1 Q Why threads block or enters to waiting state on I/O?
A Threads enters to waiting state or block on I/O because other threads can execute while the I/O operations are performed.
2 Q What are transient variables in java?
A Transient variables are variable that cannot be serialized.
3 Q How Observer and Observable are used?
A Subclass of Observable class maintain a list of observers. Whenever an Observable object is updated, it invokes the update() method of each of its observers to notify the observers that it has a changed state. An observer is any object that implements the interface Observer.
4 Q What is synchronization
A Synchronization is the ability to control the access of multiple threads to shared resources. Synchronization stops multithreading. With synchronization , at a time only one thread will be able to access a shared resource.
5 Q What is List interface ?
A List is an ordered collection of objects.
6 Q What is a Vector
A Vector is a grow able array of objects.
7 Q What is the difference between yield() and sleep()?
A When a object invokes yield() it returns to ready state. But when an object invokes sleep() method enters to not ready state.
8 Q What are Wrapper Classes ?
A They are wrappers to primitive data types. They allow us to access primitives as objects.
9 Q Can we call finalize() method ?
A Yes. Nobody will stop us to call any method , if it is accessible in our class. But a garbage collector cannot call an object's finalize method if that object is reachable.
10 Q What is the difference between time slicing and preemptive scheduling ?
A In preemptive scheduling, highest priority task continues execution till it enters a not running state or a higher priority task comes into existence. In time slicing, the task continues its execution for a predefined period of time and reenters the pool of ready tasks.
11 Q What is the initial state of a thread when it is created and started?
A The thread is in ready state.
12 Q Can we declare an anonymous class as both extending a class and implementing an interface?
A No. An anonymous class can extend a class or implement an interface, but it cannot be declared to do both
13 Q What are the differences between boolean & operator and & operator
A When an expression containing the & operator is evaluated, both operands are evaluated. And the & operator is applied to the operand. When an expression containing && operator is evaluated, the first operand is evaluated. If the first operand returns a value of true then only the second operand is evaluated otherwise the second part will not get executed. && is also called short cut and.
14 Q What is the use of the finally block?
A Finally is the block of code that executes always. The code in finally block will execute even if an exception is occurred. finally will not execute when the user calls System.exit().
15 Q What is an abstract method ?
A An abstract method is a method that don't have a body. It is declared with modifier abstract.
16 Q what is a the difference between System.err and System.out
A We can redirect System.out to another file but we cannot redirect System.err stream
17 Q What are the differences between an abstract class and an interface?
A An abstract class can have concrete method, which is not allowed in an interface. Abstract class can have private or protected methods and variables and only public methods and variables are allowed in interface. We can implement more than one interface , but we can extend only one abstract class. Interfaces provides loose coupling where as abstract class provides tight coupling.
18 Q What is the difference between synchronized block and synchronized method ?
A Synchronized blocks place locks for the specified block where as synchronized methods place locks for the entire method.
19 Q How can you force garbage collection in java?
A You cannot force Garbage Collection, but you can request for it by calling the method System.gc(). But it doesn't mean that Garbage Collection will start immediately. The garbage collection is a low priority thread of JVM.
20 Q How can you call a constructor from another constructor ?
A By using this() reference.
21 Q How can you call the constructor of super class ?
A By using super() syntax.
22 Q What's the difference between normal methods and constructors?
A Constructors must have the same name of the class and can not have a return type. They are called only once, while regular methods can be called whenever required. We cannot explicitly call a constructor.
23 Q What is the use of packages in java ?
A Packages are a way to organize files in java when a project consists of more than one module. It helps in resolving name conflicts when different modules have classes with the same names.
24 Q What must be the order of catch blocks when catching more than one exception?
A The sub classes must come first. Otherwise it will give a compile time error.
25 Q How can we call a method or variable of the super class from child class ?
A We can use super.method() or super.variable syntax for this purpose.
26 Q If you are overriding equals() method of a class, what other methods you might need to override ?
A hashCode
27 Q How can you create your own exception ?
A Our class must extend either Exception or its sub class
28 Q What is serialization ?
A Serialization is the process of saving the state of an object.
29 Q What is de-serialization?
A De-serialization is the process of restoring the state of an object.
30 Q What is externalizable ?
A It is an interface that extends Serializable. It is having two different methods writeExternal() and readExternal. This interface allows us to customize the output.
