Java Interview Questions

Front 1

Is Java pass by value or pass by reference?

Back 1

In Java, Objects are passed by reference, and primitives are passed by value.

This is half incorrect. Everyone can easily agree that primitives are passed by value; there's no such thing in Java as a pointer/reference to a primitive.

However, Objects are not passed by reference. A correct statement would be Object references are passed by value.

Front 2

Explain Java class loaders?

Back 2

Class loaders are hierarchical. Classes are introduced into the JVM as they are referenced by name in a class that
is already running in the JVM. So, how is the very first class loaded? The very first class is especially loaded with
the help of static main( ) method declared in your class. All the subsequently loaded classes are loaded by the
classes, which are already loaded and running. A class loader creates a namespace. All JVMs include at least one
class loader that is embedded within the JVM called the primordial (or bootstrap) class loader. Now let’s look at
non-primordial class loaders. The JVM has hooks in it to allow user defined class loaders to be used in place of
primordial class loader. Let us look at the class loaders created by the JVM.

Front 3

What happens if you do not provide a constructor?

Back 3

ava does not actually require an explicit constructor in
the class description. If you do not include a constructor, the Java compiler will create a default constructor in the
byte code with an empty argument. This default constructor is equivalent to the explicit “Pet(){}”. If a class includes
one or more explicit constructors like “public Pet(int id)” or “Pet(){}” etc, the java compiler does not create the
default constructor “Pet(){}”.

Front 4

Can you call one constructor from another?

Back 4

Yes, by using this() syntax

Front 5

How to call the superclass constructor?

Back 5

If a class called “SpecialPet” extends your “Pet” class then you can
use the keyword “super” to invoke the superclass’s constructor.

Front 6

What are the advantages of Object Oriented Programming Languages (OOPL)?

Back 6

The Object Oriented Programming Languages directly represent the real life objects like Car, Jeep, Account,
Customer etc. The features of the OO programming languages like polymorphism, inheritance and
encapsulation make it powerful. [Tip: remember pie which, stands for Polymorphism, Inheritance and
Encapsulation are the 3 pillars of OOPL]

Front 7

What is the difference between aggregation and composition?

Back 7

Aggregation is an association in which one class belongs to a collection. This is a part of a whole
relationship where a part can exist without a whole. For example a line item is a whole and product is a part. If a line item is deleted then corresponding product need not be deleted. So aggregation has a weaker relationship.

Composition is an association in which one class belongs to a
collection. This is a part of a whole relationship where a part
cannot exist without a whole. If a whole is deleted then all parts are
deleted. For example An order is a whole and line items are parts.
If an order is deleted then all corresponding line items for that
order should be deleted. So composition has a stronger
relationship.

Front 8

What do you mean by polymorphism?

Back 8

Polymorphism – means the ability of a single variable of a given type to be used to reference objects of
different types, and automatically call the method that is specific to the type of object the variable references. In a
nutshell, polymorphism is a bottom-up method call.

Front 9

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

Back 9

In design, you want the base class to present only an interface for its derived classes. This means, you don’t want anyone to actually instantiate an object of the base class. You only want to upcast to it (implicit upcasting, which
gives you polymorphic behavior), so that its interface can be used. This is accomplished by making that class
abstract using the abstract keyword. If anyone tries to make an object of an abstract class, the compiler prevents
it.

The interface keyword takes this concept of an abstract class a step further by preventing any method or function
implementation at all. You can only declare a method or function but not provide the implementation. The class,
which is implementing the interface, should provide the actual implementation

Front 10

When to use an abstract class?

Back 10

In case where you want to use implementation inheritance then it is usually provided by an abstract base class. Abstract classes are excellent candidates inside of application frameworks. Abstract classes let you define some default behavior and force subclasses to provide any specific behavior.

Front 11

When to use an interface?

Back 11

For polymorphic interface inheritance, where the client wants to only deal with a type and does not care about the actual implementation use interfaces. If you need to change your design frequently, you should prefer using interface to abstract. Coding to an interface reduces coupling and
interface inheritance can achieve code reuse with the help of object composition.

Front 12

Why there are some interfaces with no defined methods (i.e. marker interfaces) in Java?

