Make Java defined functions nullary when we override them (#24461)

To preserve the same source compatibility, we mark `toString` and
`hashCode` as methods with empty parameter lists in all the inherited
files from Scala 2. Note that I checked for other java defined methods
that would qualify for this change but I couldn't find any (reviewer
should double check too since we cannot fix this after 3.8.0).

Closes #19616
Supersedes #24452

Author: hamzaremmal

library/src/scala/Boolean.scala

@@ -136,7 +136,7 @@ object Boolean extends AnyValCompanion {
   def unbox(x: java.lang.Object): Boolean = ???
 
   /** The String representation of the scala.Boolean companion object. */
-  override def toString = "object scala.Boolean"
+  override def toString() = "object scala.Boolean"
 
 }
 

library/src/scala/Byte.scala

@@ -477,7 +477,7 @@ object Byte extends AnyValCompanion {
   def unbox(x: java.lang.Object): Byte = ???
 
   /** The String representation of the scala.Byte companion object. */
-  override def toString = "object scala.Byte"
+  override def toString() = "object scala.Byte"
   /** Language mandated coercions from Byte to "wider" types. */
   import scala.language.implicitConversions
   implicit def byte2short(x: Byte): Short = x.toShort

library/src/scala/Char.scala

@@ -477,7 +477,7 @@ object Char extends AnyValCompanion {
   def unbox(x: java.lang.Object): Char = ???
 
   /** The String representation of the scala.Char companion object. */
-  override def toString = "object scala.Char"
+  override def toString() = "object scala.Char"
   /** Language mandated coercions from Char to "wider" types. */
   import scala.language.implicitConversions
   implicit def char2int(x: Char): Int = x.toInt

library/src/scala/Double.scala

@@ -252,6 +252,6 @@ object Double extends AnyValCompanion {
   def unbox(x: java.lang.Object): Double = ???
 
   /** The String representation of the scala.Double companion object. */
-  override def toString = "object scala.Double"
+  override def toString() = "object scala.Double"
 }
 

library/src/scala/Enumeration.scala

@@ -98,11 +98,11 @@ abstract class Enumeration (initial: Int) extends Serializable {
 
   /** The name of this enumeration.
    */
-  override def toString: String =
+  override def toString(): String =
     getClass.getName
       .stripSuffix(MODULE_SUFFIX_STRING)
       .split('.')
-      .last 
+      .last
       .split(Regex.quote(NAME_JOIN_STRING))
       .last
 
@@ -241,7 +241,7 @@ abstract class Enumeration (initial: Int) extends Serializable {
       case that: Enumeration#Value  => (outerEnum eq that.outerEnum) && (id == that.id)
       case _                        => false
     }
-    override def hashCode: Int = id.##
+    override def hashCode(): Int = id.##
 
     /** Create a ValueSet which contains this value and another one */
     def + (v: Value): ValueSet = ValueSet(this, v)

library/src/scala/Float.scala

@@ -252,7 +252,7 @@ object Float extends AnyValCompanion {
   def unbox(x: java.lang.Object): Float = ???
 
   /** The String representation of the scala.Float companion object. */
-  override def toString = "object scala.Float"
+  override def toString() = "object scala.Float"
   /** Language mandated coercions from Float to "wider" types. */
   import scala.language.implicitConversions
   implicit def float2double(x: Float): Double = x.toDouble

library/src/scala/Int.scala

@@ -477,7 +477,7 @@ object Int extends AnyValCompanion {
   def unbox(x: java.lang.Object): Int = ???
 
   /** The String representation of the scala.Int companion object. */
-  override def toString = "object scala.Int"
+  override def toString() = "object scala.Int"
   /** Language mandated coercions from Int to "wider" types. */
   import scala.language.implicitConversions
   @deprecated("Implicit conversion from Int to Float is dangerous because it loses precision. Write `.toFloat` instead.", "2.13.1")

library/src/scala/Long.scala

@@ -474,7 +474,7 @@ object Long extends AnyValCompanion {
   def unbox(x: java.lang.Object): Long = ???
 
   /** The String representation of the scala.Long companion object. */
-  override def toString = "object scala.Long"
+  override def toString() = "object scala.Long"
   /** Language mandated coercions from Long to "wider" types. */
   import scala.language.implicitConversions
   @deprecated("Implicit conversion from Long to Float is dangerous because it loses precision. Write `.toFloat` instead.", "2.13.1")

library/src/scala/Predef.scala

@@ -442,7 +442,7 @@ object Predef extends LowPriorityImplicits {
     def length: Int                                     = sequenceOfChars.length
     def charAt(index: Int): Char                        = sequenceOfChars(index)
     def subSequence(start: Int, end: Int): CharSequence = new SeqCharSequence(sequenceOfChars.slice(start, end))
-    override def toString                               = sequenceOfChars.mkString
+    override def toString()                             = sequenceOfChars.mkString
   }
 
   /** @group char-sequence-wrappers */
@@ -453,7 +453,7 @@ object Predef extends LowPriorityImplicits {
     def length: Int                                     = arrayOfChars.length
     def charAt(index: Int): Char                        = arrayOfChars(index)
     def subSequence(start: Int, end: Int): CharSequence = new runtime.ArrayCharSequence(arrayOfChars, start, end)
-    override def toString                               = arrayOfChars.mkString
+    override def toString()                             = arrayOfChars.mkString
   }
 
   /** @group char-sequence-wrappers */

library/src/scala/Proxy.scala

@@ -29,14 +29,14 @@ import scala.language.`2.13`
 trait Proxy extends Any {
   def self: Any
 
-  override def hashCode: Int = self.hashCode
+  override def hashCode(): Int = self.hashCode
   override def equals(that: Any): Boolean = that match {
     case null  => false
     case _     =>
       val x = that.asInstanceOf[AnyRef]
       (x eq this.asInstanceOf[AnyRef]) || (x eq self.asInstanceOf[AnyRef]) || (x.equals(self))
   }
-  override def toString = "" + self
+  override def toString() = "" + self
 }
 
 @deprecated("All members of this object are deprecated.", "2.13.0")