JAVA PROGRAMMING COURSE NO. IT-705(A) SECTION - A

Transcription

1 Q1. Define the following terms: JAVA PROGRAMMING COURSE NO. IT-705(A) SECTION - A i. Event An event in Java is an object that is created when something changes within a graphical user interface. If a user clicks on a button, clicks on a combo box, or types characters into a text field, etc., then an event triggers, creating the relevant event object. This behavior is part of Java's Event Handling mechanism and is included in the Swing GUI library. For example, let's say we have a JButton. If a user clicks on the JButton, a button click event is triggered, the event will be created, and it will be sent to the relevant event listener (in this case, the ActionListener). The relevant listener will have implemented code that determines the action to take when the event occurs. Note that an event source must be paired with an event listener, or its triggering will result in no action. How Events Work Event handling in Java is comprised of two key elements: The event source, which is an object that is created when an event occurs. Java provides several types of these event sources, discussed in the section Types of Events below. The event listener, the object that "listens" for events and processes them when they occur. There are several types of events and listeners in Java: each type of event is tied to a corresponding listener. For this discussion, let's consider a common type of event, an action event represented by the Java class ActionEvent, which is triggered when a user clicks a button or the item of a list. At the user's action, an ActionEvent object corresponding to the relevant action is created. This object contains both the event source information and the specific action taken by the user. This event object is then passed to the corresponding ActionListener object's method: void actionperformed(actionevent e) This method is executed and returns the appropriate GUI response, which might be to open or close a dialog, download a file, provide a digital signature, or any other of the myriad actions available to users in an interface. Page 1

2 Types of Events Here are some of the most common types of events in Java: ActionEvent: Represents a graphical element is clicked, such as a button or item in a list. Related listener: ActionListener. ContainerEvent: Represents an event that occurs to the GUI's container itself, for example, if a user adds or removes an object from the interface. Related listener: ContainerListener. KeyEvent: Represents an event in which the user presses, types or releases a key. Related listener: KeyListener. WindowEvent: Represents an event relating to a window, for example, when a window is closed, activated or deactivated. Related listener: WindowListener. MouseEvent: Represents any event related to a mouse, such as when a mouse is clicked or pressed. Related listener: MouseListener. Note that multiple listeners and event sources can interact with one another. For example, multiple events can be registered by a single listener, if they are of the same type. This means that, for a similar set of components that perform the same type of action, one event listener can handle all the events. Similarly, a single event can be bound to multiple listeners, if that suits the program's design (although that is less common). ii. Thread A thread, in the context of Java, is the path followed when executing a program. All Java programs have at least one thread, known as the main thread, which is created by the Java Virtual Machine (JVM) at the program s start, when the main() method is invoked with the main thread. In Java, creating a thread is accomplished by implementing an interface and extending a class. Every Java thread is created and controlled by the java.lang.thread class. Java is a multi-threaded application that allows multiple thread execution at any particular time. In a single-threaded application, only one thread is executed at a time because the application or program can handle only one task at a time. For example, a single-threaded application may allow for the typing of words. However, this single thread requires an additional single thread allowing for the recording of keystrokes in order to type the words. Thus, a single-threaded application records the keystrokes, allowing the next single-threaded application (the typing of words) to follow. However, a multi-threaded application allows for the handling of both tasks (recording and typing the keystrokes) within one application. When a thread is created, it is assigned a priority. The thread with higher priority is executed first, followed by lower-priority threads. The JVM stops executing threads under either of the following conditions: Page 2

3 If the exit method has been invoked and authorized by the security manager All the daemon threads of the program have died iii. Applet An applet is a Java program that runs in a Web browser. An applet can be a fully functional Java application because it has the entire Java API at its disposal. There are some important differences between an applet and a standalone Java application, including the following An applet is a Java class that extends the java.applet.applet class. A main() method is not invoked on an applet, and an applet class will not define main(). Applets are designed to be embedded within an HTML page. When a user views an HTML page that contains an applet, the code for the applet is downloaded to the user's machine. A JVM is required to view an applet. The JVM can be either a plug-in of the Web browser or a separate runtime environment. The JVM on the user's machine creates an instance of the applet class and invokes various methods during the applet's lifetime. Applets have strict security rules that are enforced by the Web browser. The security of an applet is often referred to as sandbox security, comparing the applet to a child playing in a sandbox with various rules that must be followed. Other classes that the applet needs can be downloaded in a single Java Archive (JAR) file. Life Cycle of an Applet Four methods in the Applet class gives you the framework on which you build any serious applet init This method is intended for whatever initialization is needed for your applet. It is called after the param tags inside the applet tag have been processed. start This method is automatically called after the browser calls the init method. It is also called whenever the user returns to the page containing the applet after having gone off to other pages. stop This method is automatically called when the user moves off the page on which the applet sits. It can, therefore, be called repeatedly in the same applet. Page 3

