Amdocs Java interview questions and answers.

Java Interview Questions and Answers covered in this post:

    What are the access modifiers and their scope?

-          There are four access modifiers which are as follows.

1.       Default

2.       Private

3.       Protected

4.       Public

-          Scope:

Access Modifier	Within Class	Within Package	Subclass in another package	Outside package
Default	Y	Y	N	N
Private	Y	N	N	N
Protected	Y	Y	Y	N
Public	Y	Y	Y	Y

 

    Can you explain the public static void main(String[] args) Method?

-          Main method is a starting point of java program execution.

-          Main method components:

1.       public: Access modifier of main() is public so the method can be invoked from outside. This method must be public so the java runtime can invoke it. If you make the main method private or protected then you won’t get a compile-time error but a runtime exception.

2.       static: We are aware of the use of static. Without creating an object we can invoke the method of the class. So java runtime can invoke the main method without instantiating the class.

3.       void: return type of the main method is void cause the main method doesn’t return anything.

4.       main: This is the fixed name of the main method. You cannot change the name of the main method otherwise you will get a runtime exception.

5.       (String args[]) : These are the command line argument. We can pass these arguments to the java program.

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    What is the ternary operator in java?

-          Ternary operator lets you write if statement in one line.

-          Ternary operator evaluates either true or false.

-        There are three parts of the ternary operator. The first part is expression end with a question mark and two options separated with a colon.

-          Variable = condition ? expression1: expression2. 

package simplifiedjava.crackedInterview;  public class TernaryOperatorDemo {        public static void main(String[] args) {             int num=20;             String typeNum = (num > 0) ? "Positive" : "Negative";             System.out.println(typeNum);              } } Output : Positive

    Can you specify the scenario where you can implement String, StringBuffer and StringBuilder?

-    String: When content is fixed and never change throughout life then String is a good option.

-        StringBuffer: When content is not fixed and the requirement is thread-safety then go for StringBuffer.

-     StringBuilder: When content is not fixed and content would be changing multiple times and no need for thread safety then go for StringBuilder.

    How to achieve Thread safety in java?

-    Thread safety means multiple threads sharing the same resource without producing an unpredictable result.

-   We can achieve thread-safety in multiple ways. Please refer to the below 8 options. 

1.       Synchronized Methods.

2.       Synchronized Blocks.

3.       Synchronized Collection. (Using Collections Utility Class).

4.       Concurrent Collection.

5.       Immutable Implementation.

6.       Thread-Local Field.

7.       Volatile Field.

8.       Stateless Implementation.

            Explanation in details.

1.       Synchronized Methods:

Ø  The method can be declared with a synchronized keyword.

Ø  Once you declared the method with a synchronized keyword then only one thread can execute shared resources at a time. Other threads will be blocked until the first thread finishes its execution.

Ø  synchronized method depends on either “Intrinsic Lock” or “Monitor Lock”.

Ø  When the thread calls the synchronized method, it acquires the intrinsic lock.

Ø  The Monitor is just a reference to the role that the lock performs on the associated objects.

Ø  Please refer to the below example.

package simplifiedjava.crackedInterview;  public class SynchronizedMethodDemo {        int counter = 0;       public synchronized void getCounter() {             counter =+ 1; // Similar to counter = counter +1;             System.out.println("Counter is "+ counter);       }            public static void main(String[] args) {                    new SynchronizedMethodDemo().getCounter();       } }

 

2.       Synchronized Block:

Ø  A synchronized block can be called a statement block.

Ø  Method synchronization is highly expensive so to avoid this situation we can perform synchronization at the statement level which is less expensive from a performance perspective.

Ø  A synchronized block generally declares inside a method.

Ø  Please refer to the below example.

package simplifiedjava.crackedInterview;  import java.util.ArrayList; import java.util.Arrays; import java.util.Collection; import java.util.Collections;  public class SynchronizedBlockDemo {            int counter = 0;       public void getCounter() {                synchronized(this) {                   counter =+ 1; // Similar to counter = counter +1;             }                       System.out.println("Counter is "+ counter);       }            public static void main(String[] args) {                    new SynchronizedMethodDemo().getCounter();                  } }

 

3.               Synchronized Collection:

Ø  We can create a thread-safe collection by using utility methods of the Collections utility class.

