CODEWITHSUNDEEP
javagenerics
user2356560
user2356560

Reputation:

Syntactical issue using generics

This is a homework problem:

I have a generic class defined as follows: 

public class PriorityQueue<T extends Comparable<T>> {
    ArrayList<T> queue; 

public PriorityQueue(){
    queue = new ArrayList<>();
}

void add(T t){
    queue.add(t);
    Collections.sort(queue);
}

<T extends Comparable<T>> T remove(){
    T t = queue.get(0);
    queue.remove(t);
    return t;
}
}

but NetBeans is showing the following error (red-underline) on the line T t = queue.get(0): 

incompatible types
required: T#2
found:    T#1
where T#1,T#2 are type-variables:
T#1 extends Comparable<T#1> declared in class PriorityQueue
T#2 extends Comparable<T#2> declared in method <T#2>remove()

It seems as though somehow it's not understanding that the T type I'm referring to in the method declaration is the same as the one referred to in the type parameter of the class. I'm guessing this is some sort of a syntax issue.

I also wonder if I'm overcomplicating things - it would seem more logical to me if I simply declared the method with T remove() {. This compiles correctly, but when I try to test it using a driver class as follows: 

    PriorityQueue pq = new PriorityQueue<Integer>();

    int a = 10;
    int b = 12;
    int c = 5; 
    int d = 9;

    pq.add(a);
    pq.add(b);
    pq.add(c);
    pq.add(d);

    Integer i = pq.remove();

I get the error: 

Incompatible types: 
required: Integer 
found:    Comparable

on the line Integer i = pq.remove();

As is probably obvious, I'm just learning how to use generics. Please help me understand where I'm going wrong here.

Upvotes: 0

Views: 55

Answers (1)

Evgeniy Dorofeev
Evgeniy Dorofeev

Reputation: 136162

change 

PriorityQueue pq = new PriorityQueue<Integer>();

to

PriorityQueue<Integer> pq = new PriorityQueue<Integer>();

remove method should be

T remove() {
...

Upvotes: 3

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