Back 12

The interfaces with no defined methods act like markers. They just tell the compiler that the objects of the classes
implementing the interfaces with no defined methods need to be treated differently. Example java.io.Serializable , java.lang.Cloneable, java.util.EventListener etc. Marker interfaces are also known as
“tag” interfaces since they tag all the derived classes into a category based on their purpose.

Front 13

When is a method said to be overloaded?

Back 13

Overloading deals with multiple methods in the same class
with the same name but different method signatures.

class MyClass {
public void getInvestAmount(int rate) {…}

public void getInvestAmount(int rate, long principal)
{ … }
}

Both the above methods have the same method names
but different method signatures, which mean the methods
are overloaded.

Overloading lets you define the same operation in
different ways for different dat

Front 14

When is a method said to be overridden?

Back 14

Overriding deals with two methods, one in the parent class and
the other one in the child class and has the same name and
signatures.

Overriding lets you define the same operation in different
ways for different object types.

class BaseClass{
public void getInvestAmount(int rate) {…}
}

class MyClass extends BaseClass {
public void getInvestAmount(int rate) { …}
}

Both the above methods have the same method names and
the signatures but the method in the subclass MyClass
overrides the method in the superclass BaseClass.

Front 15

What is the difference between an Interface and an Abstract class?

Back 15

An abstract class can have instance methods that implement a default behavior. An Interface can only declare constants and instance methods, but cannot implement default behavior and all methods are implicitly abstract. An interface has all public members and no implementation. An abstract class is a class which may have the usual flavors of class members (private, protected, etc.), but has some abstract methods.

Front 16

What is the purpose of garbage collection in Java, and when is it used?

Back 16

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.

Front 17

Describe synchronization in respect to multithreading.

Back 17

With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchonization, it is possible for one thread to modify a shared variable while another thread is in the process of using or updating same shared variable. This usually leads to significant errors.

Front 18

Explain different way of using thread?

Back 18

he thread could be implemented by using runnable interface or by inheriting from the Thread class. The former is more advantageous, 'cause when you are going for multiple inheritance..the only interface can help.

Front 19

What are pass by reference and passby value?

Back 19

Pass By Reference means the passing the address itself rather than passing the value. Passby Value means passing a copy of the value to be passed.

Front 20

What is HashMap and Map?

Back 20

Map is Interface and Hashmap is class that implements that.

Front 21

Difference between HashMap and HashTable?

Back 21

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 doesnt allow). HashMap does not guarantee that the order of the map will remain constant over time. HashMap is unsynchronized and Hashtable is synchronized.

Front 22

Difference between Vector and ArrayList?

Back 22

Vector is synchronized whereas arraylist is not

Front 23

What is the difference between a constructor and a method?

Back 23

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.

Front 24

What is an Iterator?

Back 24

Some of the collection classes provide traversal of their contents via a 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.

Front 25

State the significance of public, private, protected, default modifiers both singly and in combination and state the effect of package relationships on declared items qualified by these modifiers.

Back 25

public : Public class is visible in other packages, field is visible everywhere (class must be public too)
private : Private variables or methods may be used only by an instance of the same class that declares the variable or method, A private feature may only be accessed by the class that owns the feature.
protected : Is available to all classes in the same package and also available to all subclasses of the class that owns the protected feature.This access is provided even to subclasses that reside in a different package from the class that owns the protected feature.
default :What you get by default ie, without any access modifier (ie, public private or protected).It means that it is visible to all within a particular package.

Front 26

What is an abstract class?

Back 26

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), 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.

Front 27

What is static in java?

Back 27

Static means one per class, not one for each object no matter how many instance 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.

Front 28

What is final?

Back 28

A final class can't be extended ie., final class may not be subclassed. A final method can't be overridden when its class is inherited. You can't change value of a final variable (is a constant).

Front 29

What if the main method is declared as private?

Back 29

he program compiles properly but at runtime it will give "Main method not public." message.

Front 30

What if the static modifier is removed from the signature of the main method?

Back 30

Program compiles. But at runtime throws an error "NoSuchMethodError".

Front 31

What if I write static public void instead of public static void?

Back 31

Program compiles and runs properly.

Front 32

What are Checked and UnChecked Exception?

Back 32

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 are 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() method· Checked exceptions must be caught at compile time. Runtime exceptions do not need to be. Errors often cannot be.

Front 33

What are different types of inner classes?

