Core Java FAQs

CORE JAVA!

Language Fundamentals How many number of non-public class definitions can a source file have A source file can contain unlimited number of non-public class definitions List primitive data types, there size and there range (min, max) Data Type Bytes bits min max boolean - 1 - - char 2 16 0 2^16-1 byte 1 8 -2^7 2^7-1 short 2 16 -2^15 2^15-1 int 4 32 -2^31 2^31-1 long 8 64 -2^63 2^63-1 float 4 32 - - double 8 64 - - What types of values does boolean variables take It only takes values true and false Which primitive datatypes are signed All except char and Boolean Is char type signed or unsigned char type is integral but unsigned. It range is 0 to 2^7-1 What forms an integral literal can be decimal, octal and hexadecimal, hence example it can be 28, 034 and 0x1c respectively What is the default value of Boolean False Why is the main method static So that it can be invoked without creating an instance of that class What is the difference between class variable, member variable and automatic(local) variable class variable is a static variable and does not belong to instance of class but rather shared across all the instances member variable belongs to a particular instance of class and can be called from any method of the class automatic or local variable is created on entry to a method and has only method scope When are static and non static variables of the class initialized The static variables are initialized when the class is loadedNon static variables are initialized just before the constructor is called When are automatic variable initialized Automatic variable have to be initialized explicitly How is an argument passed in java, by copy or by reference If the variable is primitive datatype then it is passed by copy. If the variable is an object then it is passed by reference What is garbage collection The runtime system keeps track of the memory that is allocated and is able to determine whether that memory is still useable. This work is usually done in background by a low-priority thread that is referred to as garbage collector. When the gc finds memory that is no longer accessible from any live thread it takes steps to release it back to the heap for reuse Does System.gc and Runtime.gc() guarantee garbage collection No Operators and assignment What are different types of operators in java Uniary ++, --, +, -, |, ~, () Arithmetic *, /, %,+, - Shift <<, >>, >>> Comparison =, instanceof, = =,!=Bitwise &, ^, |Short Circuit &&, ||Ternary ?:Assignment = How does bitwise (~) operator work It converts all the 1 bits in a binary value to 0s and all the 0 bits to 1s, e.g 11110000 coverts to 00001111 What is a modulo operator % This operator gives the value which is related to the remainder of a divisione.g x=7%4 gives remainder 3 as an answer Can shift operators be applied to float types. No, shift operators can be applied only to integer or long types What happens to the bits that fall off after shifting They are discarded What values of the bits are shifted in after the shift In case of signed left shift >> the new bits are set to zero But in case of signed right shift it takes the value of most significant bit before the shift, that is if the most significant bit before shift is 0 it will introduce 0, else if it is 1, it will introduce 1 Modifiers What are access modifiers These public, protected and private, these can be applied to class, variables, constructors and methods. But if you don’t specify an access modifier then it is considered as Friendly Can protected or friendly features be accessed from different packages No when features are friendly or protected they can be accessed from all the classes in that package but not from classes in another package How can you access protected features from another package You can access protected features from other classes by subclassing the that class in another package, but this cannot be done for friendly features What are the rules for overriding Private method can be overridden by private, friendly, protected or public methods Friendly method can be overridden by friendly, protected or public methods Protected method can be overridden by protected or public methods Public method can be overridden by public method Explain modifier final Final can be applied to classes, methods and variables and the features cannot be changed. Final class cannot be subclassed, methods cannot be overridden Can you change the reference of the final object No the reference cannot be change, but the data in that object can be changed Can abstract modifier be applied to a variable No it is applied only to class and methods Can abstract class be instantiated No abstract class cannot be instantiated i.e you cannot create a new object of this class When does the compiler insist that the class must be abstract If one or more methods of the class are abstract. If class inherits one or more abstract methods from the parent abstract class and no implementation is provided for that method If class implements an interface and provides no implementation for those methods How is abstract class different from final class Abstract class must be subclassed and final class cannot be subclassed Where can static modifiers be used They can be applied to variables, methods and even a block of code, static methods and variables are not associated with any instance of class When are the static variables loaded into the memory During the class load time When are the non static variables loaded into the memory They are loaded just before the constructor is called How can you reference static variables Via reference to any instance of the class Computer comp = new Computer (); comp.harddisk where hardisk is a static variable comp.compute() where compute is a method Via the class name Computer.harddisk Computer.compute() Can static method use non static features of there class No they are not allowed to use non static features of the class, they can only call static methods and can use static data What is static initializer code A class can have a block of initializer code that is simply surrounded by curly braces and labeled as static e.g. public class Demo{ static int =10; static{ System.out.println(“Hello world’); } } And this code is executed exactly once at the time of class load Where is native modifier used It can refer only to methods and it indicates that the body of the method is to be found else where and it is usually written in non java language What are transient variables A transient variable is not stored as part of objects persistent state and they cannot be final or static What is synchronized modifier used for It is used to control access of critical code in multithreaded programs What are volatile variables It indicates that these variables can be modified asynchronously Conversion Casting and Promotion What are wrapped classes Wrapped classes are classes that allow primitive types to be accessed as objects. What are the four general cases for Conversion and Casting Conversion of primitives Casting of primitives Conversion of object references Casting of object references When can conversion happen It can happen during Assignment Method call Arithmetic promotion What are the rules for primitive assignment and method call conversion A boolean can not be converted to any other type A non Boolean can be converted to another non boolean type, if the conversion is widening conversion A non Boolean cannot be converted to another non boolean type, if the conversion is narrowing conversion See figure below for simplicity What are the rules for primitive arithmetic promotion conversion For Unary operators : If operant is byte, short or a char it is converted to an int If it is any other type it is not converted For binary operands : If one of the operands is double, the other operand is converted to double Else If one of the operands is float, the other operand is converted to float Else If one of the operands is long, the other operand is converted to long Else both the operands are converted to int What are the rules for casting primitive types You can cast any non Boolean type to any other non boolean type You cannot cast a boolean to any other type; you cannot cast any other type to a boolean What are the rules for object reference assignment and method call conversion An interface type can only be converted to an interface type or to object. If the new type is an interface, it must be a superinterface of the old type A class type can be converted to a class type or to an interface type. If converting to a class type the new type should be superclass of the old type. If converting to an interface type new type the old class must implement the interface An array maybe converted to class object, to the interface cloneable, or to an array. Only an array of object references types may be converted to an array, and the old element type must be convertible to the new element What are the rules for Object reference casting Casting from Old types to Newtypes Compile time rules When both Oldtypes and Newtypes are classes, one should be subclass of the other When both Oldtype ad Newtype are arrays, both arrays must contain reference types (not primitive), and it must be legal to cast an element of Oldtype to an element of Newtype You can always cast between an interface and a non-final object Runtime rules If Newtype is a class. The class of the expression being converted must be Newtype or must inherit from Newtype If NewType is an interface, the class of the expression being converted must implement Newtype Flow Control and exception What is the difference between while and do while loop Do while loop walways executes the body of the loop at least once, since the test is performed at the end of the body When do you use continue and when do you use break statements When continue statement is applied it prematurely completes the iteration of a loop. When break statement is applied it causes the entire loop to be abandoned. What is the base class from which all exceptions are subclasses All exceptions are subclasses of a class called java.lang.Throwable How do you intercept and thereby control exceptions We can do this by using try/catch/finally blocks You place the normal processing code in try block You put the code to deal with exceptions that might arise in try block in catch block Code that must be executed no matter what happens must be place in finally block When do we say an exception is handled When an exception is thrown in a try block and is caught by a matching catch block, the exception is considered to have been handled When do we say an exception is not handled There is no catch block that names either the class of exception that has been thrown or a class of exception that is a parent class of the one that has been thrown, then the exception is considered to be unhandled, in such condition the execution leaves the method directly as if no try has been executed In what sequence does the finally block gets executed If you put finally after a try block without a matching catch block then it will be executed after the try block If it is placed after the catch block and there is no exception then also it will be executed after the try block If there is an exception and it is handled by the catch block then it will be executed after the catch block What can prevent the execution of the code in finally block The death of thread Use of system.exit() Turning off the power to CPU An exception arising in the finally block itself What are the rules for catching multiple exceptions A more specific catch block must precede a more general one in the source, else it gives compilation error Only one catch block, that is first applicable one, will be executed What does throws statement declaration in a method indicate This indicates that the method throws some exception and the caller method should take care of handling it What are checked exception Checked exceptions are exceptions that arise in a correct program, typically due to user mistakes like entering wrong data or I/O problems What are runtime exceptions Runtime exceptions are due to programming bugs like out of bond arrays or null pointer exceptions. What is difference between Exception and errors Errors are usually compile time and exceptions can be runtime or checked How will you handle the checked exceptions You can provide a try/catch block to handle it. OR Make sure method declaration includes a throws clause that informs the calling method an exception might be thrown from this particular method When you extend a class and override a method, can this new method throw exceptions other than those that were declared by the original method No it cannot throw, except for the subclasses of those exceptions Is it legal for the extending class which overrides a method which throws an