Implementing equals(), hashCode(), toString() in the hierarchy like the Optional<?> from Guava

I have simple hierarchy which looks [a little bit] like the Optional from Guava:

```
public abstract class Result {

public static Result get(ResultType result) {
return new PresentResult(result);
}

public static Result absent(Throwable exception) {
return new AbsentResult(exception);
}

Result() {
// clients of the package can't extend this class
}

public abstract boolean isPresent();
public abstract T get();
public abstract Throwable getException();
}

class PresentResult extends Result {

private final T result;

public PresentResult(T result) {
this.result = result;
}

@Override
public boolean isPresent() {
return true;
}

@Override
public T get() {
return result;
}

@Override
public Throwable getException() {
throw new UnsupportedOperationException();
}

@Override
public boolean equals(Object obj) {
if (obj == this) {
return true;
}
if (!(obj instanceof PresentResult)) {
return false;
}
PresentResult other = (PresentResult) obj;
return Objects.equal(get(), other.get());
}

@Override
public int hashCode() {
return Objects.hashCode(get());
}

@Override
public String toString() {
return Objects.toStringHelper(this)
.add("result", get())
.toString();
}
}

class AbsentResult extends Result {

private final Throwable exception;

public AbsentResult(Throwable exception) {
this.exception = exception;
}

@Override
public boolean isPresent() {
return false;
}

@Override
public Object get() {
throw new UnsupportedOperationException();
}

@Override
public Throwable getException() {
return exception;
}

@Override
public boolean equals(Object obj) {
if (obj == this) {
retu

Some remarks:

-
Why does getException() return a Throwable instead of an Exception? Or even better, a RuntimeException? More generally, what is the purpose of this method? If I were you I'd "drop" it and make .get() throw the wanted exception instead. Since the exception would be a RuntimeException, there would be no need to declare it.

-
Is it on purpose that PresentResult and AbsentResult are not final? What do you expect further subclasses to extend if anything?

-
Is it legal to initialize a PresentResult with a null value, an AbsentResult with a null exception?

StackExchange Code Review Q#70025, answer score: 2