Back 33

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.

Front 34

Can a top level class be private or protected?

Back 34

No. A top level class can not be private or protected. It can have either "public" or no modifier. If it does not have a modifier it is supposed to have a 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. Same is the case with protected.

Front 35

What type of parameter passing does Java support?

Back 35

In Java the arguments are always passed by value .

Front 36

Primitive data types are passed by reference or pass by value?

Back 36

Primitive data types are passed by value.

Front 37

Objects are passed by value or by reference?

Back 37

Java only supports pass by value. 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 .

Front 38

What is serialization?

Back 38

Serialization is a mechanism by which you can save the state of an object by converting it to a byte stream.

Front 39

How do I serialize an object to a file?

Back 39

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.

Front 40

Which methods of Serializable interface should I implement?

Back 40

The serializable interface is an empty interface, it does not contain any methods. So we do not implement any methods.

Front 41

How can I customize the seralization process? i.e. how can one have a control over the serialization process?

Back 41

Yes it is possible to have control over serialization process. The class should implement 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.

Front 42

What is the common usage of serialization?

Back 42

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 serilazed.

Front 43

What is Externalizable interface?

Back 43

Externalizable is an interface which contains two methods readExternal and writeExternal. These methods give you a control over the serialization mechanism. Thus if your class implements this interface, you can customize the serialization process by implementing these methods.

Front 44

When you serialize an object, what happens to the object references included in the object?

Back 44

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 alongwith the original obect.

Front 45

What one should take care of while serializing the object?

Back 45

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.

Front 46

What happens to the static fields of a class during serialization?

Back 46

There are three exceptions in which serialization doesnot necessarily read and write to the stream. These are
1. Serialization ignores static fields, because they are not part of ay particular state state.
2. Base class fields are only hendled if the base class itself is serializable.
3. Transient fields.

Front 47

Why do we need wrapper classes?

Back 47

It is sometimes easier to deal with primitives as objects. Moreover most of the collection classes store objects and not primitive data types. And also the wrapper classes provide many utility methods also. Because of these resons we need wrapper classes. And since we create instances of these classes we can store them in any of the collection classes and pass them around as a collection. Also we can pass them around as method parameters where a method expects an object.

Front 48

What are checked exceptions?

Back 48

Checked exception are those which the Java compiler forces you to catch. e.g. IOException are checked Exceptions.

Front 49

What are runtime exceptions?

Back 49

Runtime exceptions are those exceptions that are thrown at runtime because of either wrong input data or because of wrong business logic etc. These are not checked by the compiler at compile time.

Front 50

What is the difference between an Abstract class and Interface?

Back 50

1. Abstract classes may have some executable methods and methods left unimplemented. Interfaces contain no implementation code.
2. An class can implement any number of interfaces, but subclass at most one abstract class.
3. An abstract class can have nonabstract methods. All methods of an interface are abstract.
4. An abstract class can have instance variables. An interface cannot.
5. An abstract class can define constructor. An interface cannot.
6. An abstract class can have any visibility: public, protected, private or none (package). An interface's visibility must be public or none (package).
7. An abstract class inherits from Object and includes methods such as clone() and equals().

Front 51

What are checked and unchecked exceptions?

Back 51

Java defines two kinds of exceptions :

* Checked exceptions : Exceptions that inherit from the Exception class are checked exceptions. Client code has to handle the checked exceptions thrown by the API, either in a catch clause or by forwarding it outward with the throws clause. Examples - SQLException, IOxception.
* Unchecked exceptions : RuntimeException also extends from Exception. However, all of the exceptions that inherit from RuntimeException get special treatment. There is no requirement for the client code to deal with them, and hence they are called unchecked exceptions. Example Unchecked exceptions are NullPointerException, OutOfMemoryError, DivideByZeroException typically, programming errors.

Front 52

What is a user defined exception?

Back 52

User-defined exceptions may be implemented by

* defining a class to respond to the exception and
* embedding a throw statement in the try block where the exception can occur or declaring that the method throws the exception (to another method where it is handled).

Front 53

What is serialization?

Back 53

Quite simply, object serialization provides a program the ability to read or write a whole object to and from a raw byte stream. It allows Java objects and primitives to be encoded into a byte stream suitable for streaming to some type of network or to a file-system, or more generally, to a transmission medium or storage facility. A seralizable object must implement the Serilizable interface. We use ObjectOutputStream to write this object to a stream and ObjectInputStream to read it from the stream.

