I wanted to ask why we use the comparable interface in java? Wouldn't it be simpler to just make the compareTo method without using the comparable interface?
Instead of doing this:
//some class that implements comparable
public int compareTo(someClass someInstanceOfThatClass) {
// do stuff that returns -1,0,or 1
}
Why can't we just do this:
//some class that does NOT implement comparable, but we still
//make a compareTo method
public int compareTo(someClass someInstanceOfThatClass) {
// do stuff that returns -1,0, or 1
}
I guess my question is why do we bother implementing comparable, if we could just make a compareTo method without being forced to by some interface (comparable)?
Comparable is an interface, hence it imposes a contract that others may follow. For example, calling Collections.sort(list) only works if the elements are instances of Comparable, and internally relies on the compareTo method to implement the sorting.
Java's type system is nominal, not structural, so simply having the method required by the interface is not enough to implement it; you also have to declare that the class implements the interface. In some other languages such as Typescript, having the method with the right signature would be enough, but Java is not like that.
If you are only calling the compareTo method from your own code then this may not matter, but if you are using classes or methods from the standard library or from other libraries which take Comparable things as arguments, then your class will need to implement the interface so you can pass your objects to them.
I think it returns to the innate concept of the interface.
You always sure that every class which has implemented Comparable interface, has ability to be compared and sometimes you need this assurance.
For example if you have a method that have a parameter with Comparable type, then you are sure that comapreTo is implemented with that parameter and this parameter issemantically comparable.
But whitout interface you can't get this assurance.
Learning Java 8 Lambdas and just wondering how the compiler knows which method in Comparator to use for the lambda expression?
It doesn't seem to be a SAM interface? It has 2 abstract methods:
#FunctionalInterface
public interface Comparator<T> {
int compare(T o1, T o2);
boolean equals(Object obj);
}
equals() is not an abstract method. This method overrides Object.equals(Object), and is there only for the Comparator interface to be able to have javadoc attached to the method, explaining how comparators should implement equals().
See the javadoc of FunctionalInterface:
If an interface declares an abstract method overriding one of the public methods of java.lang.Object, that also does not count toward the interface's abstract method count since any implementation of the interface will have an implementation from java.lang.Object or elsewhere.
equals() is inherited from Object, and inherited public methods are not counted when you’re determining whether an interface is a functional interface. So even though equals() is abstract in Comparator, because it’s inherited, it doesn’t count.
RULE:
A functional interface is an interface that has one abstract method. Default methods don’t count; static methods don’t count; and methods inherited from Object don’t count.
All classes descend from Object class and Object contains an equal method.
So, this means that every instance that implements Comparator will already have an implementation of equal method.
Therefore Only one method is required to override by the Implanting class of Comparator Interface.
This makes only one abstract method in Comparator interface
This is why Comparator is a functional interface
I've been trying to learn the comparable class for sometime now, I know the correct syntax and the how it is used in most cases. Such as:
int result = ObjectA.compareTo(ObjectB);
would return a value of 0 if both object is the same; a negative value if object A is less then object B ,or a positive value if A is greater then B.
However when I go to actually write a program that uses the compareTo method, the compiler is saying that it can not find the compareTo method.
my question is: Do I have to directly inherit from the Comparable class in order to use the compareTo method? only reason I'm asking is because you do not have to explicitly inherit methods like toString or equals...because everything inherit from object. Where does CompareTo fall under?
You need to implement the Comparable interface:
public class MyClass implements Comparable<MyClass>
and then declare the compareTo() method:
public int compareTo(MyClass myClass){
//compare and return result
}
Comparable is an interface, not a class. So you would implement the interface, not subclass the class. Additionally, implementing the interface means implementing the compareTo method yourself in your implementing class.
First, it will only work in instances. Don't know if your compare is comparing objects or the classes itself because of your naming. I will assume you are using objects :)
The class you want to compare using #compareTo MUST implement Comparable. Another way to achieve this without having to implement Comparable is providing your sort method a Comparator expecting your class.