31 Q Does garbage collection guarantee that a program will not run out of memory?
A Garbage collection does not guarantee that a program will not run out of memory. It is also possible for programs to create objects that are not subject to garbage collection. And there is no guarantee that Garbage Collection thread will be executed.
32 Q What is a native method?
A A native method is a method that is implemented in a language other than Java.
33 Q What are different type of exceptions in Java?
A There are two types of exceptions in java. Checked exceptions and Unchecked exceptions. Any exception that is is derived from Throwable and Exception is called checked exception except RuntimeException and its sub classes. The compiler will check whether the exception is caught or not at compile time. We need to catch the checked exception or declare in the throws clause. Any exception that is derived from Error and RuntimeException is called unchecked exception. We don't need to explicitly catch a unchecked exception.
34 Q Can we catch an error in our java program ?
A Yes. We can . We can catch anything that is derived from Throwable. Since Error is a sub class of Throwable we can catch an error also.
35 Q What is thread priority?
A Thread Priority is an integer value that identifies the relative order in which it should be executed with respect to others. The thread priority values ranging from 1- 10 and the default value is 5. But if a thread have higher priority doesn't means that it will execute first. The thread scheduling depends on the OS.
36 Q How many times may an object's finalize() method be invoked by the garbage collector?
A Only once.
37 Q What is the difference between a continue statement and a break statement?
A Break statement results in the immediate termination of the statement to which it applies (switch, for, do, or while). A continue statement is used to end the current loop iteration and return control to the loop statement.
38 Q What must a class do to implement an interface?
A It must identify the interface in its implements clause. Also it must provide definition for all the methods in the interface otherwise it must be declared abstract.
39 Q What is an abstract class?
A An abstract class is an incomplete class. It is declared with the modifier abstract. We cannot create objects of the abstract class. It is used to specify a common behavioral protocol for all its child classes.
40 Q What is the difference between notify and notifyAll method ?
A notify wakes up a single thread that is waiting for object's monitor. If any threads are waiting on this object, one of them is chosen to be awakened. The choice is arbitrary and occurs at the discretion of the implementation. notifyAll Wakes up all threads that are waiting on this object's monitor. A thread waits on an object's monitor by calling one of the wait methods.
41 Q What does wait method do ?
A It causes current thread to wait until either another thread invokes notify or notifyAll method of the current object, or a specified amount of time has elapsed.
42 Q What are the different states of a thread ?
A The different thread states are ready, running, waiting and dead.
43 Q What is the difference between static and non static inner class ?
A A non-static inner class can have an object instances that are associated with instances of the class's outer class. A static inner class can not have any object instances.
44 Q What is the difference between String and StringBuffer class ?
A Strings are immutable (constant), their values cannot be changed after they are created. StringBuffer supports mutable objects.
45 Q Which is the base class for all classes ?
A java.lang.Object.
46 Q What is the difference between readers and streams?
A Readers are character oriented where streams are byte oriented. The readers are having full support for Unicode data.
47 Q What is constructor chaining ?
A When a constructor of a class is executed it will automatically call the default constructor of the super class (if no explicit call to any of the super class constructor) till the root of the hierarchy.
48 Q What are the different primitive data type in java ?
A There are 8 primitive types in java. boolean , char, byte, short, int long, float, double.
49 Q What is static ?
A static means one per class. static variables are created when the class loads. They are associated with the class. In order to access a static we don't need objects. We can directly access static methods and variable by calling classname.variablename.
50 Q Why we cannot override static methods?
A Static means they are associated with a class. In static methods , the binding mechanism is static binding. So it must be available at the compile time.
51 Q What is the difference between static and non static variables ?
A A static variable is associated with the class as a whole rather than with specific instances of a class. There will be only one value for static variable for all instances of that class. Non-static variables take on unique values with each object instance.
52 Q When does a compiler supplies a default constructor for a class?
A If there is no other constructor exist in a class, the compiler will supply a default constructor.
53 Q What are the restrictions placed on overriding a method ?
A The overridden method have the exact signature of the super class method, including the return type. The access specified cannot be less restrictive than the super class method. We cannot throw any new exceptions in overridden method.
54 Q What are the restrictions placed on overloading a method ?
A Overloading methods must differ in their parameter list, or number of parameters.
55 Q What is casting ?
A Casting means converting one type to another. There are mainly two types of casting. Casting between primitive types and casting between object references. Casting between primitive numeric types is used to convert larger data types to smaller data types. Casting between object references is used to refer to an object by a compatible class, interface, or array type reference.
