Are you planning to attend an interview for a Java role but confused on how to crack that interview and also what would be the most probable Advanced Java Interview Questions that the interviewer may ask? Well, you have reached the right place. Tekslate has collected the most frequently asked Java Advanced Interview Questions which are often asked in multiple interviews.
Ans. Java object is an object that is provided by the execution of an application. When an application is compiled an object of that application is being made. Java application on the other hand is a program that is being written in Java and being read by the Java virtual machine.
Ans. The purpose of garbage collection is to identify and discard objects that are no longer needed by a program so that their resources can be reclaimed and reused. A Java object is subject to garbage collection when it becomes unreachable to the program in which it is used.
Ans:
Advantages of Java Sockets:
Sockets are flexible and easy to implement for general communications. - Sockets cause low network traffic unlike HTML forms and CGI scripts that generate and transfer whole web pages for each new request.
Disadvantages of Java Sockets:
Socket-based communications allow only to send packets of raw data between applications. - Both the client side and server side have to provide mechanisms to make the data useful in any way.
Ans. With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchronization, it is possible for one thread to modify a shared variable while another thread is in the process of using or updating the same shared variable. This usually leads to significant errors.
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Ans. The thread could be implemented by using a runnable interface or by inheriting from the Thread class. The former is more advantageous, 'cause when you are going for multiple inheritances. the only interface that can help.
Ans. An immutable class is a class that once created, contents can not be changed. - Immutable objects are objects whose state can not be changed once constructed. - Since the state of the immutable objects can not be changed once they are created they are automatically synchronized/thread-safe. - Immutable objects are automatically thread-safe since the state of the immutable objects can not be changed once they are created - All wrapper classes in Java. lang are immutable, i.e. String, Integer, Boolean, Character, Byte, Short, Long, Float, Double, BigDecimal, BigInteger
Ans. Pass By Reference means passing the address itself rather than passing the value. Pass by Value means passing a copy of the value to be passed.
Ans. ArrayList is not thread-safe whereas Vector is thread-safe. - In the Vector class each method is surrounded by a synchronized block and thus making Vector class thread-safe. - Both the ArrayList and Vector hold onto their contents using an Array. - When an element is inserted into an ArrayList or a Vector, the object will need to expand its internal array if it runs out of room. - A Vector defaults to doubling the size of its array, while the ArrayList increases its array size by 50 percent.
Ans. The map is Interface and Hashmap is the class that implements that.
Ans. The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls. (HashMap allows null values as key and value whereas Hashtable doesn't allow). HashMap does not guarantee that the order of the map will remain constant over time. HashMap is unsynchronized and Hashtable is synchronized.
Ans. 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. 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.
Ans. Some of the collection classes provide traversal of their contents via java. util.Iterator interface. This interface allows you to walk through a collection of objects, operating on each object in turn. Remember when using Iterators that they contain a snapshot of the collection at the time the Iterator was obtained; generally, it is not advisable to modify the collection itself while traversing an Iterator.
Ans.
Multitasking includes two ways of representation:
1. Preemptive multitasking: where the system terminates the idle process without asking the user.
For example Unix/Linux, Windows NT
2. Non-preemptive multitasking: where the system asks the process to give control to other processes for execution.
For example Windows 3.1 and Mac OS 9.
Multithreading:
1. Multithreaded programs are program that extends the functionality of multitasking by dividing the program in the thread and then executing the task as individual threads.
2. Threads run in a different area and each thread utilizes some amount of CPU and memory for execution.
Ans. An abstract class can have instance methods that implement a default behaviour. An Interface can only declare constants and instance methods, but cannot implement default behaviour and all methods are implicitly abstract. An interface has all public members and no implementation. An abstract class is a class that may have the usual flavours of class members (private, protected, etc.), but has some abstract methods.
Ans. With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchronization, it is possible for one thread to modify a shared object while another thread is in the process of using or updating that object's value. This often leads to significant errors.
Ans. Vector is synchronized whereas ArrayList is not.
