Passing "const" variable to method in Java

Question

Is there an equivalent in Java to the passing on const references in C++?
Isn't leaving out the "constness" misleading in regard to the method signature?

Answer 1

See this for achieving this in Java: Immutable objects in Java

Answer 2

The closest Java equivalent for const is final.

void func(final SomeClass x) {
  // The following causes a compiler error
  x = ...;

  // The following works. If you don't want it to, then you have to make
  // somevar final or write a getter (but not a setter) for it in SomeClass.
  x.somevar = ...;
}

Answer 3

Java doesn't have anything like C++'s notion of const. It's a point of some contention, although it's interesting to note that .NET doesn't either. I believe the reasons are:

• Getting understandable syntax becomes pretty tricky (IMO) - telling the difference between a const list of mutable StringBuilders and a mutable list of const StringBuilders etc is difficult. (I've certainly found it hard in C++, although that could be due to a lack of practice.)
• I strongly suspect that it wouldn't be acceptable to allow the constness to be cast away in managed languages. It would go against the grain of the whole thing. That could make them harder to work with in many cases.
• Propagating the information about constness at execution time (to avoid the possibility of it being cast away) may be difficult and/or expensive in performance terms
• As Java (and .NET) haven't had constness in the past, introducing it late on in the game is hugely difficult - unless all common libraries support it (and get it right at every step!) you end up with a nasty mishmash, and indeed it may be completely unworkable if you need to use two libraries, one of which doesn't know about constness and the other of which only supplies const values.

Answer 4

No, there isn't.

Java "final" is not an exact equivalent of C++ "const". The following (delayed initialization of a final variable) works in Java:

final double x;
int w = 1;
if (w > 2)
{
    x = 0.5;
}
else
{
    x = - 0.5;
}

but it doesn't work in C++ with "final" replaced by "const".

Using "final" on a variable in the method declaration can be useful in Java, because allows you to use this variable inside any anonymous class created inside your method.

PS. I was first disappointed by the lack of "const" in Java but later learned to live with "final".

PS2. The Java glossary (http://mindprod.com/jgloss/immutable.html) linked to in this thread has one thing wrong: no, you are not given a 100% guaranntee that the final variable doesn't change its value:

1) it changes from "undefined" to "defined", but the compiler will tell you if you reference it before initialization

2) on Linux, a double has 80-bit precision when stored in a register, but 64-bit when stored in memory. When a final double variable is pushed out of the register, it will be truncated and change its value. As Joel Spolsky says, "abstraction has sprung a leak".

Answer 5

As above,no there is no const in Java. But when we want to achieve 'close' to the same result in Java we use

public static final Object x = somedata;

This sets the data at that point and baring the abstraction leaks and so forth, you have as close to an equivalent as you can get.

Answer 6

BTW: Java does have const as a keyword, but you cannot use it anywhere.