1. Value types
2. Generic Specialization.
In this Article, I will cover the talk on Value type given by Oogways.let us get started what Oogways told us-- me and 5 Ninja warriors.
Oogways : we all know, to support primitives in Collection API--The Wrapper Object concept is created and java5 introduced autoboxing /unboxing feature, basically the motto of Autoboxing is to uplift primitives as an Object, so we can treat them as Object and Object's methods(equals, hashcode,toString) can be inherited to the Boxed Object so it can act as an Object and user(coder) does not have to bother about Boxing the primitives in a wrapper class, It will be done automatically and Vice versa.
So in developer's perspective, it is a huge improvement as you can play with collections and primitives and under the hood, all primitives get promoted to Object(Wrapper class, int->Integer). But in case of performance, it is not so good, to make developer happy Oracle compromised on the performance. Let us take a detailed look why the performance is degraded.
package com.example.valhalla;
public class PerformanceTest {
Integer[] boxedArray = new Integer[1000];
int[] primitiveArray = new int[1000];
public void insert() {
for (int i = 0; i < 1000; i++) {
boxedArray[i] = i;
primitiveArray[i] = i;
}
}
public void traveseBoxedArray() {
Long start = System.currentTimeMillis();
for(int i=0;i<boxedArray.length;i++) {
System.out.print(i);
}
System.out.println();
Long end = System.currentTimeMillis();
System.out.println("Time elapsed :: " + (end-start));
}
public void travesePrimitiveArray() {
Long start = System.currentTimeMillis();
for(int i=0;i<primitiveArray.length;i++) {
System.out.print(i);
}
System.out.println();
Long end = System.currentTimeMillis();
System.out.println("Time elapsed :: " + (end-start));
}
public static void main(String[] args) {
PerformanceTest test = new PerformanceTest();
test.insert();
test.traveseBoxedArray();
test.travesePrimitiveArray();
}
}
Output : Case of Boxed Time elapsed:: 9
Case of primitive Time elapsed:: 6
So, Just in case of traversing boxed version and primitive version, there is a 3 Ms lag for 1000 elements. Not only in traversal there is also the side effect of memory consumption.
Why this performance degradation in case of Boxed version?
The performance degradation occurs due to the following reasons
Memory consumption : One of the differences between Java primitives and Object is --there is a cost associated with Object, just to box an int to Integer , it has to create a header(8-16 byte) and reference(4-8 bytes) in case of a reachable object, so instead of 4 bytes(int-4 bytes) it takes 4+8+4=16 bytes minimum. Imagine the same for 1000 elements each, so heap space is consumed rapidly as the elements in the array are increased (Not considering JIT optimization).
Traversing for Payload: When we wrap primitives into a wrapper class, it will act as Object so now array of Integers stores the pointers unlike the array of primitives where array store the value itself, so there is an unnecessary cost to traverse through the pointer to fetch the actual value/payload. It will certainly take much more time than storing the value directly in the array.
Lack of Inherent Locality : Think about the Java memory model map , in case of primitives , value will get stored in register and is pushed to stack memory. In case of array of primitives it will occupy same contiguous memory , so cache hits is bigger than cache miss and traversal will be optimized and faster. On the another hand in case of Object (Wrapper class) it will get stored in heap memory and we don't know where it belongs in heap memory as JVM there are different spaces like young generation, eden etc and Objects are moved to that spaces so there will no contiguous memory allocated for array of Objects. So the array of objects lacks the inherent locality and also cache miss is greater than cache hits, so traversing through pointer will take more time than directly find value from a contiguous location.
Due to the above reasons, Boxing is bad in terms of performance. But in Java, there are many use cases where we use Boxing to achieve some functionality which can't be achieved by only primitives.
I will tell you what are those areas where we use unnecessary boxing and compromise performance to achieve some programmatic design decision.
Value Types : In java, Often we design some Object which just carrier of some data, its primary goal represents a data structure nothing else, It does not hold any state so there is no identity check once the values are same we can say two objects are same , so they are only caring for value, not the reference and state. think about a Point object or a Money Object or a Pin Code Object, which are the associations of some primitives / Objects, If the value is same we can say those two objects are equal, think about Money Object, it is the combination of currency and value which is char and double respectively.
package com.example.valhalla;
public final class Money {
public final double value;
public final char currency;
public Money(double value,char currency) {
this.currency=currency;
this.value=value;
}
public final void display() {
System.out.println(value + currency);
}
}
So, Money is immutable in nature and we wrap the value and currency into a Money Object which acts as Wrapper Object, But this Money does not do anything apart from encapsulating the primitives values. So performance got affected as Money is an Object rather than a collection of primitives, and we can't store value and currency into an array as those are the heterogeneous types and array support homogeneous typing.To support heterogeneous type we have to opt for an object and compromise the performance.Wrapper Object: All primitives Wrapper types are degrading the performance.
Iterator: Iterator creates a snapshot of the collections to be traversed so it wraps the collection into an Iterator Objects which is nothing but a wrapper and degrades the performance.
Multi valued returns : Some time we need to return more than one value and those are heterogeneous type( like int, char,Object etc) from a method , in other languages we can do it by Tuples concept but in case of java we have to wrap the return value in to an object and return that Object, say I have a method which returns Address of a customer, and Address contains flat number, location, zip code, city, country -- so we have to create a wrapper Object which contains all these properties and return the Address so here Address Object acts as wrapper/boxed Object.
