1. Introduction  
To understand the difference between an abstract class and a trait, we must first define each term. Traits are similar to Java interfaces, but they allow for the implementation of their members. Abstract classes, on the other hand, are created using the `abstract` keyword and can include both abstract and concrete methods.

2. Commonalities  
Identifying the similarities helps clarify the differences. Let’s examine these features.  
A trait that has a declared and implemented method along with a variable:  
scala
trait mytrait { 
  def myMethod(): Unit
  val aVal: Int 
  val myVal = 10 
  def myImpMethod = { println("I am from trait") } 
}

An abstract class that contains both an abstract and a concrete method along with a variable:  
scala
abstract class abstClass { 
  def myMethod(): Unit 
  val aVal: Int 
  val myVal = 10 
  def myImpMethod: Unit = { println("I am from abstClass") } 
}

In the main method, neither a trait nor an abstract class can be instantiated:  
scala
val myTObj = new mytrait // compilation error
val myAObj = new abstClass() // compilation error

Classes that extend a trait or an abstract class must implement the declared methods:  
scala
class myTClass extends mytrait {
  val aVal = 10 
  def myMethod = { println("I am from myTclass") } 
} 
class myAClass extends abstClass {
  val aVal = 10 
  def myMethod = { println("I am from myAclass") } 
}


3. Distinctions  
3.1 Constructor Parameters  
scala
trait myTraitWithParam(name: String){} 
abstract class abstClassWithParam(name: String){}

The `name` serves as a constructor parameter; using a constructor parameter in a trait leads to a compilation error, while it works correctly in an abstract class.

3.2 Modifying Object Instances  
scala
trait objectInstanceTrait { 
  def myObjectMethod = { println("I originate from the object instance trait") } 
} 
class myTClass {} 
def main(args: Array[String]): Unit = { 
  val classObjWithTrait = new myTClass with objectInstanceTrait
  classObjWithTrait.myObjectMethod 
} 

Output: "I originate from the object instance trait"  
The method `myObjectMethod` from the trait can be accessed through a class instance. An abstract class cannot be mixed into an object instance, resulting in a compilation error.

3.3 Multiple Inheritances  
In multiple inheritance, a single class can inherit from more than one superclass, gaining features from all parent classes. While Scala does not allow multiple inheritance with classes, it can be achieved using traits:  
scala
class myClass extends trait1 with trait2{} 
class myNewClass extends abstClass1 with abstClass2{}

Using an abstract class for multiple inheritances results in a compilation error. Scala resolves the diamond problem in multiple inheritance through trait linearization, where the rightmost trait takes precedence:  
scala
trait Printer { 
  def print(msg: String) = println(msg) 
} 
trait DelimitWithHyphen extends Printer { 
  override def print(msg: String) = { println("-------------") } 
} 
trait DelimitWithStar extends Printer { 
  override def print(msg: String) = { println("*") } 
} 

class CustomPrinter extends Printer with DelimitWithHyphen with DelimitWithStar 

new CustomPrinter().print("Hello World!") 

Output: "*"  

3.4 Stackability  
Stackable traits in Scala allow for the combination of multiple traits that collaboratively modify a method. This involves calling `super.theMethod`:  
scala
trait base { 
  def baseMethod(s: String): Unit = println(s) 
}

trait stack1 extends base {
  override def baseMethod(s: String): Unit = println("from stack1") 
} 

trait replace1 extends base { 
  abstract override def baseMethod(s: String): Unit = {
    super.baseMethod(s); 
    println("from replace1")
  } 
}

class Stackable { 
  this: base => 
  def apply() { 
    println(baseMethod("bottom")) 
  } 
} 

(new Stackable with stack1 with replace1)() 

Output:  
from stack1  
from replace1  
The invocation starts with `replace1`, which then calls `stack1` via `super.baseMethod(s)`.

3.5 Interoperability with Java  
Traits can seamlessly integrate with Java as long as they lack any implementation; abstract classes can be used directly.