56 Q What is the difference between == and equals ?
A The equals method can be considered to perform a deep comparison of the value of an object, whereas the == operator performs a shallow comparison. If we are not overriding the equals method both will give the same result. == will is used to compare the object references. It is used to check whether two objects are points to the same reference.
57 Q What is a void return type ?
A A void indicates that the method will not return anything.
58 Q What will happen if an exception is not caught ?
A An uncaught exception results in the uncaughtException() method of the thread's ThreadGroup, which results in the termination of the program.
59 Q What are the different ways in which a thread can enter into waiting state?
A There are three ways for a thread to enter into waiting state. By invoking its sleep() method, by blocking on I/O, by unsuccessfully attempting to acquire an object's lock, or by invoking an object's wait() method.
60 Q What is a ResourceBundle class?
A The ResourceBundle class is used to store locale-specific resources that can be loaded by a program to create the program's appearance to the particular locale in which it is being run.
61 Q What is numeric promotion?
A Numeric promotion is the conversion of a smaller numeric type to a larger numeric type. In numerical promotion, byte, char, and short values are converted to int values. The int, long and float values are converted to the desired types if required.
62 Q What is the difference between the prefix and postfix forms of the ++ operator?
A The prefix form first performs the increment operation and then returns the value of the increment operation. The postfix form first returns the current value of the expression and then performs the increment operation on that value.
63 Q What are synchronized methods and synchronized statements?
A Synchronized methods are methods that are declared with the keyword synchronized. A thread executes a synchronized method only after it has acquired the lock for the method's object or class. Synchronized statements are similar to synchronized methods. It is a block of code declared with synchronized keyword. A synchronized statement can be executed only after a thread has acquired the lock for the object or class referenced in the synchronized statement.
64 Q How can we create a thread?
A A thread can be created by extending Thread class or by implementing Runnable interface. Then we need to override the method public void run().
65 Q What is the difference between a switch statement and an if statement?
A If statement is used to select from two alternatives. It uses a boolean expression to decide which alternative should be executed. The expression in if must be a boolean value. The switch statement is used to select from multiple alternatives. The case values must be promoted to an to int value.
66 Q What is hashCode?
A The hashcode of a Java Object is simply a number (32-bit signed int) that allows an object to be managed by a hash-based data structure. A hashcode should be, equal for equal object (this is mandatory!) , fast to compute based on all or most of the internal state of an object, use all or most of the space of 32-bit integers in a fairly uniform way , and likely to be different even for objects that are very similar. If you are overriding hashCode you need to override equals method also.
67 Q What is an I/O filter?
A An I/O filter is an object that reads from one stream and writes to another, usually altering the data in some way as it is passed from one stream to another.
68 Q What is the difference between RandomAccessFile and File?
A The File class contains information the files and directories of the local file system. The RandomAccessFile class contains the methods needed to directly access data contained in any part of a file.
69 Q What is final ?
A A final is a keyword in java. If final keyword is applied to a variable, then the variable will become a constant. If it applied to method, sub classes cannot override the method. If final keyword is applied to a class we cannot extend from that class.
70 Q What is the difference among JVM Spec, JVM Implementation, JVM Runtime ?
A The JVM spec is the blueprint for the JVM generated and owned by Sun. The JVM implementation is the actual implementation of the spec by a vendor and the JVM runtime is the actual running instance of a JVM implementation
71 Q How is the difference between thread and process?
A A process runs in its own address space. No two processes share their address space. Threads will run in the same address space of the process that owns them.
72 Q What is the difference between Vector and ArrayList ?
A Vector is synchronized, ArrayList is not. Vector is having a constructor to specify the incremental capacity. But ArrayList don't have. By default Vector grows by 100% but ArrayList grows by 50% only.
73 Q What is the difference between Hashtable and HashMap ?
A Hashtable is synchronized . but HashMap is not synchronized. Hashtable does not allow null values , but HashMap allows null values.
74 Q What are the access modifiers available in Java.
A Access modifier specify where a method or attribute can be used. Public is accessible from anywhere. Protected is accessible from the same class and its subclasses. Package/Default are accessible from the same package. Private is only accessible from within the class.
75 Q Why java is said to be pass-by-value ?
A When assigning an object to a variable, we are actually assigning the memory address of that object to the variable. So the value passed is actually the memory location of the object. This results in object aliasing, meaning you can have many variables referring to the same object on the heap.
