比较两个字符串空,两种方法性能比较
测试 str.isEmpty()和 "".equals(str)两种判断的效率
比较次数999999999
str.isEmpty()用时2735(2.7秒)
"".equals(str)用时10516(10秒)

差别如此之大

public boolean equals(Object anObject) {if (this == anObject) {    return true;}if (anObject instanceof String) {    String anotherString = (String)anObject;    int n = count;    if (n == anotherString.count) {char v1[] = value;char v2[] = anotherString.value;int i = offset;int j = anotherString.offset;while (n-- != 0) {    if (v1[i++] != v2[j++])return false;}return true;    }}return false;    }