CODEWITHSUNDEEP
java
vijay
vijay

Reputation: 321

how does compareTo and compare work?

I know that compare and compareTo returns an int value.

For example:

Returns
0 if a equal b
-1 if a < b
+1 if a > b

sort method makes a call to either compareTo or compare() methods. But how does sort method arranges the list when compare or compareTo returns a int value. What is background scenario running after a compare or compareTo returns an int value to sort?how does sort method makes use of int values(-1 or 0 or 1) returned to it from compare and compareTo

Upvotes: 29

Views: 40657

Answers (4)

Yash P Shah
Yash P Shah

Reputation: 809

Hopefully, these implementations of compare() method would help you understand it better.

public int compare(Object obj1, Object obj2)
{
 Integer I1 = (Integer)obj1; // typecasting object type into integer type
 Integer I2 = (Integer)obj2; // same as above ..
 // 1.
 return I1.compareTo(I2); // ascending order [0, 5, 10, 15, 20]
 // 2.
 return -I1.compareTo(I2); // descending order [20, 15, 10, 5, 0]
 // 3.
 return I2.compareTo(I1); // descending order [20, 15, 10, 5, 0]
 // 4.
 return -I2.compareTo(I1); // ascending order [0, 5, 10, 15, 20]
 // 5.
 return +1; // insertion order [10, 0, 15, 5, 20, 20]
 // 6.
 return -1; // reverse of insertion order [20, 20, 5, 15, 0, 10]
 // 7.
 return 0; // only first element [10]
}

How compare() method works in Java

Upvotes: 0

amit
amit

Reputation: 178431

General case sorting algorithms are based on comparisons. If you compare between a and b - there are exactly 3 possibilities: a == b, a > b, a < b.

The compare() or compareTo() method provides exactly this information.

Now, to use this information we have designed numerous sorting algorithms, starting with naive bubble sort, and some are more advances such as quick sort. Each providing a bit different approach to the sorting problem.

Java chose the TimSort algorithm for its sorting implementation.

As an exercise, you can design your own1 sorting algorithm. Can you find the maximum element of an array using the compare() method? when you found it where should it be? what should you do next?

(1) Well, think of it by yourself, but it already exists, actually :)

Upvotes: 4

user207421
user207421

Reputation: 310884

If the two elements (a,b) being compared are already in the right order, compare(a, b) and a.compareTo(b) both return a value that is <= 0, so nothing has to happen.

If they aren't in the right order, the return value is > 0, indicating that they must be interchanged.

Upvotes: 28

ashwinsakthi
ashwinsakthi

Reputation: 1956

Read this document:

  • 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 interface and define this method. All Java classes that have a natural ordering implement this (String, Double, BigInteger, ...).

  • compare(a, b):
    Comparator interface. Compares values of two objects. This is implemented as part of the Comparator 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.

Upvotes: 2

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