/*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.facebook.presto.sql.planner.assertions;
import com.facebook.presto.sql.planner.Symbol;
import com.facebook.presto.sql.tree.SymbolReference;
class AnySymbol
extends Symbol
implements PlanTestSymbol
{
AnySymbol()
{
super("*");
}
@Override
public Symbol toSymbol(SymbolAliases aliases)
{
return this;
}
@Override
public SymbolReference toSymbolReference()
{
return new AnySymbolReference();
}
@Override
public int hashCode()
{
/*
* It is impossible to implement hashCode in a way that is honors the general equals()/hashCode() contract:
*
* 0) If two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result.
* 1) AnySymbol.equals() returns true when the other Object passed to it is of type Symbol.
* 2) Assume there is an instance of Symbol s1, such that s1.hashCode() returns h1
* 3) Assume there is an instance of Symbol s2, such that s2.hashCode() returns h2, where h1 != h2
* 4) Assume there is an instance of AnySymbol, a1
* 5) By 1, a1.equals(s1) returns true
* 6) By 1, a1.equals(s1) returns true
* 7) By 0, 2, a1.hashCode() must return h1
* 8) By 0, 3, a2.hashCode() must return h2
* 9) We have arrived at a contradiction -> AnySymbol cannot implement hashCode() in a way that honors the general equals/hashCode contract.
*
* In general, overriding hashCode() is unlikely to be useful. Because hashCode() can't be made to work in a way that's consistent with Symbol's hashCode() method, you
* can't put AnySymbol instances in e.g. a HashSet and do a meaningful comparison to a HashSet of Symbol instances if the implementation of HashSet.equals() relies on the
* general equals()/hashCode() contract holding.
*
* If you find a use case for putting AnySymbol in a hash table, feel free to
* implement this.
*/
throw new UnsupportedOperationException("Test object");
}
@Override
public boolean equals(Object obj)
{
if (obj == this) {
return true;
}
if (obj == null) {
return false;
}
return obj instanceof Symbol;
}
}