C has pointers and Java have what is called references. They have some things in common in the sense that they all point to something. I know that pointers in C store the addresses they point to. Do reference also store the address? How they are different except that pointer is more flexible and error-prone?
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References might be implemented by storing the address. Usually Java references will be implemented as pointers, but that's not required by the specification. They may be using an additional layer of indirection to enable easier garbage collection. But in the end it will (almost always) boil down to (C-style) pointers being involved in the implementation of (Java-style) references. You can't do pointer arithmetic with references. The most important difference between a pointer in C and a reference in Java is that you can't actually get to (and manipulate) the underlying value of a reference in Java. In other words: you can't do pointer arithmetic. In C you can add something to a pointer (i.e. the address) or substract something to point to things that are "nearby" or point to places that are at any place. In Java, a reference points to one thing and that thing only. You can make a variable hold a different reference, but you can't just ask it to point to "the thing after the original thing". References are strongly typed. Another difference is that the type of a reference is much more strictly controlled in Java than the type of a pointer is in C. In C you can have an Those differences make C pointers more powerful, but also more dangerous. Both of those possibilities (pointer arithmetic and re-interpreting the values being pointed to) add flexibility to C and are the source of some of the power of the language. But they are also big sources of problems, because if used incorrectly they can easily break assumptions that your code is built around. And it's pretty easy to use them incorrectly. |
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C++ references are different again. They have to be initialized and can't be null (at least not in a well formed program) and can't be reseated to refer to something else. a C++ reference is much more like an alias for an object. Another important difference between pointers and Java/C++ references is that you can take the address of a pointer you cannot access the address of a reference (indeed a C++ reference need not actually exist as an object in memory at all) consequently you can have a pointer to a pointer but not a reference to a reference |
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They're really not different, in the sense that the memory occupied the variable contains the memory location of the object itself. In C, however, a variable doesn't have to be a pointer - if you have a variable whose value is a pointer, you can have another variable whose value is the thing being pointed to. You can't do this in Java - there's no way to make a variable whose value is the object itself (as opposed to a reference/pointer to the object). In fact, Java even uses the term "Null Pointer Exception" for what happens when you try to use a variable which is a reference to nothing, as if it were a reference to something. |
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