76 Q What do you mean by immutable ? How to create an immutable object ?
A Immutability means an object cannot be modified after it has been initialized. There will not be any setter methods in an immutable class. And normally these classes will be final.
77 Q What is class loader in java ?
A A class loader is a class that is responsible for loading the class. All JVM contains one class loader called primordial class loader.
78 Q What is a weak reference ?
A A weak reference is the one that does nor prevent the referenced object from being garbage collected. The weak reference will not keep the object that it refers to alive. A weak reference is not counted as a reference in garbage collection. This will make the memory use more effective.
79 Q What is object cloning?
A It is the process of duplicating an object so that two identical objects will exist in the memory at the same time.
80 Q What is object pooling?
A Creating a large number of identical short lived objects is called object pooling. This helps to minimize the need of garbage collection and makes the memory use more effective.
81 Q What is garbage collection?
A Garbage collection is the process of releasing memory used by unreferenced objects. It relieves the programmer from the process of manually releasing the memory used by objects .
82 Q What is the disadvantage of garbage collection?
A It adds an overhead that can affect performance. Additionally there is no guarantee that the object will be garbage collected.
83 Q What is a Dictionary?
A Dictionary is a parent class for any class that maps keys to values., In a dictionary every key is associated with at most one value.
84 Q What is JAR file ?
A JAR stands for Java Archive. This is a file format that enables you to bundle multiple files into a single archive file. A jar file will contains a manifest.mf file inside META-INF folder that describes the version and other features of jar file.
85 Q Why Java is not fully objective oriented ?
A Due to the use of primitives in java, which are not objects.
86 Q What is a marker interface ?
A An interface that contains no methods. Eg: Serializable, Cloneable, SingleThreadModel etc. It is used to just mark java classes that support certain capability.
87 Q What are tag interfaces?
A Tag interface is an alternate name for marker interface.
88 Q What are the restrictions placed on static method ?
A We cannot override static methods. We cannot access any object variables inside static method. Also the this reference also not available in static methods.
89 Q What is JVM?
A JVM stands for Java Virtual Machine. It is the run time for java programs. All are java programs are running inside this JVM only. It converts java byte code to OS specific commands. In addition to governing the execution of an application's byte codes, the virtual machine handles related tasks such as managing the system's memory, providing security against malicious code, and managing multiple threads of program execution.
90 Q What is JIT?
A JIT stands for Just In Time compiler. It compiles java byte code to native code.
91 Q What is java byte code?
A Byte code is an sort of intermediate code. The byte code is processed by virtual machine.
92 Q What is method overloading?
A Method overloading is the process of creating a new method with the same name and different signature.
93 Q What is method overriding?
A Method overriding is the process of giving a new definition for an existing method in its child class.
94 Q What is finalize() ?
A Finalize is a protected method in java. When the garbage collector is executes , it will first call finalize( ), and on the next garbage-collection it reclaim the objects memory. So finalize( ), gives you the chance to perform some cleanup operation at the time of garbage collection.
95 Q What is multi-threading?
A Multi-threading is the scenario where more than one threads are running.
96 Q What is deadlock?
A Deadlock is a situation when two threads are waiting on each other to release a resource. Each thread waiting for a resource which is held by the other waiting thread.
97 Q What is the difference between Iterator and Enumeration?
A Iterator differ from enumeration in two ways Iterator allow the caller to remove elements from the underlying collection during the iteration with well-defined semantics. And , method names have been improved.
98 Q What is the Locale class?
A A Locale object represents a specific geographical, political, or cultural region
99 Q What is internationalization?
A Internationalization is the process of designing an application so that it can be adapted to various languages and regions without changes.
100 Q What is anonymous class ?
A An anonymous class is a type of inner class that don't have any name.
101 Q What is the difference between URL and URLConnection?
A A URL represents the location of a resource, and a URLConnection represents a link for accessing or communicating with the resource at the location.
102 Q What are the two important TCP Socket classes?
A ServerSocket and Socket. ServerSocket is useful for two-way socket communication. Socket class help us to read and write through the sockets. getInputStream() and getOutputStream() are the two methods available in Socket class.
103 Q Strings are immutable. But String s="Hello"; String s1=s+"World" returns HelloWorld how ?
A Here actually a new object is created with the value of HelloWorld
104 Q What is classpath?
A Classpath is the path where Java looks for loading class at run time and compile time.
