What is the difference between compare and compareTo

Question

What is the difference between Java s compare   and compareTo   methods  Do those methods give same answer

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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 lt Car gt        public int compare Car o1  Car o2             return o1 getModelNo   - o2 getModelNo               Comparator  2 based on Model Year  public class CarModelYearComparator implements Comparator lt Car gt         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

User · Answer

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

User · Answer

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

User · Answer

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

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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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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 lt Car gt        public int compare Car o1  Car o2             return o1 getModelNo   - o2 getModelNo               Comparator  2 based on Model Year  public class CarModelYearComparator implements Comparator lt Car gt         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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

Use Comparable interface for sorting on the basis of more than one value like age name dept name    For one value use Comparator interface

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

One more point      compareTo   is from the Comparable interface and compare   is from the Comparator interface  Comparable is used to define a default ordering for objects within a class while Comparator is used to define a custom ordering to be passed to a method

User · Answer

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

User · Answer

Use Comparable interface for sorting on the basis of more than one value like age name dept name    For one value use Comparator interface

User · 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 lt T gt  interface and define this method  All Java classes that have a natural ordering implement Comparable lt T gt  - Example  String  wrapper classes  BigInteger compare a  b   Comparator interface    Compares values of two objects  This is implemented as part of the Comparator lt T gt  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 lt T gt  interface  which defines a compareTo   method  You will want to implement Comparable lt T gt  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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

One more point      compareTo   is from the Comparable interface and compare   is from the Comparator interface  Comparable is used to define a default ordering for objects within a class while Comparator is used to define a custom ordering to be passed to a method

User · Answer

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 lt Pokemon gt  comparable  Pokemon opponent            if  comparable compareTo opponent   gt  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

User · Answer

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

User · Answer

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

User · Answer

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 lt Pokemon gt  comparable  Pokemon opponent            if  comparable compareTo opponent   gt  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

User · Answer

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

User · Answer

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

User · 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 lt T gt  interface and define this method  All Java classes that have a natural ordering implement Comparable lt T gt  - Example  String  wrapper classes  BigInteger compare a  b   Comparator interface    Compares values of two objects  This is implemented as part of the Comparator lt T gt  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 lt T gt  interface  which defines a compareTo   method  You will want to implement Comparable lt T gt  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

User · Answer

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 lt Employee gt     int compare Employee obj1  Employee obj2       Sample code  public class NameComparator implements Comparator lt Employee gt     public int compare Employee e1  Employee e2            some conditions here         return e1 getName   compareTo e2 getName        returns 1 since  T omas  gt   D an      return e1 getSalary   compareTo e2 getSalary        returns -1 since 400  gt  300

User · Answer

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

User · 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 lt T gt  interface and define this method  All Java classes that have a natural ordering implement Comparable lt T gt  - Example  String  wrapper classes  BigInteger compare a  b   Comparator interface    Compares values of two objects  This is implemented as part of the Comparator lt T gt  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 lt T gt  interface  which defines a compareTo   method  You will want to implement Comparable lt T gt  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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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 lt Employee gt     int compare Employee obj1  Employee obj2       Sample code  public class NameComparator implements Comparator lt Employee gt     public int compare Employee e1  Employee e2            some conditions here         return e1 getName   compareTo e2 getName        returns 1 since  T omas  gt   D an      return e1 getSalary   compareTo e2 getSalary        returns -1 since 400  gt  300

User · 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 lt T gt  interface and define this method  All Java classes that have a natural ordering implement Comparable lt T gt  - Example  String  wrapper classes  BigInteger compare a  b   Comparator interface    Compares values of two objects  This is implemented as part of the Comparator lt T gt  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 lt T gt  interface  which defines a compareTo   method  You will want to implement Comparable lt T gt  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

[java] What is the difference between compare() and compareTo()?

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