“ T”在此代码中代表什么？

Question

T get(int i) {
    if (i < 0 || i > n - 1) throw new IndexOutOfBoundsException();
    return a[i];
}
T set(int i, T x) {
    if (i < 0 || i > n - 1) throw new IndexOutOfBoundsException();
    T y = a[i];
    a[i] = x;
    return y;
}

I'm looking over some coding examples from my textbook, but they never mention what T is. I'm not sure how to search about this since I don't know what it's called or it's purpose. I was wondering if someone can show me a write-up or some info about this. Thank you

Answer 1

T represents an object type using generics .

Whatever type x is in the set call, the set method will return that same type. In the get call, the return type is T , too. That type must be defined somewhere else, probably in the type of the class, where it might show up as <T> .

So if this is a class that is some sort of collections (since it is dealing with indexes), it might be defined as:

public class SomeCollection<T> {

Then it might get instantiated like:

SomeCollection<String> arr = new SomeCollection<String>();

In that case, T would be String , and the return type of the get and set methods, and the type of set parameter x , would all be String .

If it gets instantiated like:

SomeCollection<Integer> arr = new SomeCollection<Integer>();

then T is an Integer .

You could have both in the same code, without having to write two (or more) different versions of SomeCollection .