Reputation: 2072

Why does my if condition not accept an integer in java?

Currently I'm using:

 int a=10;
 if(a=20)
     printf("TRUE");
 else
     printf("false");

Which prints, in C, the value TRUE.

But in case of java:

int a=10;
if(a=20)
    System.out.println("TRUE");
else
    System.out.println("FALSE");

I'll get a compile time error about an incompatible type.

Upvotes: 1

Answers (7)

Ashwini Raman

Reputation: 986

for comparison you should use ==

if(a==20) instead of a=20, which is assignment

Upvotes: 0

Leonidas K

Reputation: 491

You need to replace = with ==.

= is an assignment operator

int a=10;
if(a==20){
System.out.println("TRUE");}
else{
System.out.println("FALSE");}

This should work fine.

Edit:

In C, the returned integer value of the assignment (in this case 20) is a positive int, which evaluates to true. In java, a boolean is expected.

(See the more thorough answer provided by Aleks G)

Upvotes: 6

paxdiablo

Reputation: 882716

A single = is an assignment. If you want to test for equality, you use the double == in both C and Java.

It should be evident that you're doing the wrong thing, even in the C version, since you set a to 10 and then, on "comparing" it to 20, you get a true value. I'm not aware of any mathematical systems where 10 and 20 are considered equal :-)

What happens in your C case is that the assignment actually "returns" a value as well.

The statement:

if (a = 20) ...

is equivalent to:

a = 20;
if (a) ...

so it first sets a to 20, then uses that as the if condition. Since 0 is false and anything else is true, the body of the if is executed.

A good compiler (like gcc) will actually warn you of what you're doing so that you don't get caught out by these little things. Some people also use the trick of putting constants first:

if (20 == a) ...

so that, if you mistakenly use assignment, it's a syntax error. But I find that sort of code ugly, especially since I mostly use those good compilers mentioned above :-)

The reason you get an error in Java is because it's much more strict on what you can do with assignments. By that, I mean there is no automatic conversion from int to boolean as with C. So this code refuses to compile, producing an error:

class Test {
    static public void main(String[] args) {
        int a = 10;
        if (a = 20)
            System.out.println ("true");
    }
}

But you can still get burnt with something like this:

class Test {
    static public void main(String[] args) {
        boolean a = false;
        if (a = true)
            System.out.println ("true");
    }
}

Because that assignment is of a boolean value inside the if, it doesn't cause a compilation error due to the wrong type. However, since it prints true, it's still not what you wanted.

Bottom line, go back an re-read that first paragraph. = is assignment, == is equality checking.

Upvotes: 2

Aleks G

Reputation: 57346

The reason for that is that in C there is no specific type boolean - instead any non-0 integer evaluates to a boolean "true". Thus in your C code:

if(a=20)

a is assigned the value 20, which is non-0 - and the condition is evaluated as true

In java, there's a fundamental type boolean and the value of the conditional inside if must be of this type.

a=20

in Java assigns 20 to a and returns the final result of evaluation as integer value 20, however type boolean is expected - hence you're getting a compile-time error about the incompatible types.

If you want to do a comparison of a with 20, however, you need to use == operator both in C and Java: