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Ans. Final variable is a variable constant that cannot be changed after initialization.

Sample Code for final variable

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Q2042. Which String class does not override the equals() and hashCode() methods, inheriting them directly from class Object?

Ans. java.lang.StringBuffer.

Sample Code for StringBuffer

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Very frequently asked. Favorite question in Walk in Drive of many Indian service companies.

Q2043. What is a final method ?

Ans. Its a method which cannot be overridden. Compiler throws an error if we try to override a method which has been declared final in the parent class.

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Q2044. Which interface does java.util.Hashtable implement?

Ans. Java.util.Map

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Frequently asked.

Q2045. What is an Iterator?

Ans. Iterator is an interface that provides methods to iterate over any Collection.

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Q2046. Which interface provides the capability to store objects using a key-value pair?

Ans. java.util.map

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Very frequently asked across all types of companies.

Q2047. Difference between HashMap and Hashtable?

Ans. Hashtable is synchronized whereas HashMap is not.HashMap allows null values whereas Hashtable doesnt allow null values.

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Q2048. Does java allow overriding static methods ?

Ans. No. Static methods belong to the class and not the objects. They belong to the class and hence doesn't fit properly for the polymorphic behavior.

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Q2049. When are static variables loaded in memory ?

Ans. They are loaded at runtime when the respective Class is loaded.

Sample Code for static variable

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Q2050. Can we serialize static variables ?

Ans. No. Only Object and its members are serialized. Static variables are shared variables and doesn't correspond to a specific object.

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Q2051. What will this code print ?

String a = new String ("TEST");
String b = new String ("TEST");
if(a == b) {
System.out.println ("TRUE");
} else {
System.out.println ("FALSE");
}

Ans. FALSE. == operator compares object references, a and b are references to two different objects, hence the FALSE. .equals method is used to compare string object content.

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Q2052. There are two objects a and b with same hashcode. I am inserting these two objects inside a hashmap.

hMap.put(a,a);
hMap.put(b,b);

where a.hashCode()==b.hashCode()

Now tell me how many objects will be there inside the hashmap?

Ans. There can be two different elements with the same hashcode. When two elements have the same hashcode then Java uses the equals to further differentation. So there can be one or two objects depending on the content of the objects.

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Q2053. Difference between long.Class and Long.TYPE ?

Ans. They both represent the long primitive type. They are exactly the same.

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Q2054. Does Java provides default copy constructor ?

Ans. No

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Q2055. What are the common uses of "this" keyword in java ?

Ans. "this" keyword is a reference to the current object and can be used for following -

1. Passing itself to another method.

2. Referring to the instance variable when local variable has the same name.

3. Calling another constructor in constructor chaining.

Sample Code for this keyword

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Frequently asked to fresh graduates.

Q2056. What are the difference between Threads and Processes ?

Ans. 1. when an OS wants to start running program it creates new process means a process is a program that is currently executing and every process has at least one thread running within it.
2). A thread is a path of code execution in the program, which has its own local variables, program counter(pointer to current execution being executed) and lifetime.
3. When the JavaVirtual Machine (JavaVM, or just VM) is started by the operating system, a new process is created. Within that process, many threads can be created.
4. Consider an example : when you open Microsoft word in your OS and you check your task manger then you can see this running program as a process. now when you write something in opened word document, then it performs more than one work at same time like it checks for the correct spelling, it formats the word you enter , so within that process ( word) , due to different path execution(thread) all different works are done at same time.
5. Within a process , every thread has independent path of execution but there may be situation where two threads can interfere with each other then concurrency and deadlock come is picture.
6. like two process can communicate ( ex:u open an word document and file explorer and on word document you drag and drop another another file from file explorer), same way two threads can also communicate with each other and communication with two threads is relatively low.
7. Every thread in java is created and controlled by unique object of java.lang.Thread class.
8. prior to jdk 1.5, there were lack in support of asynchronous programming in java, so in that case it was considered that thread makes the runtime environment asynchronous and allow different task to perform concurrently.

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Q2057. How can we run a java program without making any object?

Ans. By putting code within static method. With Java 6 and earlier versions, even static block can be used.

Sample Code for static block

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Q2058. Explain multithreading in Java ?

Ans. 1. Multithreading provides better interaction with the user by distribution of task

2. Threads in Java appear to run concurrently, so it provides simulation for simultaneous activities.The processor runs each thread for a short time and switches among the threads to simulate sim-ultaneous execution (context-switching) and it make appears that each thread has its own processor.By using this feature, users can make it appear as if multiple tasks are occurring simultaneously when, in fact, each is running for only a brief time before the context is switched to the next thread.

3. We can do other things while waiting for slow I/O operations.In the java.iopackage, the class InputStreamhas a method, read(), that blocks until a byte is read from the stream or until an IOExceptionis thrown. The thread that executes this method cannot do anything elsewhile awaiting the arrival of another byte on the stream.

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Q2059. Can constructors be synchronized in Java ?

Ans. No. Java doesn't allow multi thread access to object constructors so synchronization is not even needed.

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Q2060. Can we use null as a key for a map collection ?

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)

Q2061. What is the use of hashcode in Java ?

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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Q2062. Why java doesn't support multiple Inheritence ?

Ans. class A {
void test() {
System.out.println("test() method");
}
}

class B {
void test() {
System.out.println("test() method");
}
}

Suppose if Java allows multiple inheritance like this,

class C extends A, B {
}

A and B test() methods are inheriting to C class.

So which test() method C class will take? As A & B class test() methods are different , So here we would Facing Ambiguity.

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Q2063. Why threads block or enters to waiting state on I/O?

Ans. Threads enters to waiting state or block on I/O because other threads can execute while the I/O operations are performed.

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Q2064. What are transient variables in java?

Ans. Transient variables are variable that cannot be serialized.

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Q2065. What is the difference between yield() and sleep()?

Ans. When a object invokes yield() it returns to ready state. But when an object invokes sleep() method enters to not ready state.

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Q2066. What are wrapper classes ?

Ans. They are wrappers to primitive data types. They allow us to access primitives as objects.

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Q2067. What is the difference between time slicing and preemptive scheduling ?

Ans. 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.

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Q2068. What is the initial state of a thread when it is created and started?

Ans. Ready state.

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Q2069. What one should take care of, while serializing the object?

Ans. One should make sure that all the included objects are also serializable. If any of the objects is not serializable then it throws a NotSerializable Exception.

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Q2070. What is a String Pool ?