@Override
    public T peek() {
        if (index == DEFAULT_INDEX) {
            return null;
        }

        if (isOverflow() && overflowIndex > 0) {
            return factory.disableInstance();
        }
        return stack[index - 1];
    }
@author netspider
@author Woonduk Kang(emeroad)
@author jaehong.kim
agent-module/profiler/src/main/java/com/navercorp/pinpoint/profiler/context/DefaultCallStack.java