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Ans. Multiple processes are the way to provide a multitasking environment to the user to allow him to switch over to different programs quickly. In these processes, it consists of a complete set of its own variables and data. Multiple threads share the same variable and the same data. Multiple processes are safe to use but multiple threads are riskier in the sense that they share the same data.
Multiple processes have much more overhead but multiple threads have less overhead and individual threads can be stand-alone if other threads are destroyed.
In multiple processes, intercommunication is slower and more restrictive, whereas communication between threads is faster.
Ans. Abstract class must be extended/subclassed (to be useful). It serves as a template. A class that is abstract may not be instantiated (ie, you may not call its constructor), an abstract class may contain static data. Any class with an abstract method is automatically abstract itself and must be declared as such. A class may be declared abstract even if it has no abstract methods. This prevents it from being instantiated.
Ans. Static means one per class, not one for each object no matter how many instances of a class might exist. This means that you can use them without creating an instance of a class. Static methods are implicitly final, because overriding is done based on the type of the object, and static methods are attached to a class, not an object. A static method in a superclass can be shadowed by another static method in a subclass, as long as the original method was not declared final. However, you can't override a static method with a nonstatic method. In other words, you can't change a static method into an instance method in a subclass.
Ans. A final class can't be extended ie., the final class may not be subclassed. A final method can't be overridden when its class is inherited. You can't change the value of a final variable (is a constant).
Ans. The program compiles properly but at runtime, it will give a "Main method not public." message.
Q22) What if the static modifier is removed from the signature of the main method?
Ans: Program compiles. But at runtime throws an error "NoSuchMethodError".
Ans. The program compiles and runs properly.
Ans. The program compiles but throws a runtime error "NoSuchMethodError".
Ans. The String array is empty. It does not have any element. This is unlike C/C++ where the first element by default is the program name.
Ans. It is empty. But not null.
Ans. Print args. length. It will print 0. That means it is empty. But if it would have been null then it would have thrown a NullPointerException on attempting to print args. length.
Ans. Yes, it is possible. While starting the application we mention the class name to be run. The JVM will look for the Main method only in the class whose name you have mentioned. Hence there is no conflict amongst the multiple classes having the main method.
Ans. No, the program fails to compile. The compiler says that the main method is already defined in the class.
Ans. No. It is by default loaded internally by the JVM.
Ans. One can import the same package or same class multiple times. Neither compiler nor JVM complains abt it. And the JVM will internally load the class only once no matter how many times you import the same class.
Ans. A checked exception is some subclass of Exception (or Exception itself), excluding class RuntimeException and its subclasses. Making an exception checked forces client programmers to deal with the possibility that the exception will be thrown. eg, IOException thrown by java.io.FileInputStream's read() method Unchecked exceptions is RuntimeException and any of its subclasses. Class Error and its subclasses also are unchecked. With an unchecked exception, however, the compiler doesn't force client programmers either to catch the exception or declare it in a throws clause. In fact, client programmers may not even know that the exception could be thrown. eg, StringIndexOutOfBoundsException thrown by String's charAt() methodChecked exceptions must be caught at compile time. Runtime exceptions do not need to be. Errors often cannot be.
Ans. When a class defines a method using the same name, return type, and arguments as a method in its superclass, the method in the class overrides the method in the superclass. When the method is invoked for an object of the class, it is the new definition of the method that is called and not the method definition from the superclass. Methods may be overridden to be more public, not more private.
Ans. Nested top-level classes, Member classes, Local classes, Anonymous classes Nested top-level classes- If you declare a class within a class and specify the static modifier, the compiler treats the class just like any other top-level class. Any class outside the declaring class accesses the nested class with the declaring class name acting similarly to a package. eg, outer. inner.
Top-level inner classes implicitly have access only to static variables. There can also be inner interfaces. All of these are of the nested top-level variety.
Member classes - Member inner classes are just like other member methods and member variables and access to the member class is restricted, just like methods and variables. This means a public member class acts similarly to a nested top-level class. The primary difference between member classes and nested top-level classes is that member classes have access to the specific instance of the enclosing class.
