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Ans. Default methods results in multiple inheritance of behavior and not of state. In case we try to implement multiple interfaces with default method having same name and signature, and don't override it in implementation class, it will throw an error.
For example -
interface MyInterface {
public void default myMethod(){
}
}
interface MyInterface2 {
public void default myMethod(){
}
}
class MyClass implements MyInterface,MyInterface2 {
}
This code will compilation error "Duplicate Default Method"
if we specify the definition of myMethod() in myClass, compiler won't complain and there is no conflict and MyClass can use overridden definition. But if we don't override myMethod() in MyClass, Java would be in conflict as to what definition should be carried to MyClass and hence throws compilation error.
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Ans. Upstream project is a project whose successful build is required for executing the existing project build whereas Downstream project is the one that is waiting for the upstream project to build as it has the upstream project as dependency.
For example - X project has an upstream project y and downstream project Z. So It means X has dependency Y and is a dependency for Z.
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Ans. Broken Build means as unsuccessful build whereas unstable build means build was created successfully but one of the Post Build report failed or could be a Test failure.
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Ans. Constructor has the same name as class name whereas instance initialization block just have a body without any name or visibility type.
instance initialization blocks are useful if we want to have some code run regardless of which constructor is used or if we want to do some instance initialization for anonymous classes.
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Methods , Variables , Initialization Blocks and Outer Classes and nested Classes.
Methods , Variables , Initialization Blocks and Outer Classes.
Methods , Variables , Initialization Blocks and nested Classes.
Q1390. What is the difference between print() and println() in Java ?
Ans. print method prints the argument passed to the method whereas println prints the argument and then prints the new line character. println is generally used if we want each output in a new line.
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Q1391. Can I run a java program without creating any class ?
Ans. No, It requires creation of atleast one Class. Creating an object of that class is not compulsory as we can write all our logic within main method which is static.
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Ans. Earlier any class implementing an interface was supposed to implement all methods declared in an interface. There was no place for optionally implementing all or subset of methods.Though we have abstract classes wherein we could have provided such a mechanism by declaring some methods as abstract while providing definition for some. But as Abstract classes have a body and are comparatively heavier than interfaces and interfaces associate closely to the concept of providing interfacing than abstract classes, Java might have though of providing optional implementation for default methods. This way same interface can be reused in variety of ways rather than making copies of an interface to suit different needs.
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Ans. When a DML is executed, the changes only stays in session and still not pushed to DB Tables, Commit is used to push those changes to the Tables.
In case we realize that we don't want to commit those changes and would like to ignore them, we can use rollback.
For example - You may like that for a banking transaction you would like to update the account balance only if the debit or credit record was correctly inserted, so you may like to encapsulate both DML's - insert for transaction and update for balance in a single transaction and would only commit if both succeeds else rollback.
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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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Q1396. What is the difference between following two if blocks ?
if(x < 10)
do this;
else if(x > 10)
do that;
else
do this and that;
if(x < 10)
do this;
if(x > 10)
do that;
if(x == 10)
do this and that;
Ans. Though Logically both code segments are same, but first one is more efficient as the 3 checks are mutually exclusive. In second code segment, all conditions will be evaluated no matter what and will never result in short circuit. In first segment , max 2 conditions will be checked if x >= 10.
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Q1397. When do we generally get the database error - Unique Constraint Violated ?
Ans. This error can result either on an insert, update or delete when any change in data results in duplicate record or subset of it having unique constraint on record or its subset. For example - If we have a unique constraint on a combination of columns and then trying to insert duplicate column values.
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Q1398. When do we generally get this database error - integrity constraint violated - parent key not found ?
Ans. This error most likely will be thrown during an insert statement, while inserting a value within a foreign key column which doesnt exist within the Parent column. For example - Trying to add a dept number reference within a Employee Table when the Dept doesnt exist in the Dept Table.
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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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LikeDiscussCorrect / Improve hashtable synchronized collections Why synchronized collections have been deprecated Why HashTable has been deprecated HashTable vs HashMap expert
Ans. When a Table Join itself , it's a Self Join. For example - we like to know the pair of department names where first dept has lesser employees than the later.
Select D1.name , D2.name from Dept D1, Dept D2 where D1.employee_count < D2.employee_count
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Ans. In first case we are trying to initialize Inner class object using the instance of Outer Class whereas in second case we are trying to initialize the Inner class object directly using the Outer class name.
In second case , Inner class is "static inner class" as we cannot access "non static inner class" using Classname alone.
In first case, the inner class could be either "static inner class" or "non static inner class".
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Ans. Multi Level Inheritance is multi level hierarchy of classes. Here is the example - http://www.buggybread.com/2015/09/java-se-class-diagram-classes-that_603.html
Class RoleList extends ArrayList which in turn extends AbstractList which in turn extends AbstractCollection.
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Q1404. Is it advisable to just hold checking in your changes to trunk if there is another release planned in between ?
Ans. If it's just smaller change, and single person is working, then this approach is fine. Otherwise there are risk on loosing it on your machine. Moreover , If there are multiple people working , it makes it hard to share code. It's better to create a separate branch and then merge it later to trunk.
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Ans. We creates separate branches for each project if development work is going on parallel and they are to be released at different times. Once the first release is done, we merge the branch changes into trunk. If they all have to go at one time, we usually would merge everything in the trunk itself.
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Ans. POJO is Plain Java Object that holds only Member Elements , getters are setters and minimal processing on that. The primary purpose of such object is to either transfer information or keeping state for a while. They are not intended to provide any processing or transformation.
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Q1410. Any real life example of Overloading and Overridding ?
Ans. All members of a family share a common last name and thats how outsiders recognize them, but to recognize individual team members , we need an overloaded name and hence we use First Name along with Last Name to uniquely identify.
Some members of a family have a Nick Name as the real name is very long or hard to call. So the Nick name overrides the Original Name in certain circumstances.
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