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();
}
Related
I'm new here and this is my first post. I've just completed my Java OCA and now moving onto studying for the OCP. I have a question regarding Comparable interface.
I have this code snippet which explains how Comparable is implemented:
import java.util.*;
public class Duck implements Comparable<Duck> {
private String name;
public Duck(String name) {
this.name = name;
}
public String toString() { // use readable output
return name;
}
public int compareTo(Duck d) {
return name.compareTo(d.name); // call String's compareTo
}
public static void main(String[] args) {
List<Duck> ducks = new ArrayList<>();
ducks.add(new Duck("Quack"));
ducks.add(new Duck("Puddles"));
Collections.sort(ducks); // sort by name
System.out.println(ducks); // [Puddles, Quack]
}
}
I more or less understand what is goin on here but below this code snippet the author quotes that:
The Duck class implements the Comparable interface. Without implementing that interface, all we have is a method named compareTo(), but it wouldn't be a Comparable object.
My question is why would it not be comparable? Is this something to do with the fact that calling code such as the Collections.sort() would internally use the Comparable type as a reference parameter to compare any object?
Thanks in advance for any help and I hope my question makes sense.
Java is a Object Oriented Based language. Which supports inheritance through classes/ polymorphism through class/abstract class/interface
interface Comparable<T> {
// methods
}
class Person implements Comparable<Person> {
//methods
}
This essentially means any object of the Type Person is also of the Comparable Type.
interface Runnable {}
class Task implements Runnable {}
this means any object created of Task class is also of the Runnable Type.
This is what the author means.
If you do not implement Comparable interface, yet define the compareTo() method, you are just defining a method inside the class, as any other method. YOU ARE NOT OVERRIDING THE compareTo() method in the Comparable interface defined.
You can still compare each object using your compareTo() method, but you need to define your own sort method which internally would call compareTo() method to get the list in a sorted way.
The Java API Collections.sort() internally converts the list to an Object[] and calls the Arrays.sort(). Now Arrays.sort() will use a modified version of the TimSort Algorithm for sorting and the contract is - it does the sorting of elements of the Array only if they are of the Comparable Type.
ComparableTimSort
Collections.sort()
You can check, for all of the internal calls, it states clearly :
#throws IllegalArgumentException (optional) if the comparator is
found to violate the {#link Comparator} contract
So to pass any Object Types to the sort() it has to be also of the type Comparable. Strings/Wrappers are already of the Comparable Type. Hence you need to take care of this contract while defining your user defined objects.
"Without implementing that interface, all we have is a method named compareTo(), but it wouldn't be a Comparable object."
-Simply put, it means without implementing the interface, you have a Duck type object, NOT comparable type
My question is why would it not be comparable? Is this something to do with the fact that calling code such as the Collections.sort() would internally use the Comparable type as a reference parameter to compare any object?
I'm not sure what you exactly mean by reference parameter.
In Java, it is not enough for a class to provide implementation for a method on an interface (or a class) to become that type. It has to be explicitly mentioned in the class declaration.
There are two overloaded sort utility methods on the Collection class.
public static <T extends Comparable<? super T>> void sort(List<T> list)
public static <T> void sort(List<T> list, Comparator<? super T> c)
When your class implements Comparable, you can pass just the list of objects of that class in the first method. If it does not implement Comparable, it will not compile as the bounds for T must extend Comparable. In that case, you will be forced to pass an explicit Comparator to compare the objects.
As the author of the book mentioned, having a method whose signature is the same as the method in an arbitrary interface (or class) does not make it of that type.
Duck Typing does not work in Java
int testing = key.compareTo(node.entry.key());
key is an object, entry is a class .java created in the same folder. I have a hard time understanding what does that line of code is doing.
The compareTo method is the sole member of the Comparable interface. There are two variants of this method. First method compares this String to another Object and second method compares two strings lexicographically.
int compareTo(Object o)
or
int compareTo(String anotherString)
Why we need compareTo() ?
Sorting is an essential part of application development, which you often required to implement in your system. in Java sorting is implemented using Comparator and Comparable in Java. Since we store java objects in Collection there are also certain Set and Map which provides automating sorting when you insert element on that e.g. TreeSet and TreeMap. to implement sorting you need to override either compareTo(Object o) method or Comparable class or compare(Object o1, Object o2) method of Comparator class. Most of the classes implement Comparable to implement natural order. for example if you are writing Employee object you probably want to implement Comparable interface and override compareTo() method to compare current employee with other employee based on ID. So essentially you need to override compareTo() because you need to sort elements in ArrayList or any other Collection.
Read more: http://javarevisited.blogspot.com/2011/11/how-to-override-compareto-method-in.html#ixzz2yc2rJ9Er
Going through docjar for String, I happened to see the following piece of code:
public static final Comparator<String> CASE_INSENSITIVE_ORDER
= new CaseInsensitiveComparator();
private static class CaseInsensitiveComparator
implements Comparator<String>, java.io.Serializable {
// use serialVersionUID from JDK 1.2.2 for interoperability
private static final long serialVersionUID = 8575799808933029326L;
public int compare(String s1, String s2) {
// ...
}
}
My question is, why can't we just implement Comparator, like Comparable, and use a private method rather than a nested class?
