Declaración de retorno faltante para la declaración if/else

Cada return statement is inside an if statement. While it may be logically impossible as written, the compiler needs a return for when none of the if evaluate true.

Yo recomiendo:

public int checkGuess(int number, int randomnumber){
    int retVal = 0;
    this.randomnumber= randomnumber;

    if (number == randomnumber) {
        retVal = 0;
    } else if (number < randomnumber) {
        retVal = -1;
    } else if (number > randomnumber) {
        retVal = 1;
    }
    return retVal;
}

This solution fixes the compiler problem and improves readability slightly, in my opinion.

Alternatively, there's this solution:

public int checkGuess(int number, int randomnumber){
    this.randomnumber= randomnumber;

    if (number == randomnumber) {
        return 0;
    } else if (number < randomnumber) {
        return -1;
    } else if (number > randomnumber) {
        return 1;
    } else {
        //throw an exception
    }
}    

Throwing an exception will allow you to get out of the method without returning anything... because arguably, if you get to the final else, something clearly went wrong.

You could try changing the las else if a else.

if (number == randomnumber)
    return 0;

else if (number < randomnumber)
    return -1;

else
    return 1;

The compiler is not required to be able to figure out whether or not your if/else tree covers every possible case. And it would be awful if you could write code that some compilers were smart enough to figure out were okay and other compilers weren't. Rather than having a precise specification for exactly how smart a compiler has to be, Java requires you to write clear, clean code.

The closing curly brace of the function is, by the definition in the Java specification, reachable. That you can prove that it is not reachable by some other definition of "reachable" doesn't matter. The Java specification notion of "reachable" is a formal notion explained in detail in the specification that compilers can actually implement. It is not the common sense notion of "reachable", which one could never teach to a computer anyway.

Basically, the compiler is not smart enough to realzie that number == randomnumber || number < randomnumber || number > randomnumber is a tautology. Since you don't have an else, it thinks it's possible you will get past the conditional and then hit the end of function without returning. The easiest fix is to change your last elseif a solo un else. You know from the previous conditions that number > randomnumber must be true if you get to that point. (You could add an assert if you're paranoid and want to be sure.)

No hay necesidad de else después de return declaración:

if (number == randomnumber)
  return 0;

if (number < randomnumber)
  return -1;

// Must be true: if (number > randomnumber)
return 1;

Note that the reason this solves the problem is because the compiler does not check for solutions that logically must return. e.g. while A must be > < or = to B, it does not check for that relationship. It is looking for possibilities that cover every path explicitly.

You need an else for that to work as you expect, it does not evaluate your conditions to know that it will always return

a return should be made inside an else or outside the entire if/else-if. It's possible a return is never made with all returns depending on a condition

if (condition 1)
    return a;
else if (condition 2)
    return b;
else if (condition 3)
    return c;

If neither condition 1, 2, or 3 are met there will no return. A return should always be made available. So a fix would be:

if (condition 1)
    return a;
else if (condition 2)
    return b;
else if (condition 3)
    return c;
else
    return d;

Or

if (condition 1)
    return a;
else if (condition 2)
    return b;
else if (condition 3)
    return c

return d;

Java requires that non-void methods are guaranteed to return something (even if it's null). If there is an if statement, by definition, only sometimes your return statements are run.

if(number == randomnumber) {
    return 0;
} else if(number < randomnumber) {
    return -1;
} else if(number > randomnumber) {
    return 1;
} else {
    return -2;
}

The else statement isn't required, but it does make it easier to understand imo.