105 Q What is path?
A It is an the location where the OS will look for finding out the executable files and commands.
106 Q What is java collections?
A Java collections is a set of classes, that allows operations on a collection of classes.
107 Q Can we compile a java program without main?
A Yes, we can. In order to compile a java program, we don't require any main method. But to execute a java program we must have a main in it (unless it is an applet or servlet). Because main is the starting point of a java program.
108 Q What is a java compilation unit.
A A compilation unit is a java source file.
109 Q What are the restrictions when overriding a method ?
A Overridden methods must have the same name, argument list, and return type (i.e., they must have the exact signature of the method we are going to override, including return type.) The overriding method cannot be less visible than the method it overrides( i.e., a public method cannot be override to private). The overriding method may not throw any exceptions that may not be thrown by the overridden method
110 Q What is static initializer block? What is its use?
A A static initializer block is a block of code that declares with the static keyword. It normally contains the block of code that must execute at the time of class loading. The static initializer block will execute only once at the time of loading the class only.
111 Q How does a try statement determine which catch clause should be used to handle an exception?
A When an exception is thrown , the catch block of the try statement are examined in the order in which they appear. The first catch block that is capable of handling the exception is executed. The remaining catch blocks are ignored
112 Q How parameters are passed to methods in java program ?
A All java method parameters in java are passed by value only. Obviously primitives are passed by value. In case of objects a copy of the reference is passed and so all the changes made in the method will persist.
113 Q If a class doesn't have any constructors, what will happen?
A If a class doesn't have a constructor, the JVM will provide a default constructor for the class.
114 Q What will happen if a thread cannot acquire a lock on an object?
A It enters to the waiting state until lock becomes available.
115 Q How does multithreading occurring on a computer with a single CPU?
A The task scheduler of OS allocates an execution time for multiple tasks. By switching between different executing tasks, it creates the impression that tasks execute sequentially. But actually there is only one task is executed at a time.
116 Q What will happen if you are invoking a thread's interrupt method while the thread is waiting or sleeping?
A When the task enters to the running state, it will throw an InterruptedException.
117 Q What are the different ways in which a thread can enter into waiting state?
A There are three ways for a thread to enter into waiting state. By invoking its sleep() method, by blocking on I/O, by unsuccessfully attempting to acquire an object's lock, or by invoking an object's wait() method.
118 Q What are the the different ways for creating a thread?
A A thread can be created by subclassing Thread, or by implementing the Runnable interface.
119 Q What is the difference between creating a thread by extending Thread class and by implementing Runnable interface? Which one should prefer?
A When creating a thread by extending the Thread class, it is not mandatory to override the run method (If we are not overriding the run method , it is useless), because Thread class have already given a default implementation for run method. But if we are implementing Runnable , it is mandatory to override the run method. The preferred way to create a thread is by implementing Runnable interface, because it give loose coupling.
120 Q What is coupling?
A Coupling is the dependency between different components of a system
121 Q How is an interface?
A An interface is a collection of method declarations and constants. In java interfaces are used to achieve multiple inheritance. It sets a behavioral protocol to all implementing classes.
122 Q What is an abstract class?
A An abstract class is an incomplete class. An abstract class is defined with the keyword abstract . We cannot create an object of the abstract class because it is not complete. It sets a behavioral protocol for all its child classes.
123 Q How will you define an interface?
A An interface is defined with the keyword interface. Eg:
public interface MyInterface { }
124 Q How will you define an abstract class?
A An abstract class is defined with the keyword abstract Eg:
public abstract class MyClass { }
125 Q What is any an anonymous class?
A An anonymous class is a local class with no name.
126 Q What is a JVM heap?
A The heap is the runtime data area from which memory for all class instances and arrays is allocated. The heap may be of a fixed size or may be expanded. The heap is created on virtual machine start-up.
127 Q What is difference between string and StringTokenizer?
A StringTokenizer as its name suggests tokenizes a String supplied to it as an argument to its constructor and the character based on which tokens of that string are to be made. The default tokenizing character is space " ".
128 Q What is the difference between array and ArrayList ?
A Array is collection of same data type. Array size is fixed, It cannot be expanded. But ArrayList is a growable collection of objects. ArrayList is a part of Collections Framework and can work with only objects.