Also on a side note, why doesn't Comparator have a method with a single parameter similar to compareTo in Comparable?
Because a String is not a Comparator. Sure, they are indeed Comparable, but they are not "comparison functions"1 themselves: it makes no sense to have String implement Comparator.
On the other hand, CaseInsensitiveComparator is a specific comparison function, only pertaining to strings. So, it is declared as a static nested class.
1 See Comparator
My question is why cannot we just implement comparator, just like comparable and use a private function rather than inner class ?
Well (pretty much) the whole point of the Comparator interface is that an implementation of the interface is a separate class to the class of the objects being compared.
In theory you could do what you are suggesting, but the net result is counter-intuitive
public class MyKey implements Comparator<MyKey> {
private String field;
public boolean compare(MyKey m1, MyKey m2) {
// We must ignore this.field! We are comparing m1 and m2 ...
return m1.field.compareTo(m2.field);
}
}
MyKey[] keys = ...
Arrays.sort(keys, new MyKey()); // Note we have to pass an instance
// to provide the Comparator.
In addition to being a bit counter-intuitive, your idea is limited in the sense that MyKey can only "provide" one comparator this way.
Frankly, if you are going to do this, it makes more sense to have MyKey implement Comparable<MyKey>.
Suppose that they had implemented String the way you proposed. Then this ...
String[] strings = new String[]{"a", "c", "B"};
Arrays.sort(strings);
... means sort case sensitive, but ...
String[] strings = new String[]{"a", "c", "B"};
Arrays.sort(strings, "weasel");
... would mean sort case insensitive. Does that really strike you as a good idea? Really?
You can only implement one interface of the same type. String already implements Comparable for lexicographical comparison:
public final class String
implements java.io.Serializable, Comparable<String>, CharSequence
{
But it needs more compareTo method (eg: to perform case insensitive comparison). Following would give you compiler error:
public final class String
implements java.io.Serializable, Comparable<String>, Comparable<String>, CharSequence
{
Hence this is probably one reason it has additional comparator.
The idea of comparator is an object that provide comparison service between two items, not a contract where one is comparable to another (comparable interface)
Implementing Comparator on a String will compile, but this will be semantically wrong
public final class String
implements java.io.Serializable, Comparable<String>, Comparator<String>, CharSequence
{
Having this be a class rather than a function allows us to polymorphically apply different comparison strategies to Collections (or other classes which may be performing comparisons). This way the method using the comparator doesn't need to know whether it's a case-sensitive comparator or a case-insensitive comparator or something else; it just performs the comparison. If this were a function however, such polymorphism would not apply. The method performing the comparison would either have to use "the" comparison method, or would have to know what kind of comparison was being performed, in order to select the correct method.
Having two parameters rather than tying the first parameter to one particular String allows us to use the same comparator on any number of Strings. Otherwise we'd have to keep track of a lot of comparators when dealing with large collections. It also allows usage of this comparator with either a subclass on either the left side or the right side (or both); with just one parameter there would not be that flexibility.
The primary reason is to expose the Comparator object for things like TreeMap, where Strings can be used as keys, or other objects where ordering or a different kind of equality is useful. It can't be a static method since the Comparator interface can't specify a static method and the Comparator has to be an object that implements the interface to be used in classes like TreeMap. We need a separate one that's case insensitive because the default comparison (implemented by Comparable's methods) is already taken by the case sensitive comparison.
why cannot we just implement comparator, just like comparable and use
a private function rather than inner class
String class implements Comparable<String> interface, which is used to compare two strings lexicographically, which is a pretty common way to compare two strings.
However, sometimes you need to compare two strings in different way, for example comparing them while ignoring case sensitivity. Comparator interface provides a way to compare two objects differently. In this case, CaseInsensitiveComparator is implemented to provide this capability and is used in String.compareToIgnoreCase() method.
The code my team is working on has several classes where equals and hashCode are not defined in the class hierarchy. We'd like to implement Comparable such that compareTo is consistent with equals using hashCode, like so:
class MyClass implements Comparable<MyClass>
{
private String myProperty;
// Other properties, etc.
....
public int compareTo(MyClass obj) {
// Natural ordering comparisons
...
// Reach here if natural ordering properties are equivalent
return new Integer(this.hashCode()).compareTo(new Integer(obj.hashCode());
}
}
Is this considered a valid means of implementing Comparable? Are there any pitfalls with using the default hashCode implementation that I should be aware of?
UPDATE: The behavior we're striving for is as follows:
The class properties are compared first, in a natural ordering we define.
If a given property for the two objects are equivalent, we move on to the next one in the ordering.
If all properties are equivalent, we return 0 only if this.equals(obj).
Yes this is a valid way. Apparently you want a fixed ordering for objects which are equal on other values (am I right? You did not explain your aim with the hashcode usage here).
The only thing i would do is copy the java code of Integer.compareTo() in your compareTo method, so you do not have to create 2 Integers for every comparison.
No, This is not the valid means of implementing Comparable. Because , suppose your all natural ordering comparison for two different objects of MyClass within equals method comes true , after that when hashcode of two objects are compared it would return false . This is so because in this case hashcode method of Object class would be called by default(as you have not provided your own hashcode method), Which will be different for different objects. Hence the two objects of MyClass will never be equal no matter if all natural ordering comparison comes out to be true.
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.