What exactly is a reference in Java?

Question

What exactly is a reference in Java? Is it a memory address? Is a Java reference the equivalent of a dereferenced C++ pointer?

In other words, given the following:

Object o1 = new Object();
Object o2 = new Object();

o1 == o2

Is the above comparison the equivalent of comparing two pointers in C++?

Answer 1

o1 == o2 is pretty much equivalent to comparing two pointers in C/C++, yes.

But there are a two main differences between references in Java and pointers in C/C++ that are quite important:

• Java references can't do pointer arithmetic: you can't "add 3" to a reference, you can only let it point to another (known) object
• Java references are stongly typed: you can't "reinterpret" what lies on the other end of a reference unless you reinterpret it as a type that that object actually is.

Also a short note about the word "reference": C++ has references that act quite differently from both pointers in C and references in Java (but I don't know enough about C++ to tell you the specifics).

For a thorough discussion of this, see this related question on programmers.SE.

Answer 2

What exactly is a reference in Java?

It is an index of an object. It can be thought of as like a pointer but it differs in the fact that it

• can change at any time.
• does not always have a direct relationship with memory addresses.
• is usually 32-bit in a 64-bit JVM.
• you can't re-interprete what the reference refers to. You can only change the type of the reference itself.

Is the above comparison the equivalent of comparing two pointers in C++?

Yes.

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On Compresses Oops which allows a 64-bit JVM to sue 32-bit references.

Java HotSpot™ Virtual Machine Performance Enhancements - Compressed Oops

Compressed oops in the Hotspot JVM

IBM V6 - More effective heap usage using compressed references

Answer 3

Yes, a reference is basically the same thing as a pointer. By the way, if you call a method on a null reference, you get... a NullPointerException.

Note that it doesn't have to be a memory address, though. A given object can be stored elsewhere during a program execution, and still keep the same reference. But you don't need to care, as pointer arithmetic doesn't exist in Java.

Answer 4

A reference is a "pointer" to a memory address in Java, even though Java eleminates direct manipulation of pointers, unlike C++. Objects in Java are never passed to methods or returned by methods, it is always a reference that is being passed.