129 Q What is difference between java.lang .Class and java.lang.ClassLoader? What is the hierarchy of ClassLoader ?
A Class 'java.lang.Class' represent classes and interfaces in a running Java application. JVM construct 'Class' object when class in loaded. Where as a ClassLoader is also a class which loads the class files into memory in order for the Java programs to execute properly. The hierarchy of ClassLoaders is:
Bootstrap ClassLoaders
Extensive ClassLoaders
System Classpath ClassLoaders
Application ClassLoaders
130 Q What is daemon thread?
A Theards which are running on the background are called deamon threads. daemon thread is a thread which doesn't give any chance to run other threads once it enters into the run state it doesn't give any chance to run other threads. Normally it will run forever, but when all other non-daemon threads are dead, daemon thread will be killed by JVM
131 Q What is a green thread?
A Native threads can switch between threads preemptively. Green threads switch only when control is explicitly given up by a thread ( Thread.yield(), Object.wait(), etc.) or a thread performs a blocking operation (read(), etc.). On multi-CPU machines, native threads can run more than one thread simultaneously by assigning different threads to different CPUs. Green threads run on only one CPU. Native threads create the appearance that many Java processes are running: each thread takes up its own entry in the process table. One clue that these are all threads of the same process is that the memory size is identical for all the threads - they are all using the same memory. The process table is not infinitely large, and processes can only create a limited number of threads before running out of system resources or hitting configured limits.
132 Q What is volatile variable?
A A volatile variable is not allowed to have a local copy of a variable that is different from the value currently held in "main" memory. Volatile modifier requests the JVM to always access the shared copy of the variable so the its most current value is always read.
133 Q Why java does not support multiple inheritance?
A Because the multiple inheritance causes the redundancy. Also we cannot solve diamond problem.
134 Q What is diamond problem?
A The diamond problem is an ambiguity that can occur when a class multiply inherits from two classes that both descend from a common super class
135 Q How many JVM's we can run in a system?
A Any number of JVMs can run in a system. Whenever we issue the command 'java' a new JVM will start.
36 Q Why Java is not 100% pure object oriented language?
A Because java uses primitives.
137 Q Why ArrayList is faster than Vector?
A Because Vector is synchronized. Synchronization reduces the performance.
138 Q What is the security mechnaism used in java?
A Java uses sand box security model.
139 Q What is sandbox?
A A sandbox is a security mechanism for safely running programs. The sandbox typically provides a tightly-controlled set of resources for guest programs to run in, such as scratch space on disk and memory.
140 Q What is phantom memory?
A Phantom memory is the memory that does not exist in reality.
141 Q What is reflection?
A Reflection is the process of finding out the different features of a class dynamically.
142 Q What are the differences between JIT and HotSpot?
A The Hotspot VM is a collection of techniques, the most important of which is called adaptive optimization. The original JVMs interpreted byte codes one at a time. Second-generation JVMs added a JIT compiler, which compiles each method to native code upon first execution, then executes the native code. Thereafter, whenever the method is called, the native code is executed. The adaptive optimization technique used by Hotspot is a hybrid approach, one that combines byte code interpretation and run-time compilation to native code. Hotspot, unlike a regular JIT compiling VM, doesn't do "premature optimization"
143 Q What are the advantages and disadvantages of reference counting in garbage collection?
A An advantage of this scheme is that it can run in small chunks of time closely linked with the execution of the program. These characteristic makes it particularly suitable for real-time environments where the program can't be interrupted for very long time. A disadvantage of reference counting is that it does not detect cycles. A cycle is two or more objects that refer to one another. Another disadvantage is the overhead of incrementing and decrementing the reference count each time. Because of these disadvantages, reference counting currently is out of favor.
144 Q How would you implement a thread pool?
A The ThreadPool class is a generic implementation of a thread pool, which takes the following input Size of the pool to be constructed and name of the class which implements Runnable (which has a visible default constructor) and constructs a thread pool with active threads that are waiting for activation. once the threads have finished processing they come back and wait once again in the pool.
145 Q What is the difference between throw and throws clause?
A throw is used to throw an exception manually, where as throws is used in the case of checked exceptions, to tell the compiler that we haven't handled the exception, so that the exception will be handled by the calling function.
156 Q What is JAR file?
A A JAR file (short for Java Archive) is a ZIP file used to distribute a set of Java classes. It is used to store compiled Java classes and associated metadata that can constitute a program
147 Q What is a classloader?
A A class loader is an object that is responsible for loading classes.
