erro：方法没有返回值 if中有返回值但报错

    public static  double abs(double num){
        if (num>=0){
            return num;
        }
        else{
            return -num;
        }
    }

    }
    public static void main(String[] args) {
        System.out.println(abs(-500));
    }

  public static  double abs(double num){
        if (num>0){
            return num;
        }
        else{
            return -num;
        }

    public static  double abs(double num){
        if (num>0){
            return num;
        }
        else if(num<0){
            return -num;
        }
    }

错误:(17, 5) java: 缺少返回语句