Front 54

Why there are some null interface in java ? What does it mean ? Give me some null interfaces in JAVA?

Back 54

Null interfaces act as markers..they just tell the compiler that the objects of this class need to be treated differently..some marker interfaces are : Serializable, Remote, Cloneable

Front 55

Is synchronised a modifier?indentifier??what is it??

Back 55

It's a modifier. Synchronized methods are methods that are used to control access to an object. A thread only executes a synchronized method after it has acquired the lock for the method's object or class. Synchronized statements are similar to synchronized methods. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement.

Front 56

What is singleton class?where is it used?

Back 56

Singleton is a design pattern meant to provide one and only one instance of an object. Other objects can get a reference to this instance through a static method (class constructor is kept private). Why do we need one? Sometimes it is necessary, and often sufficient, to create a single instance of a given class. This has advantages in memory management, and for Java, in garbage collection. Moreover, restricting the number of instances may be necessary or desirable for technological or business reasons--for example, we may only want a single instance of a pool of database connections.

Front 57

Is string a wrapper class?

Back 57

String is a class, but not a wrapper class. Wrapper classes like (Integer) exist for each primitive type. They can be used to convert a primitive data value into an object, and vice-versa.

Front 58

Why java does not have multiple inheritance?

Back 58

The Java design team strove to make Java:

* Simple, object oriented, and familiar
* Robust and secure
* Architecture neutral and portable
* High performance
* Interpreted, threaded, and dynamic

Front 59

What is transient variable?

Back 59

Transient variable can't be serialize. For example if a variable is declared as transient in a Serializable class and the class is written to an ObjectStream, the value of the variable can't be written to the stream instead when the class is retrieved from the ObjectStream the value of the variable becomes null.

Front 60

What is Collection API?

Back 60

The Collection API is a set of classes and interfaces that support operation on collections of objects. These classes and interfaces are more flexible, more powerful, and more regular than the vectors, arrays, and hashtables if effectively replaces.

Example of classes: HashSet, HashMap, ArrayList, LinkedList, TreeSet and TreeMap.

Example of interfaces: Collection, Set, List and Map.

Front 61

s Iterator a Class or Interface? What is its use?

Back 61

Iterator is an interface which is used to step through the elements of a Collection.

Front 62

What is similarities/difference between an Abstract class and Interface?

Back 62

Differences are as follows:

* Interfaces provide a form of multiple inheritance. A class can extend only one other class.
* Interfaces are limited to public methods and constants with no implementation. Abstract classes can have a partial implementation, protected parts, static methods, etc.
* A Class may implement several interfaces. But in case of abstract class, a class may extend only one abstract class.
* Interfaces are slow as it requires extra indirection to to find corresponding method in in the actual class. Abstract classes are fast.

Similarities:

* Neither Abstract classes or Interface can be instantiated.

Front 63

Why do threads block on I/O?

Back 63

Threads block on i/o (that is enters the waiting state) so that other threads may execute while the i/o Operation is performed.

Front 64

How are Observer and Observable used?

Back 64

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.

Front 65

What is synchronization and why is it important?

Back 65

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.

Front 66

an a lock be acquired on a class?

Back 66

Yes, a lock can be acquired on a class. This lock is acquired on the class's Class object..

Front 67

What state does a thread enter when it terminates its processing?

Back 67

When a thread terminates its processing, it enters the dead state.

Front 68

What is the List interface?

Back 68

The List interface provides support for ordered collections of objects.

Front 69

How does Java handle integer overflows and underflows?

Back 69

It uses those low order bytes of the result that can fit into the size of the type allowed by the operation.

Front 70

What is the Vector class?

Back 70

The Vector class provides the capability to implement a growable array of objects

Front 71

What modifiers may be used with an inner class that is a member of an outer class?

Back 71

A (non-local) inner class may be declared as public, protected, private, static, final, or abstract.

Front 72

What is an Iterator interface?

Back 72

The Iterator interface is used to step through the elements of a Collection.

Front 73

What is the difference between the >> and >>> operators?

Back 73

The >> operator carries the sign bit when shifting right. The >>> zero-fills bits that have been shifted out.

