Java 200 Interview Questions

Front 1

Why is java so popular?

Back 1

Because it is

1)platform indipendent

Build once run anywhere through the bytecode

2) it is an OO language , which is good for code maintenance

Front 2

Compare JVM , JRE and JDK

Back 2

JVM --> java virtual machine , it is the Just in Time Compiler , which run the java code

JRE --> java runtime environment (jvm+libraries)

JDK-->java development kit (jvm+jre+compiler+debug tools)

Front 3

What is the role for a ClassLoader in Java?

Back 3

When a program is executed, JVM needs to load the content of all the needed class. JVM uses a ClassLoader to find the classes in the following order

1)System Class Loader search in the classpath for the jar,ear,war

2)Extension ClassLoader search jre ext lib ...

3)BootStrap ClassLoader search in core java files

Front 4

What are Wrapper Classes?

Back 4

A wrapper class is one of the eight classes of the package java.lang to provide methods for the 8 primitives type (byte,double,int,long,float,boolean,char,short)

It gives a primitive type an object appearance

They are immutable

Front 5

Why do we need Wrapper Classes in Java?

Back 5

1)null is a possible value

2)they can be used in a Collection

3)They can be created starting from other types

es:Integer number2 = new Integer("55");//String

Front 6

What are the different ways of creating Wrapper Class Instances and the differences?

Back 6

1) using the new () operator

always a new Object

2) using the static method valueOf()

a cached value(good for memory saving)

Two wrapper objects created using new are not same object.

Integer nineA = new Integer(9); Integer nineB = new Integer(9); System.out.println(nineA == nineB);//false System.out.println(nineA.equals(nineB));//true

Two wrapper objects created using boxing are same object.

Integer nineC = 9; Integer nineD = 9; System.out.println(nineC == nineD);//true System.out.println(nineC.equals(nineD));//true

Front 7

What is Auto Boxing?

Back 7

It is the automatic conversion made by the java compiler between the primitive type and the corresponding wrapper class.

es:Integer n = 9;n++ // allowed as well

Front 8

What are the advantages of Auto Boxing?

Back 8

It makes use of valueOf() method so objects already created are reused , with new() a new object is created everytime

Front 9

Explain java primitive datatypes

Back 9

byte (+ - 127),short(+ - 32767),int(+ - 2^31),long(+ - 2^63)

float(32 bit floating point),double(64 bit floating point)

char (16 bit unicode)

boolean (1 bit)

Front 10

What is Casting?

Back 10

It is the automatic conversion from one type to another

Front 11

What is Implicit Casting?

Back 11

It is done by the compiler when smaller values are stored in larger variable types

int value = 100;

long number = value; //Implicit Casting

Front 12

What is Explicit Casting?

Back 12

It is done through code, to store larger values into smaller variables

//int x = 35.35;//COMPILER ERROR

int x = (int)35.35;//Explicit Casting

Explicit casting would cause truncation of value if the value stored is greater than the size of the variable.

int bigValue = 280; byte small = (byte) bigValue; System.out.println(small);//output 24. Only 8 bits remain.

Front 13

Are all String’s immutable?

Back 13

Yes , it is not possible to modify a String without creating a new object

String str3 = "value1"; str3.concat("value2"); System.out.println(str3); //value1

Front 14

Where are string values stored in memory?

Back 14

It depends on the way the String is created

1) String s = "value" // literal

It is stored in the string constant pool and if the compiler finds a literal it checks if exists in the constant pool , so the instruction

String s1 = "value" points to the same object

s1==s2 // true

2)String s3 = new String("value") // new operator

in this case an object is created in the heap and not reused

s3 == s1 // false

Front 15

Why should you be careful about String Concatenation(+) operator in Loops and how you solve the problem?

String s3 = "Value1"; String s2 = "Value2";

for (int i = 0; i < 100000; ++i) {

s3 = s3 + s2;

}

Back 15

More than 10000 objs are created in memory with a huge impact on the memory.

We must use StringBuffer

Front 16

What are differences between StringBuilder and StringBuffer?

Back 16

StringBuilder is not thread safe. So, it performs better in situations where thread safety is not required.

Front 17

What is a Class?

Back 17

A class is a template for creating multiple objects.

It defines the behavior that an object can exibit

Front 18

What is an Object?

Back 18

It an instance of a class

Front 19

What is state of an Object?

Back 19

Values assigned to instance variable of an object

Front 20

What is behavior of an Object?

Back 20

Are the methods supported by an object

Front 21

Explain about toString method

Back 21

it is used to write the content of an object.By default , if not overriden the Object method is called, it writes the package and hashcode

Front 22

What is the use of equals method in Java?

Back 22

It is used to compare 2 objs

Default in Object : 2 objs ref are equal if are pointing to the same object.

we can override this behavior writing our own logic

Front 23

What are the important things to consider when implementing equals method?

Back 23

@Override

public boolean equals(Object obj) {

if (this == obj) return true;

if (obj == null) return false;

if (getClass() != obj.getClass()) return false;

Client other = (Client) obj;

if (id != other.id) return false;

return true; }

Front 24

What is the hashCode method used for in Java?

