Boolean Que

[Hemant Gupt]

Source : Marcus Green

What will happen when you attempt to compile and run this code?

public class MyIf{
public static void main(String argv[]){
MyIf mi = new MyIf();
}
MyIf(){
boolean b = false;
if(b=false){
System.out.println("The value of b is"+b);
}
}
}

1) Run time error, a boolean cannot be appended using the + operator
2) Compile time error the parameter to the if operator must evaluate
to a boolean
3) Compile time error, cannot simultaneously create and assign value for boolean
value
4) Compilation and run with no output



Answer is 4)
I know that compilation will suceed, but why there is no output. As per me,
System.out.println("The value of b is"+b);
should be executed.

[Minu Jain]

This is something you must really pay attention to while preparing for SCJP.
Notice that the if statement is NOT doing any comparison(as it is NOT using ==) but it is doing an assignment instead(as it is using =).
Though there is nothing wrong with such a syntax, but ultimately in your code the condition for if is false. Hence it will not go inside the if block at all.
Hope my explanation is clear enough

[Madhukar Ojha]

Hi Rancher ,

You should keep in mind two cases .

class{
// some codes
boolean b ;
if(false) {
// some codes
}
if(b=false) {
// some codes
}
// other stuffs .
}
Inside if , assignment to boolean variable is valid .
Both if statements are true in java .Since both expression will result in false , there will not be any output .

[Krishnamoorthy Vuyala Muralidharan]

Hi

In the if statement, if you use the assignment operator it will first assign the right-hand side value to the left-hand side variable. Then the if statement checks the variable 'b'(in your example). So it can be split up into following steps to make it simple:

if(b=false) is split up into following:

1. b=false;
2. if(b)

In the second step b evaluates to false so it will not enter into the 'if' block and it will enter in the else block if there is one.

Hope it is clear.

Kris

[vaibhav mishra]

Thank you guys I also had a similar doubt but now it is cleared because of your explanation

[vaibhav panghal]

thank you krishnamoorthy i had this doubt too since things don't work the same way in c/c++ . thank you again .

[Bob Ruth]

Any time that you have an expression like this

(b=false)

yes, the value false does get assigned to b. But the other effect is that the expression evaluates to the same value.

In other words

int x;
int y = (x = 3);

then both x and y take the value 3 because, for the y = statement (x = 3) evaluated to the value 3.

so (b = false) both set b to false AND evaluated to false.

[Krishnamoorthy Vuyala Muralidharan]

Originally posted by vaibhav panghal:
thank you krishnamoorthy i had this doubt too since things don't work the same way in c/c++ . thank you again .

Vaibhav, You are welcome.