Front 74

What is the difference between yielding and sleeping?

Back 74

When a task invokes its yield() method, it returns to the ready state. When a task invokes its sleep() method, it returns to the waiting state.

Front 75

Which java.util classes and interfaces support event handling?

Back 75

The EventObject class and the EventListener interface support event processing.

Front 76

What are wrapped classes?

Back 76

Wrapped classes are classes that allow primitive types to be accessed as objects.

Front 77

Does garbage collection guarantee that a program will not run out of memory?

Back 77

Garbage collection does not guarantee that a program will not run out of memory. It is possible for programs to use up memory resources faster than they are garbage collected. It is also possible for programs to create objects that are not subject to garbage collection

Front 78

Can an object's finalize() method be invoked while it is reachable?

Back 78

An object's finalize() method cannot be invoked by the garbage collector while the object is still reachable. However, an object's finalize() method may be invoked by other objects.

Front 79

What is the difference between preemptive scheduling and time slicing?

Back 79

Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into existence. Under time slicing, a task executes for a predefined slice of time and then reenters the pool of ready tasks. The scheduler then determines which task should execute next, based on priority and other factors.

Front 80

Can a for statement loop indefinitely?

Back 80

Yes, a for statement can loop indefinitely. For example, consider the following:

for(;;) ;

Front 81

When a thread blocks on I/O, what state does it enter?

Back 81

A thread enters the waiting state when it blocks on I/O.

Front 82

o what value is a variable of the String type automatically initialized?

Back 82

null

Front 83

What is the catch or declare rule for method declarations?

Back 83

If a checked exception may be thrown within the body of a method, the method must either catch the exception or declare it in its throws clause.

Front 84

When a thread is created and started, what is its initial state?

Back 84

A thread is in the ready state after it has been created and started.

Front 85

Can an anonymous class be declared as implementing an interface and extending a class?

Back 85

An anonymous class may implement an interface or extend a superclass, but may not be declared to do both.

Front 86

What is the purpose of finalization?

Back 86

The purpose of finalization is to give an unreachable object the opportunity to perform any cleanup processing before the object is garbage collected.

Front 87

What invokes a thread's run() method?

Back 87

After a thread is started, via its start() method or that of the Thread class, the JVM invokes the thread's run() method when the thread is initially executed.

Front 88

What is the difference between the Boolean & operator and the && operator?

Back 88

If an expression involving the Boolean & operator is evaluated, both operands are evaluated. Then the & operator is applied to the operand. When an expression involving the && operator is evaluated, the first operand is evaluated. If the first operand returns a value of true then the second operand is evaluated. The && operator is then applied to the first and second operands. If the first operand evaluates to false, the evaluation of the second operand is skipped.

Front 89

How many times may an object's finalize() method be invoked by the garbage collector?

Back 89

An object's finalize() method may only be invoked once by the garbage collector.

Front 90

What is the purpose of the finally clause of a try-catch-finally statement?

Back 90

The finally clause is used to provide the capability to execute code no matter whether or not an exception is thrown or caught.

Front 91

What is the argument type of a program's main() method?

Back 91

A program's main() method takes an argument of the String[] type.

Front 92

What is the difference between a break statement and a continue statement?

Back 92

A break statement results in the termination of the statement to which it applies (switch, for, do, or while). A continue statement is used to end the current loop iteration and return control to the loop statement.

Front 93

What must a class do to implement an interface?

Back 93

It must provide all of the methods in the interface and identify the interface in its implements clause.

Front 94

What method is invoked to cause an object to begin executing as a separate thread?

Back 94

The start() method of the Thread class is invoked to cause an object to begin executing as a separate thread.

Front 95

What is the purpose of the wait(), notify(), and notifyAll() methods?

Back 95

The wait(),notify(), and notifyAll() methods are used to provide an efficient way for threads to wait for a shared resource. When a thread executes an object's wait() method, it enters the waiting state. It only enters the ready state after another thread invokes the object's notify() or notifyAll() methods..

Front 96

What are the high-level thread states?

Back 96

The high-level thread states are ready, running, waiting, and dead.

Front 97

What value does read() return when it has reached the end of a file?

Back 97

The read() method returns -1 when it has reached the end of a file.

Front 98

What is the difference between a static and a non-static inner class?

Back 98

A non-static inner class may have object instances that are associated with instances of the class's outer class. A static inner class does not have any object instances.

Front 99

What is the difference between the String and StringBuffer classes?

Back 99

String objects are constants. StringBuffer objects are not.

Front 100

If a variable is declared as private, where may the variable be accessed?

Back 100

A private variable may only be accessed within the class in which it is declared.

Front 101

What is an object's lock and which object's have locks?

Back 101

An object's lock is a mechanism that is used by multiple threads to obtain synchronized access to the object. A thread may execute a synchronized method of an object only after it has acquired the object's lock. All objects and classes have locks. A class's lock is acquired on the class's Class object.

Front 102

When can an object reference be cast to an interface reference?

Back 102

An object reference be cast to an interface reference when the object implements the referenced interface.

Front 103

Can an object be garbage collected while it is still reachable?

Back 103

A reachable object cannot be garbage collected. Only unreachable objects may be garbage collected..

Front 104

Is the ternary operator written x : y ? z or x ? y : z ?

Back 104

It is written x ? y : z.

Front 105

How is rounding performed under integer division?

Back 105

The fractional part of the result is truncated. This is known as rounding toward zero.

Front 106

What happens when a thread cannot acquire a lock on an object?

Back 106

If a thread attempts to execute a synchronized method or synchronized statement and is unable to acquire an object's lock, it enters the waiting state until the lock becomes available.

Front 107

What is the difference between the Reader/Writer class hierarchy and the InputStream/OutputStream class hierarchy?

Back 107

The Reader/Writer class hierarchy is character-oriented, and the InputStream/OutputStream class hierarchy is byte-oriented.

Front 108

What classes of exceptions may be caught by a catch clause?

Back 108

A catch clause can catch any exception that may be assigned to the Throwable type. This includes the Error and Exception types.

Front 109

If a class is declared without any access modifiers, where may the class be accessed?

Back 109

A class that is declared without any access modifiers is said to have package access. This means that the class can only be accessed by other classes and interfaces that are defined within the same package.

Front 110

What is the Map interface?

Back 110

The Map interface replaces the JDK 1.1 Dictionary class and is used associate keys with values.

Front 111

Does a class inherit the constructors of its superclass?

Back 111

A class does not inherit constructors from any of its superclasses.

Front 112

Name the eight primitive Java types?

Back 112

The eight primitive types are byte, char, short, int, long, float, double, and boolean.

Front 113

What restrictions are placed on the values of each case of a switch statement?

Back 113

During compilation, the values of each case of a switch statement must evaluate to a value that can be promoted to an int value.

Front 114

What modifiers may be used with an interface declaration?

Back 114

An interface may be declared as public or abstract.

Front 115

Is a class a subclass of itself?

Back 115

Yes, A class is a subclass of itself.

Front 116

What is the difference between a while statement and a do statement?

Back 116

A while statement checks at the beginning of a loop to see whether the next loop iteration should occur. A do statement checks at the end of a loop to see whether the next iteration

of a loop should occur. The do statement will always execute the body of a loop at least once.

Front 117

What modifiers can be used with a local inner class?

Back 117

A local inner class may be final or abstract.

Front 118

What is the difference between static and non-static variables?

Back 118

A static variable is associated with the class as a whole rather than with specific instances of a class. Non-static variables take on unique values with each object instance.

Front 119

Can an exception be rethrown?

Back 119

Yes, an exception can be rethrown.

Front 120

When is the finally clause of a try-catch-finally statement executed?

Back 120

The finally clause of the try-catch-finally statement is always executed unless the thread of execution terminates or an exception occurs within the execution of the finally clause.

Front 121

If a method is declared as protected, where may the method be accessed?

Back 121

A protected method may only be accessed by classes or interfaces of the same package or by subclasses of the class in which it is declared.

Front 122

What restrictions are placed on method overloading?

Back 122

Two methods may not have the same name and argument list but different return types.

Front 123

What happens when you invoke a thread's interrupt method while it is sleeping or waiting?

Back 123

When a task's interrupt() method is executed, the task enters the ready state. The next time the task enters the running state, an InterruptedException is thrown.

Front 124

What is casting?