148 Q What is the difference between Comparable and Comparator ?
A The Comparable is for natural ordering and Comparator is for custom ordering. But we can override the compareTo method of comparable interface to give a custom ordering.
149 Q What is the difference between List, Set and Map?
A A Set is a collection that has no duplicate elements. A List is a collection that has an order associated with its elements. A map is a way of storing key/value pairs. The way of storing a Map is similar to two-column table.
150 Q What is the difference between Exception and Error ?
A Error is unrecoverable.
151 Q What is meant by Open Source ?
A In general, open source refers to any program whose source code is made available for use or modification as users or other developers see fit. Open source software is usually developed as a public collaboration and made freely available.
152 Q How do you send data from an applet to Servlet ? What are the steps involved in it ?
A You can use the java.net.URLConnection and java.net.URL classes to open a standard HTTP connection to the web server. The server then passes this information to the servlet in the normal way.
Basically, the applet pretends to be a web browser, and the servlet doesn't know the difference. As far as the servlet is concerned, the applet is just another HTTP client.
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153 Q What is polymorphism?
A It is the ability of an object to behave differently on different situations for the same message.
154 Q What is a class, member and local variable?
A Variables declared within a method are local variables. Variables declared within the class are member variables. Variables declared within the class with static modifier are class variables
155 Q How do I convert a numeric IP address like 66.29.36.130 into a hostname like www.javacertificate.net
A String hostname = InetAddress.getByName("66.29.36.130").getHostName();
156 Q What is the difference between a constructor and a method?
A A constructor is a member function of a class that is used to create objects of that class. It has the same name as the class itself, has no return type, and is invoked using the new operator. We cannot invoke a constructor directly. A method is an ordinary member function of a class. It has its own name, a return type (which may be void), and is invoked using the dot operator.
157 Q What are the different inner classes types?
A There are mainly four types available. They are Member classes, Nested top-level classes, Local classes, Anonymous classes
158 Q What is Nested top-level classes?
A A class declared within a class with static modifier is called nested top level class. Any class outside the declaring class can access the nested top level class with the declaring class dot nested top level class. Top-level inner classes have access to static variables only .
159 Q What is Member classes?
A A class declared inside a class without static modifier is called member class. Member classes are just like any other member methods or member variables.
160 Q What is Local inner classes ?
A Local inner classes are class declared inside a block of code. They are visible only within the block of their declaration.
161 Q Can a top level class be private or protected?
A No. A top level class can not be private or protected. It can have either "public" or no modifier.
162 Q How will you invoke any external process in Java?
A By using Runtime.getRuntime().exec(….)
163 Q What is a package?
A To group set of classes into a single unit is known as packaging. Packages provides wide namespace visibility.
164 Q What is the use of assert keyword
A Assert keyword validates certain expressions. It replaces the if block effectively and throws an AssertionError on failure. The assert keyword should be used only for critical arguments (means without that the method does nothing).
165 Q What is composition?
A Holding the reference of the other class within some other class is known as composition.
166 Q What is aggregation?
A It is a special type of composition. If you expose all the methods of a composite class and route the method call to the composite method through its reference, then it is called aggregation
167 Q What are the methods in Object?
A clone, equals, wait, finalize, getClass, hashCode, notify, notifyAll, toString
168 Q What is the relationship between synchronized and volatile keyword?
A The JVM is guaranteed to treat reads and writes of data of 32 bits or less as atomic. For long or double variable, programmers should take care in multi-threading environment. Either put these variables in a synchronized method or block, or declare them volatile.
169 Q What factors are used to decide using synchronized or volatile?
A You can make a variable synchronized or volatile under the following cases: 1) if you are not updating many variables often in a multithread environment, consider using volatile. 2) If you are updating many variables, consider using synchronized, because using volatile might be slower.
170 Q What are the drawbacks of inheritance?
A Since inheritance inherits everything from the super class and interface, it may make the subclass too clustering and sometimes error-prone when dynamic overriding or dynamic overloading in some situation. In addition, the inheritance may make peers hardly understand your code if they don't know how your super-class acts.
171 Q What is the difference between static synchronized and synchronized methods?
A Both are synchronized methods. One is instance method, the other is class method. Method with static modifier is a class method. That means the method belongs to class itself and can be accessed directly with class name and is also called Singleton design. The method without static modifier is an instance method. That means the instance method belongs to its object. Every instance of the class gets its own copy of its instance method.
172 Q What is the purpose of the Runtime class?
A The purpose of the Runtime class is to provide access to the Java runtime system.
