Why does Eclipse complain about dead code? [closed]

Question

Eclipse keep stating that the last elseif and the else is dead code but I don't get it.

if (img0 != null && img1 != null) {
    code;

} else if (img0 != null) {
    code;
} else if (img1 != null) {
    code;
} else {
    code;
}

I reason like this:

1. the if evaluates true if bote img0 and img1 is not null
2. if it evaluates false then
   • img0 is null OR
   • img1 is null OR
   • both img0 AND img1 is null.
3. the first elseif evaluates true if img0 is not null, if it evaluates false img1 could be not null OR both img0 AND img1 could be null

What am I missing, where is the "deadness"?

Thanks in advance.

Answer 1

Take a look at the use of your code in these two ways: -

Way 1: -

public static void main(String[] args)
{
    String img0 = null;
    String img1 = "Asdf";

    /** Currently there is no code here, that can modify the value of 
        `img0` and `img1` and Compiler is sure about that. 
    **/

    /** So, it's sure that the below conditions will always execute in a
        certain execution order. And hence it will show `Dead Code` warning in 
        either of the blocks depending upon the values.
    **/

    if (img0 != null && img1 != null) {
       // code;

    } else if (img0 != null) {
        //code;

    } else if (img1 != null) {
        //code;
    } else {
       // code;
    }
}

In this case, you would certainly get dead code warning on one block or the other, because you are setting the value just before the blocks, and the compiler is sure that the values won't change between the initialization and execution of those blocks.

Way 2: -

public static void main(String[] args)
{
    String img0 = null;
    String img1 = "Asdf";

    show(img0, img1);
}

public static void show(String img0, String img1) {

    /** Now here, compiler cannot decide on the execution order, 
        as `img0` and `img1` can have any values depending upon where 
        this method was called from. And hence it cannot give dead code warning.
    **/

    if (img0 != null && img1 != null) {
       // code;

    } else if (img0 != null) {
        //code;

    } else if (img1 != null) {
        //code;
    } else {
       // code;
    }
}

Now, in this case, you won't get dead code warnings, because Compiler is not sure, from where the show method might get invoked. Values of img0 and img1 can be anything inside the method.

• In case both of them are null, the last else will be executed.
• In case one of them is null, one of the else if will be executed.
• And, in case none of them are null, your if will be executed.

---

Note : -

If you want, you can configure Eclipse not to show warnings for certain cases like - Unneccessary else, Unused Imports, etc.

Go to Windows -> Preferences -> Java (on Left Panel) -> Compiler -> Errors/ Warnings