Why Is My Newly Initialized int Array in Java Not Containing Zeros?

Why do newly initialized int arrays in Java sometimes contain non-zero elements?

public static void main(String[] args) {
    int[] a;
    int n = 0;
    for (int i = 0; i < 100000000; ++i) {
        a = new int[10];
        for (int f : a)
            if (f != 0)
              throw new RuntimeException("Array just after allocation: " + Arrays.toString(a));
        Arrays.fill(a, 0);
        for (int j = 0; j < a.length; ++j)
            a[j] = (n - j)*i;
        for (int f : a)
            n += f;
    }
    System.out.println(n);
}

In Java, when an integer array is initialized, it is expected that all elements will have a default value of zero. However, peculiar behavior may occur due to JVM optimizations that can lead to instances where non-zero values appear immediately after allocation. This situation seems to have arisen specifically in certain versions of the HotSpot JVM.

Arrays.fill(a, 0); // Ensure the array is filled with zeros after creation

Causes

• JVM optimizations leading to unexpected states in memory allocations.
• Possible issues related to Just-In-Time (JIT) compilation affecting the execution order of statements.

Solutions

• Avoid relying on the initial state of the array immediately after initialization; explicitly fill the array with zeros before use.
• Disable JIT compilation with the `-Xint` flag in the JVM, which may prevent the unexpected behavior from manifesting.

Mistake: Assuming all elements in an int array are zero immediately after allocation.

Solution: Explicitly initialize or fill the array to ensure all values are set as desired.

Mistake: Ignoring the effects of JIT compilation on array initialization.

Solution: Test the code with different JVM flags or configurations to understand their impact.