1.     NSString *s4 = [[NSString alloc] initWithContentsOfFile:@"/Users/liuthou/Desktop/liye.txt" encoding:NSUTF8StringEncoding error:nil];
   
2.    
   
3.     NSLog(@"%@", s4);

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用xcode编译没错误，但是一运行
2014-04-17 05:45:38.770 foundation-NSString[503:303] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** initialization method -initWithFormat:locale:arguments: cannot be sent to an abstract object of class __NSCFConstantString: Create a concrete instance!'

求解释

1. NSString *str1 = [NSString stringWithContentsOfFile:path encoding:NSUTF8StringEncoding error:nil];

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stringWithContentsOfFile: