Core Java Interview - Classes, Objects And Interfaces

Front 1

What is the difference between an abstract class and an interface?

Back 1

Interfaces don't define non-default methods. Also, classes cannot inherit from more than one class, abstract or not, while a class may implement many interfaces. Both can have constant declarations but in an interface they are implicitly public static and final.

Front 2

How do you derive a new class from a class?

Back 2

Where A is a subclass of B,
public class A extends B {
// class contents
}

Front 3

Given an interface, how would you make your class behave like the interface.

Back 3

//Where Class A uses interface B
public class A implements B {
// Must define methods declared in B
}

Front 4

What is the common superclass from which all Java classes derive directly or indirectly?

Back 4

Object.

Front 5

What are some advantages of all classes deriving from a common class like Object?

Back 5

Every class inherits all the methods defined in Object. These include methods to get the class name, getClass() and various methods related to synchronization like, wait() or notify() and methods useful for storing an object in a collection like hashCode() or equals().

Front 6

What are some methods defined in the Object class and how are they useful?

Back 6

getClass() returns the class object.
hashCode() and equals() are useful for comparison and in collections.
notify(), notifyAll() and wait() are used in thread synchronization.
toString() is used to get a string representation of the object.
clone() is used to get a deep copy of the object.
finalize() is useful for clean up just before garbage collection.

Front 7

What does the default toString() method implementation do?

Back 7

Returns the class name and hash code. If this is not the desired representation of the object, the developer should override it.

Front 8

What are primitive wrapper classes?

Back 8

Wrapper classes are used to wrap a primitive type in an Object, mainly to make it easier to use them in a collection and for using reflection. The action of converting from a primitive type to a wrapper type is called boxing. The other way around ( wrapper to primitive ) is called unboxing. Some examples of wrapper classes are Integer class, Boolean class, Double class.

Front 9

What is the difference between constructors and other methods?

Back 9

A constructor has no return type and has the same name as the class.

Front 10

What is a static method?

Back 10

A static method is associated with a class rather than an object of that class. It can be invoked without using an instance of that class. For example:

public class A {
static public doStuff(){
}
}

//Usage: A.doStuff();

Front 11

What is an initializer block?

Back 11

It's a block defined outside of a method that gets appended to the end of each constructor. Variables declared final may be initialized in these blocks. You can define any number of these and they will be appended in the order in which they occur in your code.

Front 12

What is a static initializer block?

Back 12

A block of code defined outside of a method and declared static. It will be executed when the class is loaded rather than when the object is instantiated. Many of these may be defined.

Front 13

Explain the access modifiers public, private protected and default.

Back 13

public members and classes may be accessed from anywhere.
private members may be accessed only by methods of the class in which they are declared.
protected members may be accessed from within the class or a subclass.
default (a.k.a package private) members and classes may be accessed from anywhere in the same package. There is no actual modifier for default.

Front 14

Explain how the final keyword is used in Java.

Back 14

A method declared final may not be overridden. A class declared final may not be extended. A variable declared final may be assigned only once. A static final variable may be used like a constant. Method arguments declared final can not be changed by the called method.

Front 15

Why would you declare a class final?

Back 15

Making a class final adds some security and consistency to the class behaviour. E.g. by making the String class final, you cannot override its length method. The compiler may optimize the code and make it run faster when a class is declared final.

Front 16

What are the four types of nested classes?

Back 16

1. static nested class
2. inner (non-static) nested class
3. local class
4. anonymous class

A static nested class is just like a normal Java class. If it is public, it can be accessed or used to create objects just like any other class.
A non-static inner class requires an object of the containing class.
A local class is declared within and scoped to a block.
An anonymous class is defined in place after the new operator without assigning any name to it. It's a way to pass functions as parameters.

Front 17

What are the primitive types in Java?

Back 17

Mnemonic:
I EIGHT a BYTE but was SHORTed 16 bits INT-to 32 of those LONG 64's that FLOATed 32 bits while DOUBLE-ing 64 of them.
They just didn't have the CHAR-acter of the 16's.

byte ( 8 bits )
short (16 bits )
int ( 32 bits )
long ( 64 bits )
float ( 32 bits )
double ( 64 bits )
boolean ( unspecified )
char ( 16 bits )

Front 18

How are primitive types different from classes in Java?

Back 18

Primitive types are created when ever a variable of these types is declared and are not classes.
Primitive types are stored on the JVM stack rather than the heap. If you need to treat them as objects, they need to be boxed first.

