错误:selection does not contain a main type

本文详细介绍了在使用Eclipse创建Java项目时遇到的两个常见错误——'LaunchError'和'selectiondoesnotcontainamaintype'的解决方法。通过调整项目设置,确保源代码被正确编译,从而避免这些错误的发生。

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今天在Eclipse上新建了一个项目,编写并运行程序的时候弹出一个“Launch Error”错误对话框,曰:selection does not contain a main type。再编写一个简单的HelloWorld程序测试,发现结果一样。
上网查找原因,找到一个解决办法:
出现这个问题,是因为java文件不在项目的src路径内,也就是说源代码未被eclipse编译,字节码不存在无法运行了。在项目名上右键 -> Builder Path -> Configure Build Path -> 选择Source面板,再点Add Folder, 把源代码所在的包路径的上层目录加进来.
操作起来挺麻烦的,弄了好一会也没弄好,干脆自己找解决办法好了。
我再次新建一个项目,编写程序并运行,发现没有该错误,奇怪了。
仔细想想两个项目的不同之处,不同的地方就是在新建项目的时候第一个项目勾选了Java Settings下的Allow output folders for source folders,而第二个项目则没有勾选,莫非问题就出现在这里?
重新再新建已项目,勾选Allow output folders for source folders选项,编写程序运行,果然再次发现selection does not contain a main type错误。
问题解决。
请查看以下的C++代码的编写要求,请根据代码要求开始编写代码 PURPOSE: This file is a proforma for the EEET2246 Laboratory Code Submission/Test 1. This file defines the assessment task which is worth 10% of course in total - there is no other documentation. At the BASIC FUNCTIONAL REQUIREMENTS level, your goal is to write a program that takes two numbers from the command line and perform and arithmetic operations with them. Additionally your program must be able to take three command line arguments where if the last argument is 'a' an addition is performed, and if 's' then subtraction is performed with the first two arguments. At the FUNCTIONAL REQUIREMENTS level you will be required to extend on the functionality so that the third argument can also be 'm' for multiplication,'d' for division and 'p' for exponential operations, using the first two arguments as the operands. Additionally, at this level basic error detection and handling will be required. The functionality of this lab is relatively simple: + - / * and "raised to the power of" The emphasis in this lab is to achieve the BASIC FUNCTIONALITY REQUIREMENTS first. Once you a basic program functioning then you should attempt the FUNCTIONALITY REQUIREMENTS and develop your code so that it can handle a full range of error detection and handling. ___________________________________________________________________________________________ ___ GENERAL SPECIFICATIONS (mostly common to all three EEET2246 Laboratory Code Submissions): G1. You must rename your file to lab1_1234567.cpp, where 1234567 is your student number. Your filename MUST NEVER EVER contain any spaces. _under_score_is_Fine. You do not need to include the 's' in front of your student number. Canvas will rename your submission by adding a -1, -2 etc. if you resubmit your solution file - This is acceptable. G2. Edit the name/email address string in the main() function to your student number, student email and student name. The format of the student ID line is CSV (Comma Separated Variables) with NO SPACES- student_id,student_email,student_name When the program is run without any operands i.e. simply the name of the executable such as: lab1_1234567.exe the program MUST print student ID string in Comma Separated Values (CSV) format with no spaces. For example the following text should be outputted to the console updated with your student details: "1234567,s1234567@student.rmit.edu.au,FirstName_LastName" G3. All outputs are a single error character or a numerical number, as specified by the FUNCTIONAL REQURMENTS, followed by a linefeed ( endl or \n). G4. DO NOT add more than what is specified to the expected console output. Do NOT add additional information, text or comments to the output console that are not defined within the SPECIFICATIONS/FUNCTIONAL REQURMENTS. G5. DO NOT use 'cin', system("pause"), getchar(), gets(), etc. type functions. Do NOT ask for user input from the keyboard. All input MUST be specified on the command line separated by blank spaces (i.e. use the argv and argc input parameters). G6. DO NOT use the characters: * / \ : ^ ? in your command line arguments as your user input. These are special character and may not be processed as expected, potentially resulting in undefined behaviour of your program. G7. All input MUST be specified on the command line separated by blank spaces (i.e. use the argc and argv[] input parameters). All input and output is case sensitive unless specified. G8. You should use the Integrated Debugging Environment (IDE) to change input arguments during the development process. G9. When your code exits the 'main()' function using the 'return' command, you MUST use zero as the return value. This requirement is for exiting the 'main()' function ONLY. A return value other than zero will indicate that something went wrong to the Autotester and no marks will be awarded. G10. User-defined functions and/or class declarations must be written before the 'main()' function. This is a requirement of the Autotester and failure to do so will result in your code scoring 0% as it will not be compiled correctly by the Autotester. Do NOT put any functions/class definitions after the 'main()' function or modify the comments and blank lines at the end of this file. G11. You MUST run this file as part of a Project - No other *.cpp or *.h files should be added to your solution. G12. You are not permitted to add any other #includes statements to your solution. The only libraries permitted to be used are the ones predefined in this file. G13. Under no circumstances is your code solution to contain any go_to labels - Please note that the '_' has been added to this description so that this file does not flag the Autotester. Code that contains go_to label like syntax will score 0% and will be treated as code that does not compile. G14. Under no circumstances is your code solution to contain any exit_(0) type functions. Please note that the '_' has been added to this description so that this file does not flag the Autotester. Your solution must always exit with a return 0; in main(). Code that contains exit_(0); label like syntax will score 0% and will be treated as code that does not compile. G15. Under no circumstances is your code solution to contain an infinite loop constructs within it. For example usage of while(1), for(int i; ; i++) or anything similar is not permitted. Code that contains an infinite loop will result in a score of 0% for your assessment submission and will be treated as code that does not compile. G16. Under no circumstances is your code solution to contain any S_l_e_e_p() or D_e_l_a_y() like statements - Please note that the '_' has been added to this description so that this file does not flag the Autotester. You can use such statements during your development, however you must remove delays or sleeps from your code prior to submission. This is important, as the Autotester will only give your solution a limited number of seconds to complete (i.e. return 0 in main()). Failure for your code to complete the required operation/s within the allotted execution window will result in the Autotester scoring your code 0 marks for that test. To test if your code will execute in the allotted execution window, check that it completes within a similar time frame as the provided sample binary. G17. Under no circumstances is your code solution to contain any characters from the extended ASCII character set or International typeset characters. Although such characters may compile under a normal system, they will result in your code potentially not compiling under the Autotester environment. Therefore, please ensure that you only use characters: a ... z, A ... Z, 0 ... 9 as your variable and function names or within any literal strings defined within your code. Literal strings can contain '.', '_', '-', and other basic symbols. G18. All output to console should be directed to the standard console (stdout) via cout. Do not use cerr or clog to print to the console. G19. The file you submit must compile without issues as a self contained *.cpp file. Code that does not compile will be graded as a non-negotiable zero mark. G20. All binary numbers within this document have the prefix 0b. This notation is not C++ compliant (depending on the C++ version), however is used to avoid confusion between decimal, hexadecimal and binary number formats within the description and specification provided in this document. For example the number 10 in decimal could be written as 0xA in hexadecimal or 0b1010 in binary. It can equally be written with leading zeroes such as: 0x0A or 0b00001010. For output to the console screen you should only ever display the numerical characters only and omit the 0x or 0b prefixes (unless it is specifically requested). ___________________________________________________________________________________________ ___ BASIC FUNCTIONAL REQUIREMENTS (doing these alone will only get you to approximately 40%): M1. For situation where NO command line arguments are passed to your program: M1.1 Your program must display your correct student details in the format: "3939723,s3939723@student.rmit.edu.au,Yang_Yang" M2. For situation where TWO command line arguments are passed to your program: M2.1 Your program must perform an addition operation, taking the first two arguments as the operands and display only the result to the console with a new line character. Example1: lab1_1234567.exe 10 2 which should calculate 10 + 2 = 12, i.e. the last (and only) line on the console will be: 12 M3. For situations where THREE command line arguments are passed to your program: M3.1 If the third argument is 'a', your program must perform an addition operation, taking the first two arguments as the operands and display only the result to the console with a new line character. M3.2 If the third argument is 's', your program must perform a subtraction operation, taking the first two arguments as the operands and display only the result to the console with a new line character. The second input argument should be subtracted from the first input argument. M4. For situations where less than TWO or more than THREE command line arguments are passed to your program, your program must display the character 'P' to the console with a new line character. M5. For specifications M1 to M4 inclusive: M5.1 Program must return 0 under all situations at exit. M5.2 Program must be able to handle integer arguments. M5.3 Program must be able to handle floating point arguments. M5.4 Program must be able to handle one integer and one floating point argument in any order. Example2: lab1_1234567.exe 10 2 s which should calculate 10 - 2 = 8, i.e. the last (and only) line on the console will be: 8 Example3: lab1_1234567.exe 10 2 which should calculate 10 + 2 = 12, i.e. the last (and only) line on the console will be: 12 Example4: lab1_1234567.exe 10 4 a which should calculate 10 + 4 = 14, i.e. the last (and only) line on the console will be: 14 ___________________________________________________________________________________________ ___ FUNCTIONAL REQUIREMENTS (to get over approximately 50%): E1. For situations where THREE command line arguments (other than 'a' or 's') are passed to your program: E1.1 If the third argument is 'm', your program must perform a multiplication operation, taking the first two arguments as the operands and display only the result to the console with a new line character. E1.2 If the third argument is 'd', your program must perform a division operation, taking the first two arguments as the operands and display only the result to the console with a new line character. E1.3 If the third argument is 'p', your program must perform an exponential operation, taking the first argument as the base operand and the second as the exponent operand. The result must be display to the console with a new line character. Hint: Consider using the pow() function, which has the definition: double pow(double base, double exponent); Example5: lab1_1234567.exe 10 2 d which should calculate 10 / 2 = 5, i.e. the last (and only) line on the console will be: 5 Example6: lab1_1234567.exe 10 2 p which should calculate 10 to power of 2 = 100, i.e. the last (and only) line on the console will be: 100 NOTE1: DO NOT use the character ^ in your command line arguments as your user input. Question: Why don't we use characters such as + - * / ^ ? to determine the operation? Answer: Arguments passed via the command line are processed by the operating system before being passed to your program. During this process, special characters such as + - * / ^ ? are stripped from the input argument stream. Therefore, the input characters: + - * / ^ ? will not be tested for by the autotester. See sections G6 and E7. NOTE2: the pow() and powl() function/s only work correctly for given arguments. Hence, your code should output and error if there is a domain error or undefined subset of values. For example, if the result does not produce a real number you code should handle this as an error. This means that if the base is negative you can't accept and exponent between (but not including) -1 and 1. If you get this then, output a MURPHY's LAW error: "Y" and return 0; NOTE3: zero to the power of zero is also undefined, and should also be treated MURPHY's LAW error. So return "Y" and return 0; In Visual Studio, the 0 to the power of 0 will return 1, so you will need to catch this situation manually, else your code will likely calculate the value as 1. ___ REQUIRED ERROR HANDLING (to get over approximately 70%): The following text lists errors you must detect and a priority of testing. NB: order of testing is important as each test is slight more difficult than the previous test. All outputs should either be numerical or upper-case single characters (followed by a new line). Note that case is important: In C, 'V' is not the same as 'v'. (No quotes are required on the output). E2. Valid operator input: If the third input argument is not a valid operation selection, the output shall be 'V'. Valid operators are ONLY (case sensitive): a addition s subtraction m multiplication d division p exponentiation i.e. to the power of: 2 to the power of 3 = 8 (base exponent p) E3. Basic invalid number detection (Required): Valid numbers are all numbers that the "average Engineering graduate" in Australia would consider valid. Therefore if the first two arguments are not valid decimal numbers, the output shall be 'X'. For example: -130 is valid +100 is valid 1.3 is valid 3 is valid 0.3 is valid .3 is valid ABC123 is not valid 1.3.4 is not valid 123abc is not valid ___ ERROR HANDLING (not marked by the autotester): E4. Intermediate invalid number detection (NOT TESTED BY AUTOTESTER - for your consideration only): If the first two arguments are not valid decimal numbers, the output shall be 'X'. Using comma punctuated numbers and scientific formatted numbers are considered valid. For example: 0000.111 is valid 3,000 is valid - NB: atof() will read this as '3' not as 3000 1,000.9 is valid - NB: atof() will read this as '1' not as 1000.9 1.23e2 is valid 2E2 is valid -3e-0.5 is not valid (an integer must follow after the e or E for floating point number to be valid) 2E2.1 is not valid e-1 is not valid .e3 is not valid E5. Advanced invalid number detection (NOT TESTED BY AUTOTESTER - for your consideration only): If the first two arguments are not valid decimal numbers, the output shall be 'X'. 1.3e-1 is valid 1,00.0 is valid - NB: if the comma is not removed atof() will read this as '1' not as 100 +212+21-2 is not valid - NB: mathematical operation on a number of numbers, not ONE number 5/2 is not valid - NB: mathematical operation on a number of numbers, not ONE number HINT: consider the function atof(), which has the definition: double atof (const char* str); Checking the user input for multiple operators (i.e. + or -) is quite a difficult task. One method may involve writing a 'for' loop which steps through the input argv[] counting the number of operators. This process could also be used to count for decimal points and the like. The multiple operator check should be considered an advanced task and developed once the rest of the code is operational. E6. Input number range checking: All input numbers must be between (and including) +2^16 (65536) or -2^16 (-65536). If the operand is out of range i.e. too small or too big, the output shall be 'R'. LARGE NUMBERS: is 1.2e+999 acceptable input ? what happens if you enter such a number ? try and see. Hint: #INF error - where and when does it come up ? SMALL NUMBERS: is 1.2e-999 acceptable input ? what happens if you enter such a number ? try and see. Test it by writing your own test program. E7. ERROR checks which will NOT be performed are: E7.1 Input characters such as: *.