Question

What is the Java equivalent of C++’s templates?

I know that there is an interface called Template. Is that related?

Answer 1

Templates as in C++ do not exist in Java. The best approximation is generics.

One huge difference is that in C++ this is legal:

<typename T> T sum(T a, T b) { return a + b; } 

There is no equivalent construct in Java. The best that you can say is

<T extends Something> T Sum(T a, T b) { return a.add(b); }

where Something has a method called add.

In C++, what happens is that the compiler creates a compiled version of the template for all instances of the template used in code. Thus if we have

int intResult = sum(5, 4);
double doubleResult = sum(5.0, 4.0);

then the C++ compiler will compile a version of sum for int and a version of sum for double.

In Java, there is the concept of erasure. What happens is that the compiler removes all references to the generic type parameters. The compiler creates only one compiled version of the code regardless of how many times it is used with different type parameters.

Other differences

• C++ does not allow bounding of type parameters whereas Java does
• C++ allows type parameters to be primitives whereas Java does not
• C++ allows templates type parameters to have defaults where Java does not
• C++ allows template specialization whereas Java does not
  And, as should be expected by this point, C++ style template metaprogramming is impossible with Java generics.
• Forget about seeing the curiously recurring template pattern in Java
• Policy-based design is impossible in Java