How to use the Comparable CompareTo on Strings in Java

Question

I can use it to sort by emp id but I'm not sure if it is possible to compare strings. I get an error the operator is undefined for strings.

public int compareTo(Emp i) {
            if (this.getName() == ((Emp ) i).getName())
                return 0;
            else if ((this.getName()) > ((Emp ) i).getName())
                return 1;
            else
                return -1;

Solution 1

What you need to use is the compareTo() method of Strings.

return this.getName().compareTo(i.getName());

That should do what you want.

Usually when implementing the Comparable interface, you will just combine the results of using other Comparable members of the class.

Below is a pretty typical implementation of a compareTo() method:

class Car implements Comparable<Car> {
    int year;
    String make, model;
    public int compareTo(Car other) {
        if (!this.make.equalsIgnoreCase(other.make))
            return this.make.compareTo(other.make);
        if (!this.model.equalsIgnoreCase(other.model))
            return this.model.compareTo(other.model);
        return this.year - other.year;
    }
}

Solution 2

Pretty sure your code can just be written like this:

public int compareTo(Emp other)
{
    return this.getName().compareTo(other.getName());
}

Solution 3

Java String already implements Comparable. So you could simply write your method as

public int compareTo(Emp emp) {
   return this.getName().compareTo(emp.getName());
}

(ofcourse make sure you add proper validations such as null checks etc)

Also in your code, do not try to compare Strings using '=='. Use 'equals' method instead. '==' only compare string references while equals semantically compares two strings.

Solution 4

You don't need to cast i to Emp, it's already an Emp:

public int compareTo(Emp i) {
    return getName().compareTo(i.getName());
}

Solution 5

Shouldn't

if (this.getName() == ((Emp ) i).getName())

be

if (this.getName().equals(i.getName()))

View more solutions

Comments

[BoltClock]

Curious, why do you need to cast i to Emp when you've already declared it to be Emp in the method signature?

[zengr]

As indicated by others, getName().compareTo(i.getName()); is a better option anyway.

[Jack]

when i replace "else if ((this.getName()) > ((Emp ) i).getName())" with "else if (this.getName().compareTo(((Emp ) i).getName() > 0)", I get a similar error saying operator > is undefine for type String,int

[Jack]

Oh I see. I must have been confused switching from comparing ints and string and used > sign. Thanks!!

[Tim Stone]

@Jack: That's because your > 0 is inside of the call to compareTo(); it needs to be outside that last parenthesis to compare the return value of the string comparison against zero (and then you need another parenthesis to complete the else if condition).

[jjnguy]

@Jack, you should probably use the version of the method that everyone else is suggesting. Just directly return the value of string.compareTo(string).