Local classes - Local classes are like local variables, specific to a block of code. Their visibility is only within the block of their declaration. In order for the class to be useful beyond the declaration block, it would need to implement a more publicly available interface. Because local classes are not members, the modifiers public, protected, private, and static are not usable.
Anonymous classes - Anonymous inner classes extend local inner classes one level further. As anonymous classes have no name, you cannot provide a constructor.
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Ans. Yes, the imports are checked for semantic validity at compile time. The code containing the above line of import will not compile. It will throw an error saying, can not resolve symbol: class ABCD location: package io import java.io.ABCD;
Ans. No, you will have to import the subpackages explicitly. Importing com.MyTest.* will import classes in the package MyTest only. It will not import any class in any of its subpackage.
Ans. In a declaration, we just mention the type of the variable and its name. We do not initialize it. But defining means declaration + initialization. e.g String s; is just a declaration while String s = new String ("abcd"); Or String s = "abcd"; are both definitions.
Ans. null unless we define it explicitly.
Ans. No. A top-level class can not be private or protected. It can have either a "public" or no modifier. If it does not have a modifier it is supposed to have default access. If a top-level class is declared as private the compiler will complain that the "modifier private is not allowed here". This means that a top-level class can not be private. The same is the case with protected.
Ans. In Java, the arguments are always passed by value.
Ans. Primitive data types are passed by value.
Ans. Java only supports pass-by values. With objects, the object reference itself is passed by value and so both the original reference and parameter copy both refer to the same object.
Ans. Serialization is a mechanism by which you can save the state of an object by converting it to a byte stream.
Ans. The class whose instances are to be serialized should implement an interface Serializable. Then you pass the instance to the ObjectOutputStream which is connected to a fileoutputstream. This will save the object to a file.
Ans. The following table lists the differences between compile-time and runtime polymorphism:
|
Compile-time polymorphism |
Runtime polymorphism |
|
Also known as static polymorphism |
Also known as dynamic polymorphism |
|
Achieved through method overloading (Allowing a class to have multiple methods with the same name but different number/type of arguments) |
Achieved through method overriding (Allowing a subclass to have a method with the same name and type signature as a superclass) |
|
The call to the overloaded method is resolved at compile-time |
The call to the overridden method is resolved at runtime
|
Ans. Encapsulation simply means keeping the data of a class hidden from the outside world. This is achieved by creating a class that has private fields and public getter/setter methods.
The following class is an example of encapsulation:
public class House { private int numRooms; private String location; public int getNumRooms() { return numRooms; } public void setNumRooms(int numRooms) { this.numRooms = numRooms; } public String getLocation() { return location; } public void setLocation(String location) { this.location = location; }}
This code defines a class called House. It has two fields called numRooms and location, both are private and so cannot be accessed directly outside the class. Both fields have getter and setter methods which can be used by code outside the class.
Ans. Other programming languages like C, C++ are compiled into platform specific binary files with .exe extension. So, the binary file for one operating system does not work on other operating systems. Unlike these languages, Java gets compiled into bytecode. Bytecode is executed within a Java Virtual Machine (JVM) and the same bytecode works on all operating systems. So, Java is platform independent.
Ans. A literal is nothing but a constant value that can be assigned to a variable. Literals can be classified according to their data type. The following are some examples of literals:
• 10,20 – These are all Integer literals since they correspond to Integer values
• 26.45,31.89 – These are all floating-point literals since they represent floating-point (decimal) values.
• ‘A’, ‘b’, ‘%’ – These are character literals since they hold an individual character. Character literals need to be enclosed in single quotes
• “Java”, “Test” – These are String literals since they hold String values. String literals need to be enclosed in double quotes
Ans. A static method can be invoked directly via the class name and does not need an object of the class to be created. The Java main method is invoked by the JVM as soon as a Java program is executed. Hence, the JVM should be able to invoke the main method even before creating an object of the class. Hence the main method is static.
