object sorting using comparable interface

Question

i have one class which have 4 int fields . i want to sort objects array by some mathematical operation on fields .I tried below code but sorting is not happening.

class Cust1 implements Comparable<Cust1>{

    int a;
    int o;
    int s;
    int p;
    @Override
    public int compareTo(Cust1 b) {
        if(this.a + this.s <= b.a + b.s)
        {
            return 1;
        }
        else {
            return 0;
        }
    }   
}

public class Test5 {

    public static void main (String args[]) throws Exception
    {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        int n = Integer.parseInt(br.readLine());
        Cust1[] cust = new Cust1[n];
        for(int i=0;i<n ;i++)
        {
            Cust1 a = new Cust1();
            String[] str = br.readLine().split(" "); 
            a.a = Integer.parseInt(str[0]);
            a.o = Integer.parseInt(str[1]);
            a.s = Integer.parseInt(str[2]);
            a.p = Integer.parseInt(str[3]);
            cust[i] =a;
        }
        Arrays.sort(cust, new Comparator<Cust1>() {
                @Override
                public int compare(Cust1 o1, Cust1 o2) {
                    return o1.compareTo(o2);
                }
        });
    }
}

Welcome to Stack Overflow. Unfortunately "sorting is not happening" doesn't really tell us much about what you do observe, or what the input data is, or the expected results, or the actual data. It would be really helpful if you could provide a minimal reproducible example with hard-coded data instead of using user input, show what you expect the result to be, the actual results, and how far you've got debugging it. Also note that your compareTo comparison doesn't meet the requirements of comparators - I suggest you consult the documentation carefully. — Jon Skeet
– Jon Skeet, Commented Feb 16, 2019 at 19:36

Oleksii Zghurskyi · Accepted Answer · 2019-02-16 19:53:59Z

1

Based on your code snippet: you don't need to provide Comparator, since Cust1 already implements Comparable. So, this should be enough:

Arrays.sort(cust);

Also, Cust1 implementation of Comparable doesn't really tell, when one object is less then other. Probably, you meant something like this:

    @Override
    public int compareTo(Cust1 b) {
        if(this.a + this.s < b.a + b.s) {
            return 1;
        } else if (this.a + this.s > b.a + b.s) {
            return -1; 
        } else {
            return 0;
        }
    }

But it's hard to tell, what exact implementation of Comparable should be without more details (for instance, for some reason fields o and p are not involved in comparison at all).

edited Feb 16, 2019 at 19:53

answered Feb 16, 2019 at 19:41

Oleksii Zghurskyi

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Comments

Oleg Cherednik · Accepted Answer · 2019-02-16 20:15:58Z

0

You comparator work wrong. It should return:

>0 - current object is greater than another one
0 - current object is equal to another one

<0 - current object is less than another one

class Cust1 implements Comparable {

int a;
int o;
int s;
int p;

@Override
public int compareTo(Cust1 b) {
    return Integer.compare(a + s, b.a + b.s);
}

}

And you do not have to provide additional comparator to Arrays.sort() since Cust1 already implements Comparable.

answered Feb 16, 2019 at 20:15

Oleg Cherednik

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