4 destroy This method is only called when the browser shuts down normally. Because applets are meant to live on an HTML page, you should not normally leave resources behind after a user leaves the page that contains the applet. paint Invoked immediately after the start() method, and also any time the applet needs to repaint itself in the browser. The paint() method is actually inherited from the java.awt. iv. AWT Abstract Window Toolkit(AWT) AWT contains large number of classes and methods that allows you to create and manage graphical user interface ( GUI ) applications, such as windows, buttons, scroll bars,etc. The AWT was designed to provide a common set of tools for GUI design that could work on a variety of platforms. The tools provided by the AWT are implemented using each platform's native GUI toolkit, hence preserving the look and feel of each platform. This is an advantage of using AWT.But the disadvantage of such an approach is that GUI designed on one platform may look different when displayed on another platform. AWT is the foundation upon which Swing is made i.e Swing is a set of GUI interfaces that extends the AWT. But now a days AWT is merely used because most GUI Java programs are implemented using Swing because of its rich implementation of GUI controls and light-weighted nature. AWT Hierarchy Page 4

5 Component class Component class is at the top of AWT hierarchy. Component is an abstract class that encapsulates all the attributes of visual component. A component object is responsible for remembering the current foreground and background colors and the currently selected text font. Container Container is a component in AWT that contains another component like button, text field, tables etc. Container is a subclass of component class. Container class keeps track of components that are added to another component. Panel Panel class is a concrete subclass of Container. Panel does not contain title bar, menu bar or border. It is container that is used for holding components. Window class Window class creates a top level window. Window does not have borders and menubar. Frame Frame is a subclass of Window and have resizing canvas. It is a container that contain several different components like button, title bar, textfield, label etc. In Java, most of the AWT applications are created using Frame window. Frame class has two different constructors, Frame() throws HeadlessException Frame(String title) throws HeadlessException Creating a Frame There are two ways to create a Frame. They are, 1. By Instantiating Frame class 2. By extending Frame class Page 5

6 Creating Frame Window by Instantiating Frame class import java.awt.*; public class Testawt { Testawt() { Frame fm=new Frame(); //Creating a frame. Label lb = new Label("welcome to java graphics"); //Creating a label fm.add(lb); //adding label to the frame. fm.setsize(300, 300); //setting frame size. fm.setvisible(true); //set frame visibilty true. } public static void main(string args[]) { Testawt ta = new Testawt(); } } Page 6

7 Creating Frame window by extending Frame class package testawt; import java.awt.*; import java.awt.event.*; public class Testawt extends Frame { public Testawt() { Button btn=new Button("Hello World"); add(btn); //adding a new Button. setsize(400, 500); //setting size. settitle("studytonight"); //setting title. setlayout(new FlowLayout()); //set default layout for frame. setvisible(true); //set frame visibilty true. } public static void main (String[] args) { Testawt ta = new Testawt(); //creating a frame. } } Page 7

8 Points to Remember: 1. While creating a frame (either by instantiating or extending Frame class), Following two attributes are must for visibility of the frame: o o setsize(int width, int height); setvisible(true); 2. When you create other components like Buttons, TextFields, etc. Then you need to add it to the frame by using the method - add(component's Object); 3. You can add the following method also for resizing the frame - setresizable(true); Page 8

9 v. Vector Vector implements a dynamic array. It is similar to Array List, but with two differences Vector is synchronized. Vector contains many legacy methods that are not part of the collections framework. Vector proves to be very useful if you don't know the size of the array in advance or you just need one that can change sizes over the lifetime of a program. Following is the list of constructors provided by the vector class. Sr.No. Constructor & Description 1 Vector( ) This constructor creates a default vector, which has an initial size of Vector(int size) This constructor accepts an argument that equals to the required size, and creates a vector whose initial capacity is specified by size. 3 Vector(int size, int incr) This constructor creates a vector whose initial capacity is specified by size and whose increment is specified by incr. The increment specifies the number of elements to allocate each time that a vector is resized upward. 4 Vector(Collection c) This constructor creates a vector that contains the elements of collection c. Apart from the methods inherited from its parent classes, Vector defines the following methods Page 9