In the all above cases, we see a common problem to associate heterogeneous types we have to wrap it into a Object which act as Boxed type and compromise the performance as there is no other way in Java we can achieve Class like behavior (tied data and behaviors) but act as primitive type so we can benefit local inherent, less memory and fast performance.
Java architects addressed these problems and came up with a remarkable idea called Value Type under Project Valhalla.
The slogan of the project is “Codes like a class, works like an int!”
Seeing this slogan we can understand what they want to achieve. They want to create a new Datatype -- From developer's perspective, this act as a class and they can tie data and method together but in JVM it will act as primitive-- means it has no reference and stays in Stack memory !!!
If they really build this new datatype, it will solve performance issue without compromising the encapsulation because all the heterogeneous types act as user-defined primitives and store into the register so no pointer traversing is needed for getting payload.
Although, it is a remarkable idea and solves all design issues stated earlier without compromising performance but the road is not smooth-- there are many assumptions involved to create the new Value type-- which code like a class and act as an int.
I will try to give an overview of what considerations are needed to create a new Datatype.
Equality Check: As Value type does not has a pointer, so how you check the equality for two value Objects, As Value Object is a composite data Structure so a probable solution will call equals or == for all associated components.
Null keyword: We know for Object null is the default value, but Value type is not an Object so what will be its default value? Probable solution would be default Value of ValueObject is setting the default value to all is sub-fields/association that is the initial state of ValueObject and that is set when Value Object initialized. And the null keyword for ValueObject should be disallowed.
Reference Cast: Value type is not an Object so it is not Reference type and it is not pointed by a reference, But to support those APIs which play with Reference casting is needed, Value type should able to upgrade to reference type and vice versa as we do in the case with primitive boxing and unboxing.
Polymorphism: One of the powerful features of java is Polymorphism where parent reference can hold all the subtypes, but this is valid for only reference types which have a pointer. So at runtime pointer points to the corresponding subclass implementation. As Value type is pointer less and immutable in nature so the question is, should Value type allow Polymorphism? The Answer is no, it is Just a Value type. Although you are coding it like a class, inheritance should be disallowed as Valuetype does not carry any header information and used for flattening the associated Objects.
Cloning: Value type carry only information so Cloning does not make sense but it can be used as Identity transformation.
Till now, we have found various use cases for using Value types, Now we will see how a Value Type may look like
package com.example.valhalla;
public final _ByValue class Money {
public final double value;
public final char currency;
public Money(double value,char currency) {
this.currency=currency;
this.value=value;
}
public final void display() {
System.out.println(value + currency);
}
public boolean equals(Money that) {
return this.currency == that.currency && this.currency == that.currency;
}
}
Here, a new keyword _ByValue is used to instruct JVM to treat the class as a Value type so instead of pointing the Money Object it extracts Money's all component and create a space in the stack.Next Question will come How we instantiate Value types.
Probably in the following way
Money money = __MakeValue(100, '$')
It will create a value type and store the currency and value properties into a stack.So it flattens the Money Value type.
As Value type is neither Object and nor primitive we have still some questions about equality check. Let's see How Valuetype treats equality check.
Money money1 = __MakeValue(100, '$);
Money money2 = __MakeValue(100, '$);
now
money1 == money2 (Do the Value equality check)
money1.equals(money2) (Do the Value equality check)
Object moneyWrapper1 = money1// Reference type
Object moneyWrapper2 = money2// Reference type
moneyWrapper1 == moneyWrapper2 // Refrence check
moneyWrapper1.equals(moneyWrapper2) // call Money Value types equals
Ok, we are got a fair bit of idea what are the challenges and how Oracle architects are trying to deal with the same to offer Value type in Java.But if it ends here that would be fine but still, it has some advanced things to deal with and the probable answer to those questions are.
Subtyping:
Value type can extends Reference type? No, because it is not an Object this is a new type.
Reference type extends a value type? No, because Valuetype does not have reference/pointer.
Can a Value type class extend another value type? No, because value types are final.)
Can a value type be abstract or non-final? No abstract value type seems not worthwhile
Can a value type implement interfaces? Yes. Boxing may occur when we treat a value type as an interface instance.
Can values participate in inheritance-based subtyping? No, polymorphism not allowed.
Containment :
Can a value class contain a field of the reference type? Yes because of a value type code like a class.
Can a reference class contain a field of value type? Yes because it is just another type.
Can a value type contain a component field of value type? Yes it is work like a class
Can an array contain elements of value type? Yes works like an int. And the array itself is an object.
Can a value class contain a non-final field? No as Value type is immutable.
Can values have member types? Yes. Non-static member types have hidden fields that record containing values.
Can values be member types? Yes. If non-static, they have hidden fields that point to containing objects.
Compatibility
Are values objects? No, although they can be boxed into objects.
Are primitives values? Possibly. (Value classes named int, boolean, etc., would provide good successors to the existing wrapper types.
Can value types be serialized? Not by default, since that would violate encapsulation.
Conclusion: Value type is a rich concept, Oracle is working hard to release this concept to efficiently manage the memory wasted for tiny objects which are acting as a wrapper or as the data carriers.
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