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javaobjectcomparablecompareto
Jaimin Modi
Jaimin Modi

Reputation: 1667

Understanding syntax for compareTo() method in Java

I have created a Student class, which contains roll number, name and age as its data variables. I created an ArrayList for storing multiple objects of class Student.

Now, I have used the Comparable interface and its compareTo method to sort the Student list data according to age.

Below is the compareTo method I have created for sorting age wise:

    public int compareTo(Student sNew)
    {
        return this.age-sNew.age;
    }

Here, I can not understand, What is -? and How it's working?

Because, I have also done it as below : 

public int compareTo(Student sNew)
    {
        int returnValue=0;
        if(this.age>sNew.age)
        {
            returnValue=1;
        }else if(this.age<sNew.age){
            returnValue=-1;
        }else if(this.age==sNew.age)
        {
            returnValue=0;
        }
        return returnValue;
    }

So, I have two doubts : How '-' is working in compareTo() method and where it returns the value (0,1,-1).

Please guide.

Upvotes: -1

Views: 1526

Answers (2)

Laurence R. Ugalde
Laurence R. Ugalde

Reputation: 182

The Comparable interface is the way Java performs a three-way comparison.

A trick to get sense of the semantics of compareTo is: a.compareTo(b) is equivalent to the question How is a compared to b? There can be only three answers (law of trichotomy): a is less than b (compareTo() returns a less than zero value), a equals b (compareTo returns a zero value), and a is greater than b (compareTo returns a greater than zero value).

For the case of total order types (integers, floating-point numbers, dates, etc), the operation a - b always works because of:

  • If a < b, the operation a - b will always produce a less than zero value
  • If a = b, the operation a - b will always produce zero
  • If a > b, the operation a - b will always produce a greater than zero value

In Java syntax, a is the current (this object), so the syntax is a.compareTo(b), but the idea is the same.

Upvotes: 0

Nir Levy
Nir Levy

Reputation: 12953

The idea behind it is that compareTo doesn't return 0,1,-1, but returns 0 (equals), positive number (bigger than) or negative (smaller).

For that reason, simply subtracting the ages will give you the correct answer

Upvotes: 6

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