedReader(new InputStreamReader(inputWorkbook1));
System.out.println("i am in inside file");
Workbook w;
int arrayLength = 0;
int i = 0;
try {

w = Workbook.getWorkbook(inputWorkbook1);
// Get the first sheets

Sheet sheet = w.getSheet(0);
// Loop over first 10 column and lines
Cell[] cell0 = sheet.getColumn(0);
Cell[] cell1 = sheet.getColumn(1);
Cell[] cell2 = sheet.getColumn(2);

System.out.println(userId + "cell0 --> " + cell0[0].getContents()
+ "\ncell1 -- >" + cell1[0].getContents() + "\ncell2 -- > "


+ cell2[0].getContents());