173 Q What is the purpose of the System class?
A The purpose of the System class is to provide access to system resources.
174 Q Does the code in finally block get executed if there is an exception and a return statement in a catch block?
A If an exception occurs and there is a return statement in catch block, the finally block is still executed. The finally block will not be executed when the System.exit(1) statement is executed earlier or the system shut down earlier or the memory is used up earlier before the thread goes to finally block.
175 Q Considering notepad/IE or any other thing as process, What will happen if you start notepad or IE 3 times? Where 3 processes are started or 3 threads are started ?
A 3 processes will start.
176 Q What are the restrictions placed on the values of each case of a switch statement?
A At compile time, each case values of switch statement must evaluate to a an int value.
177 Q If aaaa is an array then why aaaa.length why not aaaa.length()?
A Because length is a property not a method.
178 Q What is dynamic typing?
A Dynamic typing means type errors are detected at run time by checking the actual data types of the values against the required data types
179 Q What is static typig?
A Static typing means type errors are detected at compile time by checking the inferred data type is a subtype of the required type
180 Q What is HashMap and Map?
A Map is Interface and HashMap is class that implements that.
181 Q What is an Object and how do you allocate memory to it?
A Object is an instance of a class and it is a software unit that combines a structured set of data with a set of operations for inspecting and manipulating that data. When an object is created using new operator, memory is allocated to it.
182 Q What is UNICODE?
A Unicode is used for internal representation of characters and strings and it uses 16 bits to represent each other.
183 Q What is adapter class?
A An adapter class provides a default implementation of all methods in an event listener interface. Adapter classes are useful when you want to process only some of the events that are handled by a particular event listener interface. You can define a new class by extending one of the adapter classes and implementing only those events relevant to us.
184 Q What is a stream and what are the types of Streams and classes of the Streams?
A A stream is a flow of data from one direction to another. . There are two types of Streams Byte Streams: Provide a convenient means for handling input and output of bytes. And Character Streams: Provide a convenient means for handling input & output of characters.
185 Q What is the difference between TCP/IP and UDP?
A TCP/IP is a two-way communication between the client and the server and it is a reliable and there is a confirmation regarding reaching the message to the destination. UDP is a one-way communication only between the client and the server and it is not a reliable and there is no confirmation regarding reaching the message to the destination.
186 Q What is Inter-Thread communication?.
A Exchange of information between two threads.
187 Q What is a policy?.
A It's an abstract class for representing the system security policy for a Java application environment (specifying which permissions are available for code from various sources). Java security properties file resides in <JAVA-HOME>/lib/security/java.security directory.
188 Q What is a thread group?
A A thread group is a data structure that controls the state of collection of thread as a whole managed by the particular runtime environment.
189 Q Why is UTFDataFormatException thrown by DataOutputStream.writeUTF() when serializing a String?
A DataOutputStream.writeUTF() does not support writing out strings larger than 64K. The first two bytes of a UTF string in the stream are the length of the string. If a java.lang.String is larger than 64K, it needs to be stored in the stream by an alternative method rather than depending on the default method of storing a String in the stream, writeUTF.
190 Q Why is OutOfMemoryError thrown after writing a large number of objects into an ObjectOutputStream?
A The ObjectOutputStream maintains a table mapping objects written into the stream to a handle. The first time an object is written to a stream, its contents are written into the stream; subsequent writes of the object result in a handle to the object being written into the stream. This table maintains references to objects that might otherwise be unreachable by an application, thus, resulting in an unexpected situation of running out of memory. A call to the ObjectOutputStream.reset() method resets the object/handle table to its initial state, allowing all previously written objects to be eligible for garbage collection.
191 Q How can I get the serialVersionUID of a class?
A By running the serialver tool with the name of the class as the command line argumet, as shown in the example that follows: serialver java.lang.String
192 Q What is serialVersionUID ?
A The serialVersionUID is a universal version identifier for a Serializable class. Deserialization uses this identifier number to ensure that a loaded class corresponds to a serialized object.
193 Q What is abstraction?
A An abstraction is an idea, concept, or word which defines the phenomena which make up the concrete events or things which the abstraction refers to, the referents.
194 Q What is encapsulation?
A Encapsulation describes the ability of an object to hide its data and methods from the rest of the world
195 Q What is inheritance?
A Inheritance is the ability to create new classes based on existing classes. It is useful to reuse existing code.