What is the difference between compare() and compareTo()?

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.

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.

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).

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.

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.

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.

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