Ø  Once you create the thread-safe collection object then only one thread can execute at a time. While other threads will be blocked until the collection object is unblocked by the first thread.

Ø  Please refer to the below example. 

package simplifiedjava.crackedInterview;  import java.util.Arrays; import java.util.Collections; import java.util.List;  public class SynchronizedCollectionDemo {        public static void main(String[] args) {             Thread t1 = new Thread(new MyClass1());             Thread t2 = new Thread(new MyClass2());             t1.start();             t2.start();       } }  class MyClass1 implements Runnable{        @Override       public void run() {                       List<Integer> list1 = Arrays.asList(10,20,30,40,50);             List<Integer> synchronizedList1 = Collections.synchronizedList(list1);                  for(Integer i : list1) {                   System.out.println(i);             }       }     }  class MyClass2 implements Runnable{        @Override       public void run() {             List<Integer> list2 = Arrays.asList(60,70,80,90,100);             List<Integer> synchronizedList2 = Collections.synchronizedList(list2);             for(Integer i : list2) {                   System.out.println(i);             }       }     }

 

4.       Concurrent Collection:

Ø  Java provides java.util.concurrent package for thread safety.

Ø  Java.util.concurrent package has some thread safe classes like ConcurrentHashMap, CopyOnWriteArrayList, CopyOnWriteArraySet.

Ø  Please refer to the below example.               

package simplifiedjava.crackedInterview;  import java.util.Map; import java.util.concurrent.ConcurrentHashMap;  public class ConcurrentCollectionDemo {        public static void main(String[] args) {             Map<String,Integer> concurrentMap = new ConcurrentHashMap<>();             concurrentMap.put("one", 1);             concurrentMap.put("two", 2);             concurrentMap.put("three", 3);       } }

 

5.       Immutable Implementation:

Ø  Immutable means its internal states can’t be changed once it is created.

Ø  If we need to share the state between different threads, we can create thread-safe classes by making them immutable classes.

Ø  If the state will be fixed forever then immutable objects can be used thread-safely.

Ø  Please refer to the below example.

package simplifiedjava.crackedInterview;  public final class ImmutableClassDemo {        private final String empCode="E001";        public String getEmpCode() {             return empCode;       } }

 

6.       Thread-Local Field:

Ø  We can easily create classes whose fields are thread-local by simply defining private fields in the Thread class.

Ø  We can create thread-safe classes that don’t share states between threads by making their fields thread-local.

Ø  Please refer to the below example.

package simplifiedjava.crackedInterview;  import java.util.Arrays; import java.util.List;  public class ThreadLocalDemo1 extends Thread {        private final List<Integer> numList = Arrays.asList(10,20,30,40,50);            @Override       public void run() {             for(int n : numList) {                   System.out.println(n);             }       } }  package simplifiedjava.crackedInterview;  import java.util.Arrays; import java.util.List;  public class ThreadLocalDemo2 extends Thread{        private final List<String> wordList = Arrays.asList("Hi","Hello","GM","GA","GN");            @Override       public void run() {             for(String str : wordList) {                   System.out.println(str);             }       } }

 

7.       Volatile Field:

Ø  If you declare a variable as volatile, JVM will store those variables on main memory.

Ø  We can make sure every time JVM reads the value from the main memory.

Ø  Every time JVM writes the value it will write on the main memory.

Ø  Volatile keyword ensures that variable will be visible to all thread and thread can read the value from main memory.

Ø  Please refer to the below example.

package simplifiedjava.crackedInterview;  public class ThreadLocalVariableDemo {        private volatile int empId;       private String name;               public static void main(String[] args) {                  } }

 

8.       Stateless Implementation:

Ø  Stateless implementation is the simplest way to achieve thread safety.

Ø  Stateless implementation is nothing but function implementation which produces the same result every time. There is no change to change the state by any thread.

Ø  Please refer to the below example.

package simplifiedjava.crackedInterview;  public class StateLessImplementationDemo {        public static void main(String[] args) {                        int i = 10;             int square = i * i ;             System.out.println(square);       } }

    What is deadlock? How to avoid a situation of dead lock?

-   Deadlocks are a set of blocked processes, each holding a resource and waiting to acquire a resource held by another process.

