java - Missing return statement in a non-void method compiles

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Top 5 Answer for java - Missing return statement in a non-void method compiles

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95

Why would a language allow us to have a non-void method having an infinite loop and not returning anything?

The rule for non-void methods is every code path that returns must return a value, and that rule is satisfied in your program: zero out of zero code paths that return do return a value. The rule is not "every non-void method must have a code path that returns".

This enables you to write stub-methods like:

IEnumerator IEnumerable.GetEnumerator()  {      throw new NotImplementedException();  } 

That's a non-void method. It has to be a non-void method in order to satisfy the interface. But it seems silly to make this implementation illegal because it does not return anything.

That your method has an unreachable end point because of a goto (remember, a while(true) is just a more pleasant way to write goto) instead of a throw (which is another form of goto) is not relevant.

Why doesn't the compiler even warn about returning something?

Because the compiler has no good evidence that the code is wrong. Someone wrote while(true) and it seems likely that the person who did that knew what they were doing.

Where can I read more about reachability analysis in C#?

See my articles on the subject, here:

ATBG: de facto and de jure reachability

And you might also consider reading the C# specification.

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86

The Java compiler is smart enough to find the unreachable code ( the code after while loop)

and since its unreachable, there is no point in adding a return statement there (after while ends)

same goes with conditional if

public int get() {    if(someBoolean) {         return 10;    }    else {      return 5;    }    // there is no need of say, return 11 here; } 

since the boolean condition someBoolean can only evaluate to either true or false, there is no need to provide a return explicitly after if-else, because that code is unreachable, and Java does not complain about it.

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73

The compiler knows that the while loop will never stop executing, hence the method will never finish, hence a return statement is not necessary.

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66

Given your loop is executing on a constant - the compiler knows that it's an infinite loop - meaning the method could never return, anyway.

If you use a variable - the compiler will enforce the rule:

This won't compile:

// Define other methods and classes here public int doNotReturnAnything() {     var x = true;      while(x == true) {         //do something     }     //no return statement - won't compile } 
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54

The Java specification defines a concept called Unreachable statements. You are not allowed to have an unreachable statement in your code (it's a compile time error). You are not even allowed to have a return statement after the while(true); statement in Java. A while(true); statement makes the following statements unreachable by definition, therefore you don't need a return statement.

Note that while Halting problem is undecidable in generic case, the definition of Unreachable Statement is more strict than just halting. It's deciding very specific cases where a program definitely does not halt. The compiler is theoretically not able to detect all infinite loops and unreachable statements but it has to detect specific cases defined in the specification (for example, the while(true) case)

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