Ans. The following table lists the differences between the abstract and final keywords:
|
Abstract |
final |
|
Can be specified with a class or method. Cannot be specified with a field |
Can be specified with a field, method or class. |
|
When specified with a method, indicates that the method is abstract. So, a subclass needs to provide an implementation for the abstract method or be declared as abstract itself |
When specified with a method, indicates that the method cannot be overridden. So, if a subclass tries to provide an implementation for the final method a compilation error occurs |
|
When specified with a class, indicates that the class is abstract. So, a subclass needs to provide an implementation for the abstract methods within the class or must be declared as abstract itself |
When specified with a class, indicates that the class cannot be extended. So, if some code tries to create a subclass of a final class, a compilation error occurs. |
|
An abstract class cannot be instantiated |
A final class can be instantiated |
short s = 10; // 1
s = s + 10; // 2
Ans. The code above is not valid and causes a compilation error at Line 2. This is because Line 2 adds the value 10 to the short variable s. By default, the literal 10 is treated to be of type int and so Line 2 produces a result of type int but tries to assign it to a variable of type short. This code can be fixed either by changing the data type of s to int or by explicitly casting the result of s+10 to int as follows:
s = (short)(s + 10);
Ans. A two-dimensional array is nothing but an array having two dimensions. In other words, it is an array of arrays, that is each array within a two-dimensional array is an array itself. The following are some examples of two-dimensional arrays:
int array1[ ][ ] = new int[3][6];
double array2[ ][ ] = new double[2][5];
array1 is a two-dimensional integer array having dimensions 3,6. array2 is a two-dimensional double array having dimensions 2,5.
Ans. Both the prefix and postfix form of the increment operator increments its operand by 1. However, when used in a statement, the prefix operator first performs the increment operation and then the assignment while the postfix operator first performs the assignment and then the increment operation.
The following code sample demonstrates this:
int n1=10;
int n2=++n1;
The code above first causes n1 to be incremented and then assigned to n2, so n2 has the value 11.
Now consider the following code:
int n1=10;
int n2=n1++;
The code above first causes n2 to be assigned with the value of n1 and then n1 is incremented, so n2 has the value 10.
int num1 = 17;
int num2 = 25;
int num3 = num1 & num2;
System.out.println(num3);
Ans. The code above prints the following output:
17
The code above uses the bitwise AND (&) operator on num1 and num2. The & operator performs a bitwise AND operation on the binary representations of its operands. So, it returns 1 only if both the bits are 1. So, the above operation can be represented in binary form as below:
00010001 (17)
00011001 (25)
--------------
00010001 (17)
Ans. Both the break and continue statements are control statements that can be used to transfer control to a different part of a program. Both can be used within a loop. When a break statement is encountered within a loop, it causes the loop to be terminated and control is transferred to the first statement outside the loop. When a continue statement is encountered within a loop, it causes the current iteration of the loop to be skipped and control is transferred to the top of the loop. The loop then continues for the next iteration.
String taskStatus = "IN_PROGRESS";
switch (taskStatus) {
case "ASSIGNED":
System.out.println("Task is assigned");
case "IN_PROGRESS":
System.out.println("Task is in progress");
case "COMPLETED":
System.out.println("Task is completed");
}
Ans. The code above uses a switch statement which matches the value of the taskStatus variable with the value in each case statement. Since taskStatus matches the second case statement, it executes the code in this case statement and the code in the subsequent case statements. So, the code above prints the following output:
Task is in progress
Task is completed
Ans. The new keyword is used along with a constructor to create a new Java object. It has the following syntax:
Class1 ob = new Class1();
The line above creates an object called ob of the type Class1. The new keyword is followed by the class name that is Class1. This allocates memory for the object ob, that is the instance fields of ob.
Integer num = Integer.valueOf("100");
Ans. Yes, the code above is valid. It creates an Integer object corresponding to the value 100. Integer is a wrapper class corresponding to the int data type. Integer class has a static method called valueOf that can accept as parameter a String and return the Integer value corresponding to the String. In case an invalid Integer is specified within the valueOf method, it throws an Exception.