exception, not o throw in the overridden class Yes it is perfectly legal Explain the user defined Exceptions? User defined Exceptions are the separate Exception classes defined by the user for specific purposed. An user defined can created by simply sub-classing it to the Exception class. This allows custom exceptions to be generated (using throw) and caught in the same way as normal exceptions. Example: class myCustomException extends Exception { // The class simply has to exist to be an exception } Objects and Classes What's the difference between constructors and other methods Constructors must have the same name as the class and can not return a value. They are only called once while regular methods could be called many times. What is the difference between Overloading and Overriding Overloading : Reusing the same method name with different arguments and perhaps a different return type is called as overloading Overriding : Using the same method name with identical arguments and return type is know as overriding What do you understand by late binding or virtual method Invocation. (Example of runtime polymorphism) When a compiler for a non object oriented language comes across a method invocation, it determines exactly what target code should be called and build machine language to represent that call. In an object oriented language, this is not possible since the proper code to invoke is determined based upon the class if the object being used to make the call, not the type of the variable. Instead code is generated that will allow the decision to be made at run time. This delayed decision making is called as late binding Can overriding methods have different return types No they cannot have different return types If the method to be overridden has access type protected, can subclass have the access type as private No, it must have access type as protected or public, since an overriding method must not be less accessible than the method it overrides Can constructors be overloaded Yes constructors can be overloaded What happens when a constructor of the subclass is called A constructor delays running its body until the parent parts of the class have been initialized. This commonly happens because of an implicit call to super() added by the compiler. You can provide your own call to super(arguments..) to control the way the parent parts are initialized. If you do this, it must be the first statement of the constructor. If you use super() or this() in a constructor where should it appear in the constructor It should always be the first statement in the constructor What is an inner class An inner class is same as any other class, but is declared inside some other class How will you reference the inner class To reference it you will have to use OuterClass$InnerClass Can objects that are instances of inner class access the members of the outer class Yes they can access the members of the outer class What modifiers may be used with an inner class that is a member of an outer class? A (non-local) inner class may be declared as public, protected, private, static, final, or abstract Can inner classes be static Yes inner classes can be static, but they cannot access the non static data of the outer classes, though they can access the static data Can an inner class be defined inside a method Yes it can be defined inside a method and it can access data of the enclosing methods or a formal parameter if it is final What is an anonymous class Some classes defined inside a method do not need a name, such classes are called anonymous classes What are the rules of anonymous class The class is instantiated and declared in the same place The declaration and instantiation takes the form new Xxxx () {// body} Where Xxxx is an interface name. An anonymous class cannot have a constructor. Since you do not specify a name for the class, you cannot use that name to specify a constructor Threads Where does java thread support reside It resides in three places The java.lang.Thread class (Most of the support resides here) The java.lang.Object class The java language and virtual machine What is the difference between Thread and a Process Threads run inside process and they share data. One process can have multiple threads, if the process is killed all the threads inside it are killed, they dont share data What happens when you call the start() method of the thread This registers the thread with a piece of system code called thread scheduler The schedulers determines which thread is actually running Does calling start () method of the thread causes it to run No it merely makes it eligible to run. The thread still has to wait for the CPU time along with the other threads, then at some time in future, the scheduler will permit the thread to run When the thread gets to execute, what does it execute The thread executes a method call run(). It can execute run() method of either of the two choices given below : The thread can execute it own run() method. The thread can execute the run() method of some other objects For the first case you need to subclass the Thread class and give your subclass a run() method For the second method you need to have a class implement the interface runnable. Define your run method. Pass this object as an argument to the Thread constructor How many methods are declared in the interface runnable The runnable method declares only one method : public void run(); Which way would you prefer to implement threading , by extending Thread class or implementing Runnable interface The preferred way will be to use Interface Runnable, because by subclassing the Thread class you have single inheritance i.e you wont be able to extend any other class What happens when the run() method returns When the run() method returns, the thread has finished its task and is considered dead. You can’t restart a dead thread. You can call the methods of dead thread What are the different states of the thread They are as follows: Running: The state that all thread aspire to be Various waiting states : Waiting, Sleeping, Suspended and Bloacked Ready : Waiting only for the CPU Dead : All done What is Thread priority Every thread has a priority, the higher priorit thread gets preference over the lower priority thread by the thread scheduler What is the range of priority integer It is from 1 to 10. 