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Ans. The external tables feature is a complement to existing SQL Loader functionality. It enables to access data in external sources as if it were in a table in the database. We have used it few times for replicating tables across different database systems. | ||||
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Like Discuss Correct / Improve  external table oracle Asked in 1 Companies   rare | ||||
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Ans. It make sense only if we intend to modify either of the object and would like to preserve original state in other. Otherwise we can reuse the original object by making it singleton. | ||||
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Like Discuss Correct / Improve  clone  clone objects  prototype design pattern   immutable  immutability  immutability  cloning | ||||
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Ans. Copy Constructor is used for creating a duplicate of another object. Duplicate object will have the same state of the copied object but will have independent values in different memory locations. Copy Constructor can be useful to create duplicates of immutable objects as the Original cannot be tampered. Moreover It can be useful if base copies are required for individual requests in threading. | ||||
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Ans. Audit Tables generally stores Raw information to be reviewed in case of problems or determining impact. If Database space is an issue , and the audit information is rarely retrieved, one design could be to use compressed file system. | ||||
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Ans. HashTable has been deprecated. As an alternative, ConcurrentHashMap has been provided. It uses multiple buckets to store data and hence much better performance than HashTable. Moreover, there is already a raw type HashMap. The only difference between the HashTable and HashMap is that Hashtable is synchronized whereas HashMap is not. Most of the synchronized collections have been deprecated and their raw alternative have been presented as preferred.Synchronization has a cost. Using synchronized collection in places where there is no need of it leads to useless utilization of resources. As these collections are rarely used in a static context or shared among threads, Java might have thought it better to just provide the raw collection and let developers implement synchronization if he feels the need to do so. HashMap is now presented as the default and the preferred way of using Map with read optimized hashing, and ConcurrentHashMap has been provided for synchronized access which provides better performance than HashTable. Because of this, Java thought it right to deprecate the use of HashTable.' Synchronization has a cost. Using synchronized collection at a place where there is hardly any need of it would means useless utilization of resources. As these collections are rarely used in static context or shared among threads, Java might have thought it better to just provide the raw collection and let developer implement synchronization if he feels the need to do so. As HashMap has been presented as default and preferred way of using Map with read optimized hashing, and ConcurrentHashMap has been provided for synchronized access which provides better performance than HashTable, Java thought it right to deprecate the use of HashTable. | ||||
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Like Discuss Correct / Improve  hashtable  synchronized collections  Why synchronized collections have been deprecated  Why HashTable has been deprecated  HashTable vs HashMap expert | ||||
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Ans. Java.util.Map | ||||
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Very frequently asked across all types of companies. | ||||
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Ans. Hashtable is synchronized whereas HashMap is not.HashMap allows null values whereas Hashtable doesnt allow null values. | ||||
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Ans. Yes , for HashMap. HashMap implements Map interface. HashMap allows one null key and any number of null values. | ||||
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Frequently asked in all types of companies especially Indian Services companies. Frequently asked in CTS (Based on 2 feedback) | ||||
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Ans. Hashcode is used for bucketing in Hash implementations like HashMap, HashTable, HashSet etc. The value received from hashcode() is used as bucket number for storing elements. This bucket number is the address of the element inside the set/map. when you do contains() then it will take the hashcode of the element, then look for the bucket where hashcode points to and if more than 1 element is found in the same bucket (multiple objects can have the same hashcode) then it uses the equals() method to evaluate if object are equal, and then decide if contain() is true or false, or decide if element could be added in the set or not. | ||||
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Like Discuss Correct / Improve  java   collections   hashcode   advanced  hashtable Asked in 33 Companies intermediate   frequent | ||||
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Advanced level question. Frequently asked in High end product companies. Frequently asked in Google , Cognizant and Deloitte ( Based on 2 feedback ) | ||||
