Java中两个判断字符串是否为空的方法的执行效率比较

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 class Test4  
{  
    public static void main(String[] args)  
    {  
        long times = 999999999;  
        String str = "";  
  
        long a1 = System.currentTimeMillis();  
        for (long i = 0; i < times && str.isEmpty(); i++)  
        {  
            // System.out.println(str.isEmpty());  
        }  
        long a2 = System.currentTimeMillis();  
        System.out.println("str.isEmpty() times: " + (a2 - a1));  
  
        long b1 = System.currentTimeMillis();  
        for (long i = 0; i < times && "".equals(str); i++)  
        {  
            // System.out.println("".equals(str));  
        }  
        long b2 = System.currentTimeMillis();  
        System.out.println("\"\".equals(str) times: " + (b2 - b1));  
    }  
}  

输出结果如下：

str.isEmpty() times: 2735  
"".equals(str) times: 10516  

其实看下源码很快就发现问题所在

isEmpty()函数的源码

public boolean isEmpty() {  
    return count == 0;  
    }  

equals的源码

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;  
    }