Converting Non-Generic Class Extended by Generic Class in C# to Java

Question

I am trying to convert the following classes in C# over to Java.

Result is a non-generic class that is extended by the generic Result<T> class.

Their sample usages are as follows:

// When we only care if the operation was successful or not.
Result result = Result.OK();

// When we also want to store a value inside the Result object.
Result<int> result = Result.OK<int>(123);    

In Java, each class needs to be defined in its own file (unless they're embedded).

Unfortunately, I cannot find a way to have the base and extended classes share the same name like they do in C#.

Is there a way to convert the following C# code to Java?

Result.cs:

using System;

namespace MyProject
{
    public class Result
    {
        private bool _isSuccess;
        private string _errorMsg = "";

        public bool IsSuccess()
        {
            return _isSuccess;
        }

        public bool IsFailure()
        {
            return !_isSuccess;
        }

        public string ErrorMsg()
        {
            return _errorMsg;
        }

        public Result(bool isSuccess, string errorMsg)
        {
            bool errorMsgIsEmpty = string.IsNullOrEmpty(errorMsg);

            if (isSuccess && !errorMsgIsEmpty)
            {
                throw new Exception("cannot have error message for successful result");
            }
            else if (!isSuccess && errorMsgIsEmpty)
            {
                throw new Exception("must have error message for unsuccessful result");
            }

            _isSuccess = isSuccess;

            if (!errorMsgIsEmpty)
            {
                _errorMsg = errorMsg;
            }
        }

        public static Result Fail(string errorMsg)
        {
            return new Result(false, errorMsg);
        }

        public static Result<T> Fail<T>(string errorMsg)
        {
            return new Result<T>(default(T), false, errorMsg);
        }

        public static Result OK()
        {
            return new Result(true, "");
        }

        public static Result<T> OK<T>(T value)
        {
            return new Result<T>(value, true, "");
        }
    }

    public class Result<T> : Result
    {
        private T _value;

        public T Value()
        {
            return _value;
        }

        public Result(T value, bool isSuccess, string errorMsg) : base(isSuccess, errorMsg)
        {
            _value = value;
        }
    }
}

---

UPDATE: Special thanks to @Juan Cristóbal Olivares below for the answer! Below are my changes:

NOTE: I had to rename the typed fail and ok functions to failT and okT respectively because Java does not allow functions that only differ by the return type.

Result.java:

public class Result<T> {
    private boolean isSuccess;
    private String errorMsg = "";
    private T value;

    public boolean isSuccess() {
        return isSuccess;
    }

    public boolean isFailure() {
        return !isSuccess;
    }

    public String errorMsg() {
        return errorMsg;
    }

    public T value() {
        return value;
    }

    public Result(boolean isSuccess, String errorMsg) throws Exception {
        boolean errorMsgIsEmpty = StringUtil.IsNullOrEmpty(errorMsg);

        if (isSuccess && !errorMsgIsEmpty) {
            throw new Exception("cannot have error message for successful result");
        } else if (!isSuccess && errorMsgIsEmpty) {
            throw new Exception("must have error message for unsuccessful result");
        }

        this.isSuccess = isSuccess;

        if (!errorMsgIsEmpty) {
            this.errorMsg = errorMsg;
        }
    }

    public Result(T value, boolean isSuccess, String errorMsg) throws Exception {
        this(isSuccess, errorMsg);
        this.value = value;
    }

    public static Result<?> fail(String errorMsg) throws Exception {
        return new Result<>(false, errorMsg);
    }

    public static <T> Result<T> failT(String errorMsg) throws Exception {
        return new Result<T>(null, false, errorMsg);
    }

    public static Result<?> ok() throws Exception {
        return new Result<>(true, "");
    }

    public static <T> Result<T> okT(T value) throws Exception {
        return new Result<T>(value, true, "");
    }
}

Sample usages:

// When we only care if the operation was successful or not.
Result<?> result = Result.ok();

// When we also want to store a value inside the Result object.    
Result<Integer> result = Result.okT(123);    

Answer 1

When you create a C# generic class with 1 generic argument, this class is generated:

SomeClass`1

See What's the meaning of “apostrophe + number” in the object type of properties with generics (eg. “Collection`1”)? .

So while the non generic class is named SomeClass , the generic versions are SomeClass`1 , SomeClass`2 , etc (depending on the number of generic arguments).

Java generics are different. Generic information is removed at compile time.

See Are generics removed by the compiler at compile time .

This means that the non generic and generic versions are just the same class ( SomeClass ).

So, for this usecase, you just may need to define the generic version. This version will work for generic and non generic cases.