Front 19

What is an abstract class?

Back 19

An abstract class is a class declared with the abstract keyword. Although references to such a class may be used, the class may not be directly instantiated. Abstract classes may define methods or just declare them and may contain variables.

Front 20

Where would you use the keyword extends and implements?

Back 20

extends is the keyword used to declare that this class is a subclass of the class that follows the extends keyword. Implements is a keyword that designates this class as an implementer of the specified interface.

Front 21

How do you call a constructor of a super class? Where would you call it?

Back 21

A call to the default super class constructor is implicitly the first line of each constructor unless a different constructor is explicitly provided by the developer. So for the first line of a constructor that calls a non-default super class constructor, one might write: super("some string");
It's a compilation error if it is not the first line.

Front 22

How do you call a method in a super class from a subclass?

Back 22

Use the super keyword like so:
super.someMethodCall("some string parameter" );
Note that in most cases the super keyword may be omitted.

Front 23

How are parameters passed in Java, by value or by reference?

Back 23

By value but in the case of objects, the value is a reference.

Front 24

What if you wanted to pass a primitive type ( say an int ) so that the called method can modify it and the caller sees the modified value?

Back 24

Wrap it in a wrapper class or make it a member of a class object and pass the object to the method.

Front 25

Why don't we call the new keyword for primitive types like int, float and double?

Back 25

Primitive types are not objects so they are not created with the new operator which allocates memory on the heap. They are created by simply declaring them and assigning a value.

Front 26

How are Arrays defined in Java?

Back 26

An array is a container object that holds a fixed number of values of a single type. The member called length is initialized when the array is created and tells how many elements are in the array.

Front 27

How do you clone an array?

Back 27

Use the clone() method of the array object.

Front 28

When we say that String's are immutable in Java, what does that mean?

Back 28

It means that String objects cannot be changed once created. Consequently, each time a String is altered, a new String must be created to get a proper result. This has a performance impact so that if a program performs lots of String manipulation, the helper classes StringBuilder and/or StringBuffer should be used.

Front 29

Describe some do's and don't's for manipulating Strings in Java.

Back 29

Because Strings are immutable, any little operation you do with them creates one or more new strings which impacts performance. One can use a StringBuilder or StringBuffer to manipulate the string instead and then convert it when the alterations / additions are complete.

Front 30

What's the difference between StringBuffer and StringBuilder?

Back 30

StringBuffer is thread safe while StringBuilder is not.

Front 31

What is the conditional or ternary operator in Java?

Back 31

It's shorthand for an if/else block.
Example:

(boolean condition) ? // if true : // else ;

So it might look like:

X.equals(Y) ? X = "if true" : Y ="else block" ;

Front 32

Is String a primitive type in Java?

Back 32

No but it is one of the most frequently used objects, consequently it has some special features. Enclosing a String of characters in double quotes creates a new String object.

Front 33

Which package is always imported by default?

Back 33

java.lang

Front 34

Which operator checks the type of an object?

Back 34

instanceof

Front 35

What is the difference between the == operator and the equals() method?

Back 35

== compares two operands and in the case of primitives, returns true if the values are equal. In the case of Objects, it checks to see if the references are the same. The default implementation of the equals() method does the same thing but the equals method is override-able and many pre-defined classes have such overrides.

Front 36

What is a marker interface?

Back 36

A Marker interface is an empty interface used to indicate that a given class implements a certain functionality. Serializable for example, indicates that a classes non-transient data can be written to an OutputStream. RandomAccess indicates that that a collection supports fast ( almost constant time ) random access.

Front 37

What are annotations and how are they useful?

Back 37

Annotations are pieces of metadata that contain information used by the compiler, associated with an object at run time or compile time. For example, @Deprecated, @Override and @SuppressWarnings.

Front 38

What is a static import?

Back 38

Declaring an import static allows unqualified access to static members. For example:

import static java.lang.Math.PI;

The above would allow you to use PI without any further qualification.

Front 39

When should static imports be used?

Back 39

They should be used sparingly and only on frequently used classes. This will save typing the extra boiler plate code while preserving the quality of the namespace.

Front 40

When should a data field be declared final?

Back 40

Objects with fields declared final can be passed from one thread to another without synchronization. It contributes to robust software design and is slightly faster. Unless there's a good reason why not, all fields should be declared final by default.