-   We can avoid the deadlock: Do not grant a resource request if this allocation has the potential to lead to deadlock.

    What is NotSerializableException? When it can be thrown?

-   The NotSerializableException is thrown when attempting to serialize or deserialize an object that does not implement the java. io.Serializable interface.

-      Please refer to the below example.

package simplifiedjava.crackedInterview;  import java.io.Serializable;  public class Parent{        public int i;            public Parent() {             System.out.println("Parent Class no-arg constructor called.");       }            public Parent(int i) {             this.i = i;       }                        public void check(){             System.out.println("Parent Class check() method");                }     }  package simplifiedjava.crackedInterview;  import java.io.NotSerializableException; import java.io.ObjectInputStream; import java.io.ObjectOutputStream;  public class Child extends Parent{            int j;            public Child() {                  }       public Child(int i,int j) {             super(i);             this.j = j;       }            public void check(){             System.out.println("Addition in Child Class "+ (i + j));                }                 }  package simplifiedjava.crackedInterview;  import java.io.File; import java.io.FileInputStream; import java.io.FileOutputStream; import java.io.IOException; import java.io.ObjectInputStream; import java.io.ObjectOutputStream;  public class SerializationInHeritanceDemo {        public static void main(String[] args) throws IOException, ClassNotFoundException {                        Child c1 = new Child(10,20);             System.out.println("Values Before Serialization : " + c1.i +" and "+ c1.j);              FileOutputStream fos = new FileOutputStream(new File("abc.ser"));             ObjectOutputStream oos = new ObjectOutputStream(fos);             oos.writeObject(c1);                        FileInputStream fis = new FileInputStream("abc.ser");             ObjectInputStream ois = new ObjectInputStream(fis);             Child c2 =(Child) ois.readObject();             System.out.println("Values After Deserialization : "+ c2.i +" and "+ c2.j);       } }  Output: Values Before Serialization : 10 and 20 Exception in thread "main" java.io.NotSerializableException: simplifiedjava.crackedInterview.Child        at java.io.ObjectOutputStream.writeObject0(Unknown Source)        at java.io.ObjectOutputStream.writeObject(Unknown Source)                at simplifiedjava.crackedInterview.SerializationInHeritanceDemo.main(SerializationInHeritanceDemo.java:19)

    What is HashSet? Can you explain internal working of Hashset?

-     HashSet of Collection framework represents the group of unique objects.

-    HashSet internally uses hashing technique to store the elements. It's not guaranteed in which sequence the elements will be stored or retrieved.

-  Whenever you insert an element into HashSet using add() method, it creates an entry in the HashMap object with the element you have specified as its key and constant called “PRESENT” as its value. This “PRESENT” is defined in the HashSet class as below.

-          private static final Object PRESENT = new Object();

-          Please refer the add method.

public boolean add(E e) {

            return map.put(e, PRESENT)==null;

          }

-          Please refer to the below example.

package simplifiedjava.crackedInterview; import java.util.HashSet;  public class HashSetDemo {        public static void main(String[] args) {             HashSet<String> dayList = new HashSet<String>();             dayList.add("Sat");             dayList.add("Sun");             dayList.add("Mon");             dayList.add("Tue");             dayList.add("Sun");                       System.out.println(dayList);       } } Output: [Tue, Sat, Sun, Mon]

    -     Graphical Representation of Internal working of HashSet

    What is rehashing?

-   Hashmap re-size itself by creating a new bucket array of size twice the previous size of hashmap and then start putting all old elements into the new bucket array this process is called rehashing.

    What will happen if I declared multiple abstract methods in one interface?

-    If you declare multiple abstract methods inside interface and interface is annotated with @FunctionalInterface then it will give a compile-time error.

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Upcoming questions in next post:

1. Can I declare final and abstract method in class.

2. What is static binding and dynamic binding.

3. What is the use of trim() method in String.

4. What are the multiple ways to create a thread. which one is recommendable.

5. What is the purpose of Inner classes.

6. What is the difference between static inner class and non static inner class.

7. What is load factor in collection API. What is the default size of load factor.

8. Can you explain internal working of ArrayList.

9. Explain the defined method in Stack.

10.What is entrySet?

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