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Ans. 1. String Pool - When a string is created and if it exists in the pool, the reference of the existing string will be returned instead of creating a new object. If string is not immutable, changing the string with one reference will lead to the wrong value for the other references. Example - String str1 = "String1"; String str2 = "String1"; // It doesn't create a new String and rather reuses the string literal from pool // Now both str1 and str2 pointing to same string object in pool, changing str1 will change it for str2 too 2. To Cache its Hashcode - If string is not immutable, One can change its hashcode and hence it's not fit to be cached. 3. Security - String is widely used as parameter for many java classes, e.g. network connection, opening files, etc. Making it mutable might possess threats due to interception by the other code segment. | ||||
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Very frequently asked. Usually asked with questions related to String. Frequently asked at CTS / Cognizant | ||||
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Ans. Object that can't be changed after instantiation. | ||||
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Very Frequently asked. Usually asked along with String Class related questions. | ||||
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Ans. Class using which only immutable (objects that cannot be changed after initialization) objects can be created. | ||||
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Ans. We can make a class immutable by 1. Making all methods and variables as private. 2. Setting variables within constructor. Public Class ImmutableClass{ private int member; ImmutableClass(int var){ member=var; } } and then we can initialize the object of the class as ImmutableClass immutableObject = new ImmutableClass(5); Now all members being private , you cant change the state of the object. | ||||
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Ans. Only declaring variables as final makes them immutable. Making objects final means that the object handler cannot be used to target some other object but the object is still mutable. | ||||
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Ans. The properties class is a subclass of Hashtable that can be read from or written to a stream. | ||||
Sample Code for Load Properties using Property Class | ||||
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Ans. Java is a portable-language because without any modification we can use Java byte-code in any platform(which supports Java). So this byte-code is portable and we can use in any other major platforms. | ||||
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Asked multiple times in Capgemini. | ||||
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Ans. String is immutable in java and stored in String pool. Once it's created it stays in the pool until unless garbage collected, so even though we are done with password it's available in memory for longer duration and there is no way to avoid it. It's a security risk because anyone having access to memory dump can find the password as clear text. | ||||
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Like Discuss Correct / Improve  java   string class   string   immutable  immutability   string pool   garbage collection   advanced Asked in 6 Companies Expert | ||||
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Ans. Since String is immutable, its hashcode is cached at the time of creation and it doesnt need to be calculated again. This makes it a great candidate for key in a Map and its processing is fast than other HashMap key objects. This is why String is mostly used Object as HashMap keys. | ||||
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Ans. Hashcode is used for bucketing in Hash implementations like HashMap, HashTable, HashSet etc. | ||||
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Ans. These are the tables that are created temporarily and are deleted once the Stored Procedure is complete. For example - we may like to pull some info from a table and then do some operations on that data and then store the output in final output table. We can store the intermediary values in a temp table and once we have final output with us, we can just delete it. | ||||
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Ans. No, We cannot have duplicate keys in HashMap. If we attempt to do so , the previous value for the key is overwritten. | ||||
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Ans. First Annotation will set the Entity name as EMPLOYEES and hence will try to map with the same Table name. The second annotation will make the Entity mapped to table EMPLOYEES irrespective of the Entity Name ( which is class name in this case ). Third Annotations will set the different names for Enitity and Table and will explicitly map them. | ||||
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Ans. NOT NULL | ||||
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Ans. The following code returns the list of Employee objects having employee name starting with A and Dept Name ( Department , Employee Mapped ). session.createCriteria(Employee.class,"emp") .createAlias("emp.department", "dept",Criteria.INNER_JOIN) .add( Restrictions.like("name", "A%") ) .add(Restrictions.eq("dept.name","Finance") .list(); | ||||
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Ans. Stateless objects are the objects without instance fields (instance variables). The class may have compile time constants i.e static final fields.Immutable objects are the objects which have state but the state cannot be changed after initialization. Both are Thread safe. | ||||
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Ans. They can be shared across multiple threads as they are thread safe. | ||||
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Ans. Map | ||||
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Ans. HashMap came before HashTable. | ||||
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Ans. HashMap | ||||
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Ans. tb | ||||
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