Saturday, March 28, 2009

java String[] is O(1) for lookup-by-index

int[] has constant by-index lookup time, because system can compute address of 55th element from base address and offset ie base address + 54 * 4 since an int takes 4 bytes.

For a String[], I believe it's the same formula. 55th string could be very long, but it's not stored in the array's memory block. I believe it's the address of 1st char that's stored in the array's memory block, assuming the c-str implementation. Each address probably takes 4 bytes in a 32-bit machine.

More generally, any array of objects is probably implemented the same way. Note an array is also an object. If the elements in the outer array are array objects, then we get a 2D array. Outer array's memory block holds 55 pointers to 55 inner arrays.

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