Back 24

HashCode's are used in hashing to decide which group (or bucket) an object should be placed into. A group of object's might share the same hashcode. The implementation of hash code decides effectiveness of Hashing. A good hashing function evenly distributes object's into different groups (or buckets)

A good hashCode method should have the following properties

• If obj1.equals(obj2) is true, then obj1.hashCode() should be equal to obj2.hashCode()

• obj.hashCode() should return the same value when run multiple times, if values of obj used in equals() have not changed.

• If obj1.equals(obj2) is false, it is NOT required that obj1.hashCode() is not equal to obj2.hashCode(). Two unequal objects MIGHT have the same hashCode.

Front 25

What is Method Overloading?

Back 25

Defining a method in a class or in a subclass having different parameters but the same name

Front 26

What is Method Overriding?

Back 26

Defining a method in a subclass having the same name and signature of the superClass method

Front 27

Can super class reference variable can hold an object of sub class?

Back 27

Yes , for example

Animal a = new Cat();

Object obj = new Animal(); // the superclass can hold all the references

Front 28

Is Multiple Inheritance allowed in Java?

Back 28

No, but we can create an inheritance chain and we can extend one class and implement multilple interfaces

Front 29

Can you extend an interface?

Back 29

yes , an interface can extend another interface,but not a class

Front 30

What is an interface and what is an abstract class?

Back 30

1) interface is a contract , implementing it the user is forced to override the methods defined in the interface.

2)abstract class provides some common functionality and some declared abstract methods for the subclass to be inherited , hence it cannot be instantiated

example : AbstractMap

Front 31

What is a Constructor?

Back 31

Constructor is the method with the same name of the class and no return type called when an instance is created

Front 32

What is a Default Constructor?

Back 32

It's the no-args constructor provided by the compiler if we don't declare any other contructors

Front 33

How do you call a Super Class Constructor from a Constructor?

Back 33

super(), it has to be the first instruction

Front 34

What is the use of this()?

Back 34

it is used to invoke a constructor of the same class inside another constructor

Front 35

Is a super class constructor called even when there is no explicit call from a sub class constructor?

Back 35

yes , if we have inheritance hierarchy we will have

Animal

Dog

Chihuahua and the constructors called will be

Animal

Dog

Chihuahua

Front 36

What is Polymorphism?

Back 36

Same code giving different behavior

Animal animal = new Animal();

animal.shout // don't know

Animal animal = new Dog;

animal.shout // uoaf

Front 37

What is the use of instanceof Operator in Java?

Back 37

instanceof operator checks if an object is of a particular type, it works also with interfaces

Front 38

What is Coupling?

Back 38

Coupling is a measure of how much a class is dependent on other classes. There should minimal dependencies between classes. So, we should always aim for low coupling between classes.

Front 39

What is Cohesion?

Back 39

Cohesion is a measure of how related the responsibilities of a class are. A class must be highly cohesive i.e. its responsibilities (methods) should be highly related to one another.

Different classes must have their own responsibilities.

Front 40

What is Encapsulation?

Back 40

Encapsulation is “hiding the implementation of a Class behind a well defined interface”. Encapsulation helps us to change implementation of a class without breaking other code.

Front 41

What is an Inner Class and a Static Inner Class?

Back 41

Inner Classes are classes which are declared inside other classes.

Consider the following example:

class OuterClass {

public class InnerClass { }

public static class StaticNestedClass { }

}

Front 42

Can you create an inner class inside a method?

Back 42

Yes , only a class , not static

Front 43

What is an Anonymous Class?

Back 43

They don't have name and they are not reusable , are used to instantiate an object on the go , let's think to the Comparator example

Front 44

What is private access modifier?

Back 44

a. Private variables and methods can be accessed only in the class they are declared.

b. Private variables and methods from SuperClass are NOT available in SubClass

Front 45

What is default or package access modifier?

Back 45

a. Default variables and methods can be accessed in the same package Classes.

b. Default variables and methods from SuperClass are available only to SubClasses in same package.

Front 46

What is protected access modifier?

Back 46

a. Protected variables and methods can be accessed in the same package Classes.

b. Protected variables and methods from SuperClass are available to SubClass in any package

Front 47

What is public access modifier?

Back 47

a. Public variables and methods can be accessed from every other Java classes.

b. Public variables and methods from SuperClass are all available directly in the SubClass

Front 48

What is the use of a final modifier on a class?

Back 48

The class cannot be extended, String and wrapper classes are final

Front 49

What is the use of a final modifier on a method?

Back 49

Final methods cannot be overridden

Front 50

What is a Final variable?

Back 50

Once initialized, the value of a final variable cannot be changed

ex: java.lang.Math.pi

Front 51

What is a final argument?

Back 51

final argument of a method cannot be modified

Front 52

What happens when a variable is marked as volatile?

Back 52

• Volatile can only be applied to instance variables.

• A volatile variable is one whose value is always written to and read from "main memory". Each thread has its own cache in Java. The volatile variable will not be stored on a Thread cache,it is like a global variable for every thread accessing the class

Front 53

What are a Static Variables and methods ?

Back 53

They belong to the class , not to the object , they can be called without making an instance , all the objects will point to this unique copy of the static variable

Front 54

Guess the output of this switch block.

int number = 2;

switch (number) {

case 1:

System.out.println(1);

case 2:

System.out.println(2);

case 3:

System.out.println(3);

default:

System.out.println("Default"); }

Back 54

Output of above switch

2

3

Default

NB: there'no break instruction