用Java创建数组（C ++风格）

Question

Why can't we make arrays in Java like this:

int marks[5];

And assign values after this declaration?

Anyone please explain the terminology or difference.

Answer 1

This is because there are no stack arrays in Java. Here is Java equivalent:

int[] marks = new int[5];

It looks a lot like allocating dynamically-sized arrays in C++. Of course you don't have to worry about calling a delete[] , because it's garbage collected.

Answer 2

Because the syntax you're citing allocates the array on the stack, and Java arrays are objects, and all Java objects are allocated on the heap (modulo recent JVM optimizations, but those are implicit).

And it pretty much has to be that way in a language without manual memory management because stack-allocated stuff disappears when the call returns, leading to dangling pointers, and a fundamental feature of Java is not to allow stuff like that.

Of course one could argue that Java should use the stack allocation syntax to do heap allocation, but that would have confused the heck out of anyone who knew C - not good.

Answer 3

In Java the size of the array is determined by the expression that creates it, eg:

int[] marks = new int[5];

or

int[] marks = {1,2,3,4,5};

---

Note also that although the syntax int marks[] is allowed in Java (Java has several such rules for compatibility with C++), it is not recommended, the syntax int[] marks is more idiomatic and thus preferred.

Answer 4

int[] marks = new int[]{1, 2, 3};

Answer 5

Because arrays are Object -s (inheriting from the Object class). And to create an object you have to use the new keyword. (Most of the time).
I think the reason behind the new keyword is to denote that the variables are references to dynamically allocated instances.