Comparable is an interface so you do not "inherit" it (extends), but implement it (implements).
You can write your own compareTo method in your class without specifying the Comparable interface, but then the methods in the API won't know if your object meets the contract that Comparable enforces. So, you won't be able to use methods like Arrays.sort() and the like that expect to work with objects that do enforce Comparable.
If you want the objects of your class A compared, possibly because you like to sort a list of those objects, you need to make the class A implement Comparable interface.
Once the class A implements Comparable it must implement and define the behavior of compareTo, which is essentially how you want two objects of class A be compared.
It it this method where you can decide which fields of the class A take part in evaluating the lesser- or greaterness of an object.
I'm trying to write a generic class in Java. A few methods in that class require that T extends Comparable<T>. How can I make it such that T is required to be comparable only if one of those methods is used? Or maybe there's some other way I should organize my class?
Here's the class I'm trying to implement. Its and array that I plan to use on both comparable and non-comparable types.
// I know Java has its own containers, but this
// is homework and I'm not allowed to use them
class Array<T>
{
// Some methods that pose no
// special restrictions on T
// These require that T be comparable
public Array<T> union(...) {...}
public Array<T> intersect(...) {...}
}
You can hide type T for method. T of Test is not the same as T of CompareMethodhere.
public static class Test<T> {
<T extends Comparable<T>> void compareMethod(T t, Class<T> classt) {
}
void normalMethod(T t) {
}
}
Now example
Test<String> test = new Test<String>();//Comparable class
test.compareMethod("",String.class);//works fine
Test<Random> tes1t = new Test<Random>();//Non Comparable class
tes1t.compareMethod(new Random(),Random.class);//Compilation error here
tes1t.normalMethod(new Random());//Works fine
new Test<Random>().compareMethod("",String.class);// Not a good but can be valid
new Test<String>().compareMethod(new Random(),Random.class);//Compilation error here
Update:
After being cursed about this solution I did some search in java API and this practice gets followed for toArray() method
ArrayList<String> string = new ArrayList<String>();
string.toArray(new Integer[5]);<--Illegal however <T> is hide by toArray method
Edit: It looks like this is possible after all (see AmitD's post). But anyway, other possible solutions are
Refactor the methods requiring comparable into a subclass
Just use casts in the relevant methods, meaning that that part will only be checked at runtime.
It wont be possible through normal method such as using comparable.
It would be better if you share what is the exact requirement.
If Sorting in ArrayList/Arrays are your goal, then Comparing Non Comparable classes is useless. Sorting can only be done in objects of the same or sub types.
But if you are going to use compare for checking if the objects are equal or not then I'll suggest that you override equals(Object O) method.
What is the difference between Java's compare() and compareTo() methods? Do those methods give same answer?
From JavaNotes:
a.compareTo(b):
Comparable interface : Compares values and returns an int which tells if the values compare less than, equal, or greater than.
If your class objects have a natural order, implement the Comparable<T> interface and define this method. All Java classes that have a natural ordering implement Comparable<T> - Example: String, wrapper classes, BigInteger
compare(a, b):
Comparator interface : Compares values of two objects. This is implemented as part of the Comparator<T> interface, and the typical use is to define one or more small utility classes that implement this, to pass to methods such as sort() or for use by sorting data structures such as TreeMap and TreeSet. You might want to create a Comparator object for the following:
Multiple comparisons. To provide several different ways to sort something. For example, you might want to sort a Person class by name, ID, age, height, ... You would define a Comparator for each of these to pass to the sort() method.
System class To provide comparison methods for classes that you have no control over. For example, you could define a Comparator for Strings that compared them by length.
Strategy pattern To implement a Strategy pattern, which is a situation where you want to represent an algorithm as an object that you can pass as a parameter, save in a data structure, etc.
If your class objects have one natural sorting order, you may not need compare().
Summary from http://www.digizol.com/2008/07/java-sorting-comparator-vs-comparable.html
Comparable
A comparable object is capable of comparing itself with another object.
Comparator
A comparator object is capable of comparing two different objects. The class is not comparing its instances, but some other class’s instances.
