CODEWITHSUNDEEP
javagenerics
Chris Dobkowski
Chris Dobkowski

Reputation: 325

Can't find what's wrong converting a sort method to use generics

I have an exercise to solve but I can't get my head around it. So the question is: The following program prints a sequence of strings in ascending order. Observe that sort sorts arrays of strings only. Rewrite it so that it sorts objects of type T, where T is introduced in the sort header as a generic type name, and T implements Comparable. Changes should be limited to modifying the header and body of sort. So the code I'm given is:

class SortNames {

static void sort(String[] s) {
    for (int i = 0; i<s.length; i++) {
        for (int j = i+1; j < s.length; j++) {
            if (s[i].compareTo(s[j])>0) {
                String t;
                t = s[i]; s[i] = s[j]; s[j] = t;
            }
            }
    }
}

public static void main(String[] args) {
    String[] names = {"Pete","Jill","May","Anne","Tim"};             
    sort(names);
    for (String s:names){ 
        System.out.println(s);
    }
}

So what I did is:

class TestSort{

static void sort(T[] s) {
    for (int i = 0; i<s.length; i++) {
        for (int j = i+1; j < s.length; j++) {
            if (s[i].compareTo(s[j])>0) {
                Object t;
                t = s[i]; s[i] = s[j]; s[j] = t;
            }
    }
    }
}

public static void main(String[] args) {
    int[] numbers = {3,6,2,7,9,1,8};
    sort(numbers);
    for (int a:numbers){ 
        System.out.println(a);
     }
}

Now when I compile my code, I get:

TestSort.java:3: error: cannot find symbol
static void sort(T[] s) {
                 ^
symbol:   class T
location: class TestSort
1 error

What is wrong? :( Can anyone recommend me some good source for reading? I have exam on that tomorrow :( Thanks in advance!

Upvotes: 1

Views: 77

Answers (1)

rgettman
rgettman

Reputation: 178263

You need to define your generic variable T on your sort method to make it generic. The T as the datatype merely refers to the generic type parameter.

static <T extends Comparable<T>> void sort(T[] s) {

Upvotes: 4

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