我为您实现了alternateSum方法：
public double alternatingSum() {
double alternatingSum = 0;
if(data != null || dataSize > 0) {
for(int i = 0; i < dataSize; i = i + 2) {
alternatingSum += data[i];
}
for(int i = 1; i < dataSize; i = i + 2) {
alternatingSum -= data[i];
}
}
return alternatingSum;
}
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