实现Comparable，compareTo名称冲突：“具有相同的擦除，但不会覆盖其他”

Question

I'd like to have a compareTo method that takes a Real (a class for working with arbitrarily large and precise real numbers [well, as long as it's less than 2^31 in length at the moment]) and a compareTo method that takes an Object, but Java isn't letting me and I'm not experienced enough to know why.

I just tried to modify the class to implement Comparable and I got these error messages below. I don't really understand what the error messages mean but I know it's got something to do with the horrible way I'm trying to give the class some flexibility with all the different method signatures for every single method I make, and I can fix it by removing the compareTo(Object other) method, but I would ideally like to keep it. So what I'm really asking is: Is there a way to make these error messages disappear without removing the compareTo(Object other) method and what exactly do these errors mean?

Also, I know there are already some built-in Java classes like BigInteger and things like that for what I'm trying to use this class for but I'm doing it for fun/satisfaction for use with Project Euler ( https://projecteuler.net/ ).

Jake@Jake-PC /cygdrive/c/Users/Jake/Documents/Java/Mathematics
$ javac Real.java
Real.java:377: error: name clash: compareTo(Real) in Real overrides a method whose erasure is the same as another method, yet neither overrides the other
  public int compareTo(Real other)
             ^
  first method:  compareTo(Object) in Real
  second method: compareTo(T) in Comparable
  where T is a type-variable:
    T extends Object declared in interface Comparable
Real.java:440: error: name clash: compareTo(Object) in Real and compareTo(T) in Comparable have the same erasure, yet neither overrides the other
  public int compareTo(Object other)
             ^
  where T is a type-variable:
    T extends Object declared in interface Comparable
2 errors

These are the compareTo methods:

  @Override
  public int compareTo(Real other)
  {
    // Logic.
  }
  public int compareTo(char givenValue) 
  { return compareTo(new Real(givenValue)); }
  public int compareTo(char[] givenValue) 
  { return compareTo(new Real(givenValue)); }
  public int compareTo(char[] givenValue, int offset, int count) 
  { return compareTo(new Real(givenValue, offset, count)); }
  public int compareTo(double givenValue) 
  { return compareTo(new Real(givenValue)); }
  public int compareTo(float givenValue) 
  { return compareTo(new Real(givenValue)); }
  public int compareTo(int givenValue) 
  { return compareTo(new Real(givenValue)); }
  public int compareTo(long givenValue) 
  { return compareTo(new Real(givenValue)); }
  public int compareTo(Object other) 
  { return compareTo(new Real(other.toString())); }

and the constructors just in case you need them:

  public Real(String givenValue)
  {
    // Logic.
  }
  public Real(char givenValue) 
  { this(String.valueOf(givenValue)); }
  public Real(char[] givenValue) 
  { this(String.valueOf(givenValue)); }
  public Real(char[] givenValue, int offset, int count) 
  { this(String.valueOf(givenValue, offset, count)); }
  public Real(double givenValue) 
  { this(String.valueOf(givenValue)); }
  public Real(float givenValue) 
  { this(String.valueOf(givenValue)); }
  public Real(int givenValue) 
  { this(String.valueOf(givenValue)); }
  public Real(long givenValue) 
  { this(String.valueOf(givenValue)); }
  public Real(Object other) 
  { this(other.toString()); }

Answer 1

The offending methods are:

@Override
public int compareTo(Real other) { ... }

public int compareTo(Object other) { ... }

These methods have the same erasure , meaning that once the compiler strips out generic type information, there will no longer be a way to differentiate them at runtime.

Your options are to either remove the compareTo(Object other) overload, or for Real to implement Comparable<Object> .

Since it looks like the implementations of all your compareTo overloads just instantiate a new Real and pass it to compareTo(Real) , I'd suggest removing them and leaving that conversion up to the caller:

Real real = ...;
Object compared = ...;

Real comparedAsReal = new Real(compared);
int result = real.compareTo(comparedAsReal);

Answer 2

Since you want to be able to compare Real object to Object , you may just replace the implements Comparable<Real> with implements Comparable<Object> . This would be consistent with Comparable<T> javadoc which says that <T> the type of objects that this object may be compared to .

Then you just have to change your current code to :

// No more @Override
public int compareToReal(Real other)
{
  // Logic.
}
public int compareTo(char givenValue) 
{ return compareToReal(new Real(givenValue)); }
public int compareTo(char[] givenValue) 
{ return compareToReal(new Real(givenValue)); }
public int compareTo(char[] givenValue, int offset, int count) 
{ return compareToReal(new Real(givenValue, offset, count)); }
public int compareTo(double givenValue) 
{ return compareToReal(new Real(givenValue)); }
public int compareTo(float givenValue) 
{ return compareToReal(new Real(givenValue)); }
public int compareTo(int givenValue) 
{ return compareToReal(new Real(givenValue)); }
public int compareTo(long givenValue) 
{ return compareToReal(new Real(givenValue)); }
@Override
public int compareTo(Object other) 
{ return compareToReal(new Real(other.toString())); }

Answer 3

This is a side effect of Java generics type-erasure .

You are implementing a generic interface , Comparable, but this its unique method, once generic type erased will become compareTo(Object) , hence it clashes with your own compareTo(Object) .

Here is a minimal code to reproduce:

class Real implements Comparable<Real>
{
    public int compareTo(Object o)
    {
        return 0;
    }

    @Override
    public int compareTo(Real o)
    {
        return 0;
    }       
}