1、判断字符串是否相等使用isEqualToString：
NSString *astring01 = @"This is a String!";
NSString *astring02 = @"This is a String!";
BOOL result = [astring01 isEqualToString:astring02];
NSLog(@"result:%d",result);
2、判断字符不相等：
NSString *astring01 = @"This is a String!";
NSString *astring02 = @"This is a String!";
if ([astring01 compare: astring02]!=NSOrderedSame )
{
NSLog(@"not equal");
}
3、判断字符串是否为空。
NSString *urlString = [urlInput stringValue];
if (!urlString) {
NSLog( @”NO INPUT.” );
} else {
if ([urlString length] == 0 ) {
NSLog( @”NO INPUT.” );
} else {
}
}
}