How to check if a Number is Positive or Negative in Java - Interview Question

Write a Java program to check if a number is positive or negative is one
of the popular Java coding interview question,
it may look easy but programmers often fumble on this question. One of the
tricky part of this question is that Java has multiple data type to support
numbers like byte, short, char, int, long, float and double,
out of those all are signed except char, which
can not represent negative numbers. Some solution work for int and long but may
not work for floating point number e.g. float and double. This is
also a tricky Java question, another tricky
point is considering special case like positive infinity, negative infinity or
NaN in case of checking floating point numbers. Things gets more complicated
when, as followup questions, interviewer put additional conditions such as you
can not use relational operator etc. Nevertheless Java provides several way to
check if a number whether integer or floating point is positive or negative. In
this Java tutorial, we will see such methods of checking sign of number in
Java.

Different ways to check if number is
positive or negative in Java

Here is quick summary of different ways I can think of checking sign of
number and finding whether a number is positive or negative in Java. All these
method assumes that you will put special check logic for handling special cases
like positive infinity, negative infinity and Nan and assuming
zero on positive side.

1) By Converting Number to String

Convert any number into String and get first character, if its equals to "-" then its
negative number otherwise its

positive one. Though this method will not work for float and double, if number
is represented in exponential form. This could be the case if you are dealing with
large floating point numbers. For long and int this method of checking sign
of number should work. Look at following example of string equivalent of
minimum values of Integer, Long, Float and Double

System.out.println(Integer.MIN_VALUE);

System.out.println(Long.MIN_VALUE);

System.out.println(Float.MIN_VALUE);

System.out.println(Double.MIN_VALUE);



-2147483648

-9223372036854775808

1.4E-45

4.9E-324

Only Integer and Long can be
checked getting first character and comparing with "-". See How to convert Integer to String
and  Long to String in Java to
convert these numbers into String. By the way this approach is fragile and
should not be used in any production code, I have just shared it here because I
have seen people checking sign of number like this.

2) By
using Relational Operators in Java

Use relational operator to check if number is positive or not,  if number >=0 means number
is positive if number<0 means number is negative in Java
this should work for double and float as well
but I haven't tested it for all values.

 public
static String
checkSignWithRelational(double number){

        if( number <
0){

            return
"negative";

        }else {

            return
"positive";

        }

 }

Testing:

System.out.println("0.0 is " + checkSignWithRelational(0.0));

System.out.println("2.0 is "
+ checkSignWithRelational(2.0));

System.out.println("-2.0 is "
+ checkSignWithRelational(-2.0));

System.out.println("Double.POSITIVE_INFINITY
is " + checkSignWithRelational(Double.POSITIVE_INFINITY));

System.out.println("Double.NEGATIVE_INFINITY
is " + checkSignWithRelational(Double.NEGATIVE_INFINITY));



Output

0.0 is positive

2.0 is positive

-2.0 is negative

Double.POSITIVE_INFINITY is positive

Double.NEGATIVE_INFINITY is negative

In my opinion this should be the right way to check whether a number is
positive or negative in Java. Let me know if you guys have seen any issue using
relational operator to check sign of a number in Java.

3) By
Using Bit Shift operator

This is an alternative way of checking
if a number is positive or negative in Java and used if Interviewer ask you
not to use relational operators. Negative numbers in Java are represented using
2's complement method and since long and int are signed
integer along with byte and short, most
significant bit represent sign of number which should be 0 for positive number
and 1 for negative number in binary format. By using bit shift operator or a
proper mask you can check if most significant bit is 1 or zero. Here is example
for checking int and long values :

  public static
String checkSign(int number){

        if(number == 0) return "positive";

        if(number >>
31 != 0){

            return
"negative";

        }else{

            return
"positive";

        }

    }

 

    public static String checkSign(long
number){

        if(number == 0) return "positive";

        if(number >>
63 != 0){

            return
"negative";

        }else{

            return
"positive";

        }

    }

Testing for checkSign(int) method

System.out.println("0 is " + checkSign(0));

System.out.println("2 is "
+ checkSign(2));

System.out.println("-2 is "
+ checkSign(-2));

System.out.println("Integer.MAX_VALUE is
" + checkSign(Integer.MAX_VALUE));

System.out.println("Integer.MIN_VALUE
is " + checkSign(Integer.MIN_VALUE));



Output

0 is positive

2 is positive

-2 is negative

Integer.MAX_VALUE is positive

Integer.MIN_VALUE is negative

This method can be extended to check double and float value by
representing double and float values
into long

using Double.longBitsToDouble() or Float.floatToIntBits(number).

4) Use Math.signum()

java.lang.Math provides a method called signum() which
returns signum function of method argument means returns zero if argument is
zero , 1.0 if argument is greater than zero and -1.0 if
argument is less than zero. This is a overloaded method to accept float
and double values.

That’s all on How to check if a number is positive or negative in Java.
From looking all approaches, seems using relational operator seems most easy and
correct solution to find out if number is positive or negative. Don’t use
String approach, its fragile and only works for some data types. 

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Source:http://javarevisited.blogspot.com/2013/01/how-to-check-if-number-is-positive-or-negative-java-example.html