Ans. Java supports the following types of inheritance:
public class House {}public class RowHouse extends House {public static void main(String[] args) {House h1 = new RowHouse();//1RowHouse h2 = new House(); //2}}Ans. This code creates a class called House which has a subclass called RowHouse. Line 1 declares a House variable h1 and assigns it a RowHouse object. This is valid since a subclass object can be assigned to a superclass variable. Line 2 declares a RowHouse object h2 and assigns it a House object. This is invalid and causes a compilation error, since a superclass object cannot be assigned to a variable of a subclass type. So, in order to fix this error, Line 2 needs to be removed from the code.
Ans. The following table lists the differences between static and default interface methods:
|
Default |
Static |
|
Has the keyword ‘default’ specified in the method declaration |
Has the keyword ‘static’ specified in the method declaration |
|
Can be overridden by an implementing class |
Cannot be overridden by an implementing class |
|
Can be invoked via an object of the implementing class |
Can only be invoked via the interface name |
public interface First {public void firstMethod();}public interface Second {public void secondMethod();}public class House implements First,Second{public void firstMethod() {System.out.println("First Method");}}Ans. The code above will cause a compilation error in the class House. Since the House class implements the interfaces First and Second, it needs to provide code for all the methods in both interfaces. However, in this case, the House class only has code for firstMethod from the interface First. In order to fix the code, it needs to include code for secondMethod from the interface Second. So, the House class needs to be fixed as follows:
public class House implements First,Second{public void firstMethod() {System.out.println("First Method");}public void secondMethod() {//code here}}Ans. Java’s access specifiers help to control where an instance field/method of a class is accessible. Java has the following access specifiers:
Ans. Both the add and the offer methods can be used to add an element to a Queue. However, there are some differences between the two as listed in the table below:
|
add |
offer |
|
Method on the java.util.Collection interface |
Method on the java.util.Queue interface |
|
Throws an exception if it is not possible to add the specified element into the Queue |
Returns false if it is not possible to add the specified element into the Queue |
Ans. The java.util.Collections class has a method called swap. It can be used to swap two elements in a List. The following code demonstrates this:
List<String> animals = Arrays.asList("Lion","Elephant","Tiger","Zebra"); Collections.swap(animals, 1, 2); //swaps elements at position 1,2
This code causes the elements at position 1,2 to be swapped, so “Tiger” is moved to position 1 and “Elephant” is moved to position 2
Ans. The following table lists the differences between a checked an unchecked exception:
|
Checked Exception |
Unchecked Exception |
|
Checked by the compiler. So, if you write code that can throw a checked exception, the compiler causes an error unless the exception is handled or declared in the throws clause |
Is not checked by the compiler. So, if you write code that can throw an unchecked exception, the compiler does not cause an error |
|
All subclasses of java.lang.Exception except java.lang.RuntimeException are unchecked exceptions |
All subclasses of java.lang.RuntimeException are unchecked exceptions |
Ans. The throw keyword can be specified within any block of code. It can be used to explicitly throw an exception (both in-built as well as custom exception). It is followed by an Exception object. The throws keyword is specified in a method declaration. It indicates that the method is capable of throwing an exception but does not handle it and that the code that invokes the method needs to handle the exception. It is followed by the exception class name.
Ans. The String class has several methods that can be used to check if a String contains another String. These are as follows:
Ans. The capacity method on the StringBuffer class returns the capacity of the StringBuffer that is the number of characters that the StringBuffer is capable of holding. The length method on the StringBuffer class returns the actual length of the StringBuffer, that is the actual number of characters in the StringBuffer.
File file = new File("C:/test");
file.mkdir();
Ans. The code above creates a File object corresponding to the C:/test path and then invokes the mkdir() method on this File object. This method creates a directory corresponding to the specified path. So, once the code above executes, a directory called “test” will be created in c:/
Ans. The file class has a method called delete. This can be used to delete a file from the file system. The following code demonstrates this:
File file1=new File("C:/test.txt");file1.delete();
This code first creates a file object corresponding to “C:/test.txt” and then invokes the delete method on this file object. This causes the “C:/test.txt” file to be deleted from the file system.
