为什么在Java中可以使用变量作为长度来初始化数组？

Question

I try use a integer array in java with the code below:

    public static void main(String[] args) {
        int[] array = testArray(100);
        System.out.println(array.length);
        for(int i = 0; i < 100; i++)
           System.out.println(array[i]);
    }

    public static int[] testArray(int size){
        int[] array = new int[size];
        for(int i = 0; i < size; i++)
           array[i] = i;
        return array;
    }

I also test an integer array in C++ as below:

       #include<iostream>
       using namespace std;

       void getArray(int size)
       {
            int array[size];
            for(int i=0; i<size; i++)
                array[i]=i; 
      }

      int main()
      {
           getArray(10);
           return 0;
      }

And I always get the right answer with the two snippets, why? since I think that the length of an array cannot be variable for language such as java, c and c++.

Answer 1

The three languages are different, and the features of the different languages will, well, differ. In particular, in C99 you can define arrays of a variable in the stack (for those that program mainly Java, this is commonly used to refer to a size that is not a compile time constant, not that the array will change sizes).

In C++ you cannot declare them in the stack (GCC allows this, with a non-conforming extension to the language that mimics C99 behavior), but you can dynamically allocate memory with new of the given size and store the pointer in a variable.

In Java you cannot create an array in the stack ever, and all you can do is dynamically allocate the array with new and store a reference.

int size = 100;

// C99
int data[size];

// C++
// int data[size];          // error, but allowed by gcc
int *data = new int[size]; 

// Java
int[] data = new int[size];

Answer 2

Java and C are different languages. What's allowed in standard C only as dynamic allocations ( malloc / calloc ) is allowed in Java by default, because all array allocations in Java are dynamic anyway; the concept of explicit stack allocation is missing from Java.

Answer 3

Length of an array in Java is not variable. It is defined upon creation of the array (that is, dynamically at runtime ), but cannot be changed after that.

Moreover, every array has a length property that can be used to make a for loop able to iterate through an array of any length:

for(int i = 0; i < array.length; i++)
   System.out.println(array[i]);

Answer 4

There is nothing variable here. You are initializing the array by passing the size to a method that creates an array of the requested size. In other words, you cannot manipulate the size of the array after initialization, unlike with ArrayList where you can vary the size.