10 beings the highest priority and 1 being the lowest What is the default priority of the thread The default priority is 5 What happens when you call Thread.yield() It caused the currently executing thread to move to the ready state if the scheduler is willing to run any other thread in place of the yielding thread. Yield is a static method of class Thread What is the advantage of yielding It allows a time consuming thread to permit other threads to execute What happens when you call Thread.sleep() It passes time without doing anything and without using the CPU. A call to sleep method requests the currently executing thread to cease executing for a specified amount of time. Does the thread method start executing as soon as the sleep time is over No, after the specified time is over the thread enters into ready state and will only execute when the scheduler allows it to do so. What do you mean by thread blocking If a method needs to wait an indeterminable amount of time until some I/O occurrence takes place, then a thread executing that method should graciously step out of the Running state. All java I/O methods behave this way. A thread that has graciously stepped out in this way is said to be blocked What threading related methods are there in object class wait(), notify() and notifyAll() are all part of Object class and they have to be called from synchronized code only What is preemptive scheduling In preemptive scheduling there are only two ways for the thread to leave the running state without ecplicitly calling wait() or suspended() It can cease t be ready to execute ()by calling a blocking I/O method) It can get moved out by CPU by a higher priorit thread that becomes ready to execute What is non-preemptive or Time sliced or round robin scheduling With time slicing the thread is allowd to execute for a limited amount of time. It is then moved to ready state, where it must contend with all the other ready threads. What are the two ways of synchronizing the code Synchronizing an entire method by putting the synchronized modifier in the methods declaration. To execute the method, a thread must acquire the lock of the object that owns the method Synchronize a subset of a method by surrounding the desired lines of code with curly brackets and inserting the synchronized expression before the opening curly. This allows you to synchronize the block on the lock of any object at all, not necessarily the object that owns the code What happens when the wait() method is called The calling thread gives up CPU The calling thread gives up the lock The calling thread goes into the monitor’s waiting pool What happens when the notify() method is called One thread gets moved out of monitors waiting pool and into the ready state The thread that was notified ust reacquire the monitors locl before it can proceed Using notify () method how you can specify which thread should be notified You cannot specify which thread is to be notified, hence it is always better to call notifyAll() method java.lang & java.util Packages What is the ultimate ancestor of all java classes Object class is the ancestor of all the java classes What are important methods of Object class wait(), notify(), notifyAll(), equals(), toString(). What is the difference between “= =” and “equals()” “= =” does shallow comparison, It retuns true if the two object points to the same address in the memory, i.e if the same the same reference “equals()” does deep comparison, it checks if the values of the data in the object are same What would you use to compare two String variables - the operator == or the method equals()? I'd use the method equals() to compare the values of the Strings and the == to check if two variables point at the same instance of a String object Give example of a final class Math class is final class and hence cannot be extended What is the difference between String and StringBuffer String is an immutable class, i.e you cannot change the values of that class Example : String str = “java”; // address in memory say 12345 And now if you assign a new value to the variable str then str = “core java”; then the value of the variable at address 12345 will not change but a new memory is allocated for this variable say 54321 So in the memory address 12345 will have value “java” And the memory address 54321 will have value “core java” and the variable str will now be pointing to address 54321 in memory StringBuffer can be modified dynamically Example: StringBuffer strt =”java” // address in memory is say 12345 And now if you assign a new value to the variable str then Str = “core java”; then value in the address of memory will get replaced, a new memory address is not allocated in this case. What will be the result if you compare StringBuffer with String if both have same values It will return false as you cannot compare String with StringBuffer What is Collection API The Collection API is a set of classes and interfaces that support operation on collections of objects. These classes and interfaces are more flexible, more powerful, and more regular than the vectors, arrays, and hashtables if effectively replaces. Example of classes: HashSet, HashMap, ArrayList, LinkedList, TreeSet and TreeMap. Example of interfaces: Collection, Set, List and Map. What are different types of collections A collection has no special order and does not reject duplicates A list is ordered and does not reject duplicates A set has no special order but rejects duplicates A map supports searching on a key field, values of which must be unique Tell me something about Arrays Arrays are fast to access, but are inefficient if the number of elements grow and if you have to insert or delete an element Difference between ArrayList and Vector Vector methods are synchronized while ArrayList methods are not Iterator a Class or Interface? What is its use? Iterator is an interface which is used to step through the elements of a Collection Difference between Hashtable and HashMap Hashtable does not store null value, while HashMap does Hashtable is synchronized, while HashMap is not This site will be helpful for someone looking for a new job or a promotion or someone looking to brush up few topics if they are moving to new project and have not been quite in touch with one of the technologies; it will also help freshers to understand the basics of java. I strongly recommend they thoroughly go through the Core java page before proceeding with advance topics. 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