Missing return statement for if/else statement

Every 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.

I recommend:

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.

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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.