Back 124

There are two types of casting, casting between primitive numeric types and casting between object references. Casting between numeric types is used to convert larger values, such as double values, to smaller values, such as byte values. Casting between object references is used to refer to an object by a compatible class, interface, or array type reference.

Front 125

What is the return type of a program's main() method?

Back 125

A program's main() method has a void return type.

Front 126

What is the difference between a field variable and a local variable?

Back 126

A field variable is a variable that is declared as a member of a class. A local variable is a variable that is declared local to a method.

Front 127

Under what conditions is an object's finalize() method invoked by the garbage collector?

Back 127

he garbage collector invokes an object's finalize() method when it detects that the object has become unreachable.

Front 128

How are this() and super() used with constructors?

Back 128

his() is used to invoke a constructor of the same class. super() is used to invoke a superclass constructor.

Front 129

What is the relationship between a method's throws clause and the exceptions that can be thrown during the method's execution?

Back 129

A method's throws clause must declare any checked exceptions that are not caught within the body of the method.

Front 130

How is it possible for two String objects with identical values not to be equal under the == operator?

Back 130

he == operator compares two objects to determine if they are the same object in memory. It is possible for two String objects to have the same value, but located indifferent areas of memory.

Front 131

What state is a thread in when it is executing?

Back 131

An executing thread is in the running state.

Front 132

What is the Set interface?

Back 132

The Set interface provides methods for accessing the elements of a finite mathematical set. Sets do not allow duplicate elements.

Front 133

What interface must an object implement before it can be written to a stream as an object?

Back 133

An object must implement the Serializable or Externalizable interface before it can be written to a stream as an object.

Front 134

How are this and super used?

Back 134

this is used to refer to the current object instance. super is used to refer to the variables and methods of the superclass of the current object instance.

Front 135

What is the purpose of garbage collection?

Back 135

The purpose of garbage collection is to identify and discard objects that are no longer needed by a program so that their resources may be reclaimed and reused.

Front 136

What restrictions are placed on method overriding?

Back 136

* Overridden methods must have the same name, argument list, and return type.
* The overriding method may not limit the access of the method it overrides.
* The overriding method may not throw any exceptions that may not be thrownby the overridden method.

Front 137

What happens if an exception is not caught?

Back 137

An uncaught exception results in the uncaughtException() method of the thread's ThreadGroup being invoked, which eventually results in the termination of the program in which it is thrown.

Front 138

What are three ways in which a thread can enter the waiting state?

Back 138

A thread can enter the waiting state by invoking its sleep() method, by blocking on I/O, by unsuccessfully attempting to acquire an object's lock, or by invoking an object's wait() method. It can also enter the waiting state by invoking its (deprecated) suspend() method.

Front 139

What happens if a try-catch-finally statement does not have a catch clause to handle an exception that is thrown within the body of the try statement?

Back 139

The exception propagates up to the next higher level try-catch statement (if any) or results in the program's termination.

Front 140

What is the difference between a public and a non-public class?

Back 140

A public class may be accessed outside of its package. A non-public class may not be accessed outside of its package.

Front 141

What are the Object and Class classes used for?

Back 141

The Object class is the highest-level class in the Java class hierarchy. The Class class is used to represent the classes and interfaces that are loaded by a Java program..

Front 142

How does a try statement determine which catch clause should be used to handle an exception?

Back 142

When an exception is thrown within the body of a try statement, the catch clauses of the try statement are examined in the order in which they appear. The first catch clause that is capable of handling the exception is executed. The remaining catch clauses are ignored.

Front 143

Can an unreachable object become reachable again?

Back 143

An unreachable object may become reachable again. This can happen when the object's finalize() method is invoked and the object performs an operation which causes it to become accessible to reachable objects.

Front 144

What method must be implemented by all threads?

Back 144

All tasks must implement the run() method, whether they are a subclass of Thread or implement the Runnable interface.

Front 145

What are synchronized methods and synchronized statements?

Back 145

Synchronized methods are methods that are used to control access to an object. A thread only executes a synchronized method after it has acquired the lock for the method's object or class. Synchronized statements are similar to synchronized methods. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement.

Front 146

What are the two basic ways in which classes that can be run as threads may be defined?

Back 146

A thread class may be declared as a subclass of Thread, or it may implement the Runnable interface.

Front 147

What is the List interface?

Back 147

The List interface provides support for ordered collections of objects.