Bug 332713

Summary:	[compiler][null]Bogus "Null comparison always yields false"
Product:	[Eclipse Project] JDT	Reporter:	till toenges <tt>
Component:	Core	Assignee:	Ayushman Jain <amj87.iitr>
Status:	VERIFIED DUPLICATE	QA Contact:
Severity:	normal
Priority:	P3	CC:	amj87.iitr, jarthana, paul
Version:	3.7
Target Milestone:	3.7 M6
Hardware:	All
OS:	All
Whiteboard:

Description till toenges

2010-12-16 05:14:24 EST

Build Identifier: 20100917-0705

I get a "Null comparison always yields false: The variable s cannot be null at this location" in the following code. That should not be the case here. The difference between the good and the bad test case is only one if.

public class NullTest {

	public static void main(String[] args) {
		good(args);
		bad(args);
	}
	public static void good(String[] args) {
		String s = "";
		for (int i = 0; i < 13; i++) {
			s = test(args, i);
			if (s == null) {
				System.out.println("null");
			}
		}
	}
	public static void bad(String[] args) {
		String s = "";
		for (int i = 0; i < 13; i++) {
			if (i != 0) {
				s = test(args, i);
			}
			if (s == null) {
				System.out.println("null");
			}
		}
	}
	public static String test(String[] args, int i) {
		if (args.length > i) {
			return args[i];
		}
		return null;
	}
}


Reproducible: Always

Steps to Reproduce:
1. Put the code into Eclipse.
2. Have all the code warnings on.
3. See bogus warning about unneccessary null check.

Comment 1 Ayushman Jain

2010-12-16 06:42:22 EST

I'll take a look.

Comment 2 Paul Wagland

2011-02-08 21:30:21 EST

I have essentially the same problem, with essentially the same reproduction test case. If the initial line = "" is changed to line= null then the warning goes away. It does not change based on whether the initial doRead is set to true or false.


package net.wagland.paul.test;

import java.io.BufferedReader;
import java.io.IOException;

public class NullWarningTest {
  public void testNullWarning(BufferedReader bufReader) throws IOException {
    String line = "";
    boolean doRead = false;
    while (true) {
      if (doRead)
        line = bufReader.readLine();
      if (line == null)
        return;
      doRead = true;
    }
  }
}

Comment 3 Paul Wagland

2011-02-08 21:32:58 EST

Actually, this bug should be marked as a duplicate of bug 313870.

Comment 4 Ayushman Jain

2011-02-09 02:16:54 EST

Closing as dup

*** This bug has been marked as a duplicate of bug 313870 ***

Comment 5 Jay Arthanareeswaran

2011-05-16 00:58:11 EDT

Verified for 3.7 RC1 with build I20110514-0800.