Use case contexts:
Comparable interface
The equals method and == and != operators test for equality/inequality, but do not provide a way to test for relative values.
Some classes (eg, String and other classes with a natural ordering) implement the Comparable<T> interface, which defines a compareTo() method.
You will want to implement Comparable<T> in your class if you want to use it with Collections.sort() or Arrays.sort() methods.
Defining a Comparator object
You can create Comparators to sort any arbitrary way for any class.
For example, the String class defines the CASE_INSENSITIVE_ORDER comparator.
The difference between the two approaches can be linked to the notion of:
Ordered Collection:
When a Collection is ordered, it means you can iterate in the collection in a specific (not-random) order (a Hashtable is not ordered).
A Collection with a natural order is not just ordered, but sorted. Defining a natural order can be difficult! (as in natural String order).
Another difference, pointed out by HaveAGuess in the comments:
Comparable is in the implementation and not visible from the interface, so when you sort you don't really know what is going to happen.
Comparator gives you reassurance that the ordering will be well defined.
compareTo() is from the Comparable interface.
compare() is from the Comparator interface.
Both methods do the same thing, but each interface is used in a slightly different context.
The Comparable interface is used to impose a natural ordering on the objects of the implementing class. The compareTo() method is called the natural comparison method. The Comparator interface is used to impose a total ordering on the objects of the implementing class. For more information, see the links for exactly when to use each interface.
Similarities:
Both are custom ways to compare two objects.
Both return an int describing the relationship between two objects.
Differences:
The method compare() is a method that you are obligated to implement if you implement the Comparator interface. It allows you to pass two objects into the method and it returns an int describing their relationship.
Comparator comp = new MyComparator();
int result = comp.compare(object1, object2);
The method compareTo() is a method that you are obligated to implement if you implement the Comparable interface. It allows an object to be compared to objects of similar type.
String s = "hi";
int result = s.compareTo("bye");
Summary:
Basically they are two different ways to compare things.
The methods do not have to give the same answers. That depends on which objects/classes you call them.
If you are implementing your own classes which you know you want to compare at some stage, you may have them implement the Comparable interface and implement the compareTo() method accordingly.
If you are using some classes from an API which do not implement the Comparable interface, but you still want to compare them. I.e. for sorting. You may create your own class which implements the Comparator interface and in its compare() method you implement the logic.
Using Comparator, we can have n number of comparison logic written for a class.
E.g.
For a Car Class
We can have a Comparator class to compare based on car model number. We can also have a Comparator class to compare based on car model year.
Car Class
public class Car {
int modelNo;
int modelYear;
public int getModelNo() {
return modelNo;
}
public void setModelNo(int modelNo) {
this.modelNo = modelNo;
}
public int getModelYear() {
return modelYear;
}
public void setModelYear(int modelYear) {
this.modelYear = modelYear;
}
}
Comparator #1 based on Model No
public class CarModelNoCompartor implements Comparator<Car>{
public int compare(Car o1, Car o2) {
return o1.getModelNo() - o2.getModelNo();
}
}
Comparator #2 based on Model Year
public class CarModelYearComparator implements Comparator<Car> {
public int compare(Car o1, Car o2) {
return o1.getModelYear() - o2.getModelYear();
}
}
But this is not possible with the case of Comparable interface.
In case of Comparable interface, we can have only one logic in compareTo() method.
Comparable interface contains a method called compareTo(obj) which takes only one argument and it compares itself with another instance or objects of the same class.
Comparator interface contains a method called compare(obj1,obj2) which takes two arguments and it compares the value of two objects from the same or different classes.
compareTo(T object)
comes from the java.lang.Comparable interface, implemented to compare this object with another to give a negative int value for this object being less than, 0 for equals, or positive value for greater than the other. This is the more convenient compare method, but must be implemented in every class you want to compare.
compare(T obj1, T obj2)
comes from the java.util.Comparator interface, implemented in a separate class that compares another class's objects to give a negative int value for the first object being less than, 0 for equals, or positive value for greater than the second object. It is needed when you cannot make a class implement compareTo() because it is not modifiable. It is also used when you want different ways to compare objects, not just one (such as by name or age).