* or / or \ or : or any of these characters: * / ^ ? will not be tested for. E7.2 Range check: some computer systems accept numbers of size 9999e999999 while others flag and infinity error. An infinity error becomes an invalid input Therefore: input for valid numbers will only be tested to the maximum 9.9e99 (Note: 9.9e99 is out of range and your program should output 'R') E8. Division by zero should produce output 'M' E9. Error precedence: If multiple errors occur during a program execution event, your program should only display one error code followed by a newline character and then exit (using a return 0; statement). In general, the precedence of the error reported to the console should be displayed in the order that they appear within this proforma. However to clarify the exact order or precedence for the error characters, the precedence of the displayed error code should occur in this order: 'P' - Incorrect number of input command line arguments (see M4) 'X' - Invalid numerical command line argument 'V' - Invalid third input argument 'R' - operand (command line argument) value out of range 'M' - Division by zero 'Y' - MURPHY'S LAW (undefined error) Therefore if an invalid numerical command line argument and an invalid operation argument are passed to the program, the first error code should be displayed to the console, which in this case would be 'X'. Displaying 'V' or 'Y' would be result in a loss of marks. E10. ANYTHING ELSE THAT CAN GO WRONG (MURPHY'S LAW TEST): If there are any other kinds of errors not covered here, the output shall be 'Y'. Rhetorical question: What for example are the error codes that the Power function returns ? If this happens then the output shall be 'Y'. See section E1.3, NOTE2. ___________________________________________________________________________________________ ___ HINTS: - Use debug mode and a breakpoint at the return statement prior to program finish in main. - What string conversion routines, do you know how to convert strings to number? Look carefully as they will be needed to convert a command line parameter to a number and also check for errors. - ERROR CHECKING: The basic programming rules are simple (as covered in lectures): 1) check that the input is valid. 2) check that the output is valid. 3) if any library function returns an error code USE IT !!! CHECK FOR IT !!! - Most conversion routines do have inbuilt error checking - USE IT !!! That means: test for the error condition and take some action if the error is true. If that means more than 50% of your code is error checking, then that's the way it has to be. ____________________________________________________________________________________________ */ // These are the libraries you are allowed to use to write your solution. Do not add any // additional libraries as the auto-tester will be locked down to the following: #include <iostream> #include <cstdlib> #include <time.h> #include <math.h> #include <errno.h> // leave this one in please, it is required by the Autotester! // Do NOT Add or remove any #include statements to this project!! // All library functions required should be covered by the above // include list. Do not add a *.h file for this project as all your // code should be included in this file. using namespace std; const double MAXRANGE = pow(2.0, 16.0); // 65536 const double MINRANGE = -pow(2.0, 16.0); // All functions to be defined below and above main() - NO exceptions !!! Do NOT // define function below main() as your code will fail to compile in the auto-tester. // WRITE ANY USER DEFINED FUNCTIONS HERE (optional) // all function definitions and prototypes to be defined above this line - NO exceptions !!! int main(int argc, char *argv[]) { // ALL CODE (excluding variable declarations) MUST come after the following 'if' statement if (argc == 1) { // When run with just the program name (no parameters) your code MUST print // student ID string in CSV format. i.e. // "studentNumber,student_email,student_name" // eg: "3939723,s3939723@student.rmit.edu.au,Yang_Yang" // No parameters on command line just the program name // Edit string below: eg: "studentNumber,student_email,student_name" cout << "3939723,s3939723@student.rmit.edu.au,Yang_Yang" << endl; // Failure of your program to do this cout statement correctly will result in a // flat 10% marks penalty! Check this outputs correctly when no arguments are // passed to your program before you submit your file! Do it as your last test! // The convention is to return Zero to signal NO ERRORS (please do not change it). return 0; } //--- START YOUR CODE HERE. // The convention is to return Zero to signal NO ERRORS (please do not change it). // If you change it the AutoTester will assume you have made some major error. return 0; } // No code to be placed below this line - all functions to be defined above main() function. // End of file.
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08-16
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