What is the difference between compare() and compa

Question

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

Answer 1

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
}

}

Answer 2

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).

Answer 3

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.

Answer 4

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.

Answer 5

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.

Answer 6

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.