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];