public class MyThread extends Thread{ public void run() { System.out.println(Thread.currentThread().getName()); } public static void main(String[] args) { MyThread t = new MyThread(); t.start(); }}
Ans. The code above creates a Thread class called MyThread. The run method invokes the Thread.currentThread().getName()which prints the name of the current thread. So in order for the code to print “MyThread”, the name of the thread should be set to “MyThread”. For this you can do either of the following:
Map<Character,String> animals = Map.of('A',"Lion",'B',"Elephant",'C',"Tiger");
String animal = animals.getOrDefault('D', "Giraffe");
Ans. The code above uses the getOrDefault method on the animals map. This returns the value corresponding to the specified key if the key is present in the Map or the specified default value. Since there is no value corresponding to the key ‘D’, it returns the value “Giraffe”. So, the following is the output of the code listed above:
Giraffe
Ans. Java supports a process known as garbage collection whereby the memory allocated to objects that are no longer used is freed up automatically. When a Java program is executed, the JVM allocates memory to the objects/variables created in the code. For example, the JVM may allocate memory to some objects declared within a method. Once the method terminates, those objects are no longer required. So, the garbage collector frees up the memory allocated to the objects within the method. The JVM periodically runs the garbage collector in the background.
Ans. The serializable interface is an empty interface, it does not contain any methods. So we do not implement any methods.
Ans. Yes, it is possible to have control over the serialization process. The class should implement the Externalizable interface. This interface contains two methods namely readExternal and writeExternal. You should implement these methods and write the logic for customizing the serialization process.
Ans. Whenever an object is to be sent over the network, objects need to be serialized. Moreover, if the state of an object is to be saved, objects need to be serialized.
Ans. Externalizable is an interface that contains two methods readExternal and writeExternal. These methods give you control over the serialization mechanism. Thus if your class implements this interface, you can customize the serialization process by implementing these methods.
Ans. The serialization mechanism generates an object graph for serialization. Thus it determines whether the included object references are serializable or not. This is a recursive process. Thus when an object is serialized, all the included objects are also serialized along with the original object.
Checkout: [Exception Handling in java]
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 NotSerializableException.
Ans. There are three exceptions in which serialization does not necessarily read and write to the stream. These are 1. Serialization ignores static fields because they are not part of a particular state. 2. Base class fields are only handled if the base class itself is serializable. 3. Transient fields
Ans. An unhandled exception moves up the method stack in search of a matching When an exception is thrown from a code that is wrapped in a try block followed by one or more catch blocks, a search is made for a matching catch block. If a matching type is found then that block will be invoked. If a matching type is not found then the exception moves up the method stack and reaches the caller method. The same procedure is repeated if the caller method is included in a try-catch block. This process continues until a catch block handling the appropriate type of exception is found. If it does not find such a block then finally the program terminates.
Ans. There are two ways to handle exceptions, 1. By wrapping the desired code in a try block followed by a catch block to catch the exceptions. and 2. List the desired exceptions in the throws clause of the method and let the caller of the method handle those exceptions.
Ans. 1> try catch block and 2> specifying the candidate exceptions in the throws clause?
Ans. In the first approach as a programmer of the method, you urself are dealing with the exception. This is fine if you are in the best position to decide what should be done in case of an exception. Whereas if it is not the responsibility of the method to deal with its own exceptions, then do not use this approach. In this case, use the second approach. In the second approach, we are forcing the caller of the method to catch the exceptions, that the method is likely to throw. This is often the approach library creators use. They list the exception in the throws clause and we must catch them. You will find the same approach throughout the java libraries we use.
Ans. It is not necessary that each try block must be followed by a catch block. It should be followed by either a catch block OR a finally block. And whatever exceptions are likely to be thrown should be declared in the throws clause of the method.
Ans. Yes even if you write return as the last statement in the try block and no exception occurs, the finally block will execute. The final block will execute and then the control return.
Ans. No in this case the finally block will not execute because when you say System.exit (0); the control immediately goes out of the program, and thus finally never executes.
Ans. Objects that subclass the Observable class maintain a list of observers. When an Observable object is updated it invokes the update() method of each of its observers to notify the observers that it has changed state. The Observer interface is implemented by objects that observe Observable objects.
Ans. It uses those low-order bytes of the result that can fit into the size of the type allowed by the operation.
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