Java 判断字符串是否为空

import java.io.File;

public class StringTest {

public static void main(String[] args) {
String str = "1111111222222222a";

if(str == null || "".equals(str))
System.out.print("str为空1\n");
if(str == null || str.length() <= 0)
System.out.print("str为空2\n");
if(str == null || str.isEmpty())
System.out.print("str为空3\n");
if(str == null || str == "")
System.out.print("str为空4\n");

long start = 0L;
long end = 0L;
start = System.currentTimeMillis();
for (int i = 0; i < 9999; i++) {
if(str == null || "".equals(str));
//System.out.print("str为空1\n");
}
end = System.currentTimeMillis();
System.out.println("使用equals的时间是:" + (end - start) + "毫秒!");

start = System.currentTimeMillis();
for (int i = 0; i < 9999; i++) {
if(str == null || str.length() <= 0);
//System.out.print("str为空1\n");
}
end = System.currentTimeMillis();
System.out.println("使用length的时间是:" + (end - start) + "毫秒!");

start = System.currentTimeMillis();
for (int i = 0; i < 9999; i++) {
if(str == null || str.isEmpty());
//System.out.print("str为空1\n");
}
end = System.currentTimeMillis();
System.out.println("使用isEmpty的时间是:" + (end - start) + "毫秒!");

start = System.currentTimeMillis();
for (int i = 0; i < 9999; i++) {
if(str == null || str == "");
//System.out.print("str为空1\n");
}
end = System.currentTimeMillis();
System.out.println("直接判断的时间是:" + (end - start) + "毫秒!");

}
}

执行结果:
使用equals的时间是:4毫秒!
使用length的时间是:0毫秒!
使用isEmpty的时间是:1毫秒!
直接判断的时间是:0毫秒!

//注意：
//1.四个方式的效率2，3，4的效率差不多，1相对慢一点，这个几种方式我试过循环1000次来比较，效率其实差不多，2，4方式最优
//2. 注意判断字符串是否为空，必须先判断null，否则用length或者equals判断会抛异常，java.lang.NullPointerException.
//3.对List、Array等进行操作时（特别是判断size、length时）先进行空引用判断

至于为啥出现效率的差异，下面附上JDK源码就非常清晰了。
/**
* Compares this string to the specified object.  The result is {@code
* true} if and only if the argument is not {@code null} and is a {@code
* String} object that represents the same sequence of characters as this
* object.
*
* @param  anObject
*         The object to compare this {@code String} against
*
* @return  {@code true} if the given object represents a {@code String}
*          equivalent to this string, {@code false} otherwise
*
* @see  #compareTo(String)
* @see  #equalsIgnoreCase(String)
*/
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;
}

/**
* Returns <tt>true</tt> if, and only if, {@link #length()} is <tt>0</tt>.
*
* @return <tt>true</tt> if {@link #length()} is <tt>0</tt>, otherwise
* <tt>false</tt>
*
* @since 1.6
*/
public boolean isEmpty() {
return count == 0;
}
//1.isEmpty方法和length其实实现的原理是相同的，jdk 1.6版本之后建议用这个比较。
//2. equals 实现就相对复杂