The relationship of the object having this method and its collaborators is different.
compareTo() is a method of the interface Comparable, so it is used to compare THIS instance to another one.
compare() is a method of the interface Comparator, so it is used to compare two different instances of another class with each other.
If you will, implementing Comparable means that instances of the class can be easily compared.
Implementing Comparator means, that instances are suited to compare different objects (of other classes).
The main difference is in the use of the interfaces:
Comparable (which has compareTo()) requires the objects to be compared (in order to use a TreeMap, or to sort a list) to implement that interface. But what if the class does not implement Comparable and you can't change it because it's part of a 3rd party library? Then you have to implement a Comparator, which is a bit less convenient to use.
compareTo() is called on one object, to compare it to another object.
compare() is called on some object to compare two other objects.
The difference is where the logic that does actual comparison is defined.
One more point:
Comparable is used to define a default ordering for objects within a class
Comparator is used to define a custom ordering to be passed to a method.
comparator-vs-comparable
When you want to sort a List which include the Object Foo, the Foo class has to implement the Comparable interface, because the sort methode of the List is using this methode.
When you want to write a Util class which compares two other classes you can implement the Comparator class.
Employee Table
Name, DoB, Salary
Tomas , 2/10/1982, 300
Daniel , 3/11/1990, 400
Kwame , 2/10/1998, 520
The Comparable interface allows you to sort a
list of objects eg Employees with reference to one primary field – for
instance, you could sort by name or by salary with the CompareTo() method
emp1.getName().compareTo(emp2.getName())
A more flexible interface for such requirements is provided by
the Comparator interface, whose only method is compare()
public interface Comparator<Employee> {
int compare(Employee obj1, Employee obj2);
}
Sample code
public class NameComparator implements Comparator<Employee> {
public int compare(Employee e1, Employee e2) {
// some conditions here
return e1.getName().compareTo(e2.getName()); // returns 1 since (T)omas > (D)an
return e1.getSalary().compareTo(e2.getSalary()); // returns -1 since 400 > 300
}
}
There is a technical aspect that should be emphasized, too. Say you need comparison behavior parameterization from a client class, and you are wondering whether to use Comparable or Comparator for a method like this:
class Pokemon {
int healthPoints;
int attackDamage;
public void battle (Comparable<Pokemon> comparable, Pokemon opponent) {
if (comparable.compareTo(opponent) > 0) { //comparable needs to, but cannot, access this.healthPoints for example
System.out.println("battle won");
} else {
System.out.println("battle lost");
}
}
}
comparable would a lambda or an object, and there is no way for comparable to access the fields of this Pokemon. (In a lambda, this refers to the outer class instance in the lambda's scope, as defined in the program text.) So this doesn't fly, and we have to use a Comparator with two arguments.
Use Comparable interface for sorting on the basis of more than one value like age,name,dept_name...
For one value use Comparator interface
Important Answar
String name;
int roll;
public int compare(Object obj1,Object obj2) { // For Comparator interface
return obj1.compareTo(obj1);
}
public int compareTo(Object obj1) { // For Comparable Interface
return obj1.compareTo(obj);
}
Here in return obj1.compareTo(obj1) or return obj1.compareTo(obj) statement
only take Object; primitive is not allowed.
For Example
name.compareTo(obj1.getName()) // Correct Statement.
But
roll.compareTo(obj1.getRoll())
// Wrong Statement Compile Time Error Because roll
// is not an Object Type, it is primitive type.
name is String Object so it worked.
If you want to sort roll number of student than use below code.
public int compareTo(Object obj1) { // For Comparable Interface
Student s = (Student) obj1;
return rollno - s.getRollno();
}
or
public int compare(Object obj1,Object obj2) { // For Comparator interface
Student s1 = (Student) obj1;
Student s2 = (Student) obj2;
return s1.getRollno() - s2.getRollno();
}