Unreachable code error vs. dead code warning in Java under Eclipse?

Question

Does anyone know why:

public void foo()
{
    System.out.println(\"Hello\");
    return;
    System.out.println(\"World!\");
}

Would be rep

Answer 1

Unreachable code is an error according to the Java Language Spec.

To quote from the JLS:

The idea is that there must be some possible execution path from the beginning of the constructor, method, instance initializer or static initializer that contains the statement to the statement itself. The analysis takes into account the structure of statements. Except for the special treatment of while, do, and for statements whose condition expression has the constant value true, the values of expressions are not taken into account in the flow analysis.

What that means, is that the if block is not taken into account, since if you go through one of the paths of the if statement, you could reach final print statement. If you changed your code to be:

public void foo() {
    System.out.println("Hello");
    if (true)
        return;
    else
        return;
    System.out.println("World!");
}

then suddenly it wouldn't compile anymore, since there is no path through the if statement that would allow the last line to be reached.

That is, a Java compliant compiler is not allowed to compile your first code fragment. To further quote the JLS:

As an example, the following statement results in a compile-time error:

while (false) { x=3; }

because the statement x=3; is not reachable; but the superficially similar case:

if (false) { x=3; }

does not result in a compile-time error. An optimizing compiler may realize that the statement x=3; will never be executed and may choose to omit the code for that statement from the generated class file, but the statement x=3; is not regarded as "unreachable" in the technical sense specified here.

The second warning that Eclipse gives, about dead code, is a warning generated by the compiler, that is not "unreachable", according to the JLS, but in practice is. This is an additional lint style check that Eclipse provides. This is entirely optional, and, by using the Eclipse configuration, can be disabled, or turned into a compiler error instead of a warning.

This second block is a "code smell", if (false) blocks are normally put in to disable code for debugging purposes, having it left behind is typically accidental, and hence the warning.

In fact, Eclipse does even more advanced tests to determine the possible values for an if statement to determine whether or not it is possible to take both paths. For example, Eclipse would also complain about dead code in the following method:

public void foo() {
    System.out.println("Hello");
    boolean bool = Random.nextBoolean();
    if (bool)
        return;
    if (bool || Random.nextBoolean())
      System.out.println("World!");
}

It will generate an unreachable code for the second if statement, since it can reason that bool must only be false at this point in the code. In such a short code fragment it is obvious that the two if statements are testing the same thing, however if there are 10-15 code lines in the middle it might not be so obvious anymore.

So in summary, the difference between the two: one is forbidden by the JLS, and one is not, but is detected by Eclipse as a service to the programmer.

Answer 2

This is to allow a kind of conditionally compilation.
It is not an error with if, but the compiler will flag an error for while, do-while and for.
This is OK:

if (true) return;    // or false
System.out.println("doing something");

This are errors

while (true) {
}
System.out.println("unreachable");

while (false) {
    System.out.println("unreachable");
}

do {
} while (true);
System.out.println("unreachable");

for(;;) {
}
System.out.println("unreachable");

---

It is explained at the end of JLS 14.21: Unreachable Statements:

The rationale for this differing treatment is to allow programmers to define "flag variables" such as:

 static final boolean DEBUG = false;

and then write code such as:

   if (DEBUG) { x=3; }

The idea is that it should be possible to change the value of DEBUG from false to true or from true to false and then compile the code correctly with no other changes to the program text.

Answer 3

I think one way to some it up is that unreachable code is most likely a mistake, and the JLS tries to protect you from such mistakes.

Allowing if (true) return; is a good way to work around the JLS limitation if you actually want to do this on purpose. If the JLS stopped this, it would be getting in the way. In addition, it should also stop:

 public static boolean DEBUG = true; //In some global class somewhere else


 ...

 if (DEBUG) return; //in a completely unrelated class.

 ...

Because the DEBUG constant is completely in-lined, and functionally equivalent to just typing a true in that if condition. From a JLS perspective those two cases are very similar.

Answer 4

If you wish to ignore the warning "dead code warning in Java under Eclipse" do the following inside eclipse*:

1. Click Window-Preferences-Java-Compiler-Errors/Warnings
2. Click on "Potential Programming Problems"
3. Choose "Ignore" the "Dead Code eg if(false)"
4. Click Apply
5. Click OK

Save and close your eclipse IDE When you reopen eclipse, these specific warnings should no longer be listed.

*For this example solution I'm using Eclipse IDE for Java Developers - Version: Mars.2 Release (4.5.2)

Answer 5

I made some try on eclipse and think there are 3 kinds of dead code handling of JDK: 1) no warn, 2) warn and 3) error.

For a typical "IF" conditional compilataion code, JDK detect this and didnt report it as dead code. For a dead code that is caused by a constant boolean flag, JDK detect this and report it at warning level. For dead code that is cause by the program's control flow, JDK detect it as error.

Below is my try:

    public class Setting {
        public static final boolean FianlDebugFlag = false;
    }


    class B {
    .....

    // no warn, it is typical "IF" conditional compilataion code
    if(Setting.FianlDebugFlag) 
        System.out.println("am i dead?");   
    if(false) 
        System.out.println("am i dead?");   


    // warn, as the dead code is caused by a constant boolean flag
    if(ret!=null && Setting.FianlDebugFlag) 
        System.out.println("am i dead?");   

    if(Setting.FinalDebug)                  
        return null;                                                            
    System.out.println("am i dea?");        

    // error, as the dead code is due to the program's control flow
    return null;
    System.out.println("am i dead");        
    }

Answer 6

The difference is in the semantics between run-time and compile-time. In your second example, the code compiles to an if-else branch in the bytecode, and eclipse is simply smart enough to tell you that the else portion will never be reached in runtime. Eclipse only warns you, because it is still legal code.

In your first example, it is an error because the code is illegal by the definition of java. The compiler doesn't allow you to create byte code with unreachable statements.