数组索引超出范围未触发

Question

public static void insertionSort(int[] data) {
    for (int i =0; i < data.length; i++) {
        int current = data[i];
        int j = i-1;
        while (j >=0 && data[j] >= current) {
            data[j+1] = data[j];
            j--;
        }
        data[j+1] = current;
    }
}

the line while (j >=0 && data[j] >= current) should throw array index of bounds when data[-1] for the first time. I dont understand why it does not. Can some one please help Thank you Ashok Pappu

Answer 1

Boolean expressions like these are never fully evaluated if the condition can be met earlier.

So...in your case since j >= 0 is false and false && ??? will always be false the second part does not need to get evaluated. This is why data[-1] will never be called.

You can use the same principle for null checks, eg

if (object != null && object.isSomething())

Answer 2

If while clause contains more, than 1 expression, splitted by && , they are executed one by one. This is done to increase performance. Ie at first in your code j >= 0 will checks. If j < 0 , it false. So, cycle will be interrupted regardless of the 2nd expression.