Uncertainity in java != with !equals

Question

I wrote a small program to read a csv file...I used buffered reader for that... My code looks like this:

package files;

import java.io.BufferedReader;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileReader;
import bk.bk;

public class QwithLinkedList{
    public static void main(String args[]) throws Exception{
        FileReader f=new FileReader("G:/bk.csv");
        BufferedReader br=new BufferedReader(f);
        String line;
        while((line=br.readLine())!=(null)){
            System.out.println(line);
        }
    }
}

Above is perfect code but my question is that I'm getting an exception with this code :

package files;

import java.io.BufferedReader;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileReader;
import bk.bk;

public class QwithLinkedList{
    public static void main(String args[]) throws Exception{
        FileReader f=new FileReader("G:/bk.csv");
        BufferedReader br=new BufferedReader(f);
        String line;
        while(!(line=br.readLine()).equals(null)){
            System.out.println(line);
        }
    }
}

The output for the above code is this :

a1
bk
abc
def
ghi
jkl
bharath
Exception in thread "main" java.lang.NullPointerException
    at files.QwithLinkedList.main(QwithLinkedList.java:14)

Someone please explain why it is giving an exception with the above code. Moreover if(a!=b) and !a.equals(b) are't they same ?

Because you end up with `null.equals(...)` but `null` doesn't have any methods, including `equals`. — Pshemo, Mar 15 '17 at 10:04
`a != b` and `!a.equals(b)` are _not_ the same. The first compares _object identity_ whereas the second simply invokes the (possibly overridden) [`public boolean equals(Object obj)`](https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html#equals-java.lang.Object-) on the `a` instance. — jensgram, Mar 15 '17 at 10:08
@bharath: The difference between `==` and `equals()` (or `!=`and `!equals()`) is vital for java programming. Please make sure you understand the difference and what @jensgram writes, otherwise you'll run into very hard to find bugs. — sruetti, Mar 15 '17 at 10:14

score 0 · Accepted Answer · answered Mar 15 '17 at 10:06

0

When checking for null, always use == or !=. Using equals will always become a NullPointerException if what you are checking is null.

The difference between == and equals is that the former checks whether or not both sides points to the same address in memory, while the latter checks for object equality.

answered Mar 15 '17 at 10:06

Tobb

11,850
6
52
77

Thanks man now I got the difference – Bharat Mar 15 '17 at 10:23

score 0 · Answer 2 · answered Mar 15 '17 at 10:08

0

When checking for a null value, never use the .equals() since it might return a NullPointerException as in your case.

answered Mar 15 '17 at 10:08

JonasJR

76
9

1

You just confirmed OP problem but didn't explain its cause. Question is "**why** it is giving an exception with the above code" (emphasis mine) – Pshemo Mar 15 '17 at 10:16

Uncertainity in java != with !equals

2 Answers2