NSString *testA = @"123";

NSString *testB = @"789";

    1. 字符串拼接

NSString *testString = [NSString stringWithFormat:@"%@%@",testA, testB];

    2. 字符转NSInteger

NSInteger testC = [testString integerValue];

    3. NSInteger转字符

NSString *testD = [NSString stringWithFormat:@"%ld", testC];

    4. 字符转int

int intString = [testString intValue];

    5. int转字符

NSString *stringInt = [NSString stringWithFormat:@"%d", intString];