How are Java generics different from C++ templates? Why can't I use int as a parameter?

Question

I am trying to create

ArrayList myList = new ArrayList();

in Java but that does not work.

Can someone explain

Answer 1

The main difference is in way they are implemented, but their names accurately describe their implementation.

Templates behave like templates. So, if you write:

template
void f(T s)
{
    std::cout << s << '\n';
}

...
int x = 0;
f(x);
...

Compiler applies the template, so in the end compiler treats the code like:

void f_generated_with_int(int s)
{
    std::cout << s << '\n';
}

...
int x = 0;
f_generated_with_int(x);
...

So, for each type which is used to call f a new code is "generated".

On the other hand, generics is only typechecked, but then all type information is erased. So, if you write:

class X {
    private T x;

    public T getX() { return x; }
    public void setX(T x) { this.x = x; }
}

...
Foo foo = new Foo();
X x = new X<>();
x.setX(foo);
foo = x.getX();
...

Java compiles it like:

class X {
    private Object x;

    public Object getX() { return x; }
    public void setX(Object x) { this.x = x; }
}

...
Foo foo = new Foo();
X x = new X();
x.setX(foo);
foo = (Foo)x.getX();
...

In the end:

• templates require instantiation of each call to templated function (in compilation of each .cpp file), so templates are slower to compile
• with generics you can't use primitives, because they are not Object, so generics is less versatile