10 Sr.No. Method & Description 1 void add(int index, Object element) Inserts the specified element at the specified position in this Vector. 2 boolean add(object o) Appends the specified element to the end of this Vector. 3 boolean addall(collection c) Appends all of the elements in the specified Collection to the end of this Vector, in the order that they are returned by the specified Collection's Iterator. 4 boolean addall(int index, Collection c) Inserts all of the elements in in the specified Collection into this Vector at the specified position. 5 void addelement(object obj) Adds the specified component to the end of this vector, increasing its size by one. 6 int capacity() Returns the current capacity of this vector. 7 void clear() Removes all of the elements from this vector. 8 Object clone() Returns a clone of this vector. 9 boolean contains(object elem) Tests if the specified object is a component in this vector. 10 boolean containsall(collection c) Page 10

11 Returns true if this vector contains all of the elements in the specified Collection. 11 void copyinto(object[] anarray) Copies the components of this vector into the specified array. 12 Object elementat(int index) Returns the component at the specified index. 13 Enumeration elements() Returns an enumeration of the components of this vector. 14 void ensurecapacity(int mincapacity) Increases the capacity of this vector, if necessary, to ensure that it can hold at least the number of components specified by the minimum capacity argument. 15 boolean equals(object o) Compares the specified Object with this vector for equality. 16 Object firstelement() Returns the first component (the item at index 0) of this vector. 17 Object get(int index) Returns the element at the specified position in this vector. 18 int hashcode() Returns the hash code value for this vector. 19 int indexof(object elem) Searches for the first occurence of the given argument, testing for equality using the equals method. 20 int indexof(object elem, int index) Searches for the first occurence of the given argument, beginning Page 11

12 the search at index, and testing for equality using the equals method. 21 void insertelementat(object obj, int index) Inserts the specified object as a component in this vector at the specified index. 22 boolean isempty() Tests if this vector has no components. 23 Object lastelement() Returns the last component of the vector. 24 int lastindexof(object elem) Returns the index of the last occurrence of the specified object in this vector. 25 int lastindexof(object elem, int index) Searches backwards for the specified object, starting from the specified index, and returns an index to it. 26 Object remove(int index) Removes the element at the specified position in this vector. 27 boolean remove(object o) Removes the first occurrence of the specified element in this vector, If the vector does not contain the element, it is unchanged. 28 boolean removeall(collection c) Removes from this vector all of its elements that are contained in the specified Collection. 29 void removeallelements() Removes all components from this vector and sets its size to zero. Page 12

13 30 boolean removeelement(object obj) Removes the first (lowest-indexed) occurrence of the argument from this vector. 31 void removeelementat(int index) removeelementat(int index). 32 protected void removerange(int fromindex, int toindex) Removes from this List all of the elements whose index is between fromindex, inclusive and toindex, exclusive. 33 boolean retainall(collection c) Retains only the elements in this vector that are contained in the specified Collection. 34 Object set(int index, Object element) Replaces the element at the specified position in this vector with the specified element. 35 void setelementat(object obj, int index) Sets the component at the specified index of this vector to be the specified object. 36 void setsize(int newsize) 37 int size() Sets the size of this vector. Returns the number of components in this vector. 38 List sublist(int fromindex, int toindex) Returns a view of the portion of this List between fromindex, inclusive, and toindex, exclusive. 39 Object[] toarray() Returns an array containing all of the elements in this vector in the Page 13

14 correct order. 40 Object[] toarray(object[] a) Returns an array containing all of the elements in this vector in the correct order; the runtime type of the returned array is that of the specified array. 41 String tostring() Returns a string representation of this vector, containing the String representation of each element. 42 void trimtosize() Trims the capacity of this vector to be the vector's current size. Example The following program illustrates several of the methods supported by this collection import java.util.*; public class VectorDemo { public static void main(string args[]) { // initial size is 3, increment is 2 Vector v = new Vector(3, 2); System.out.println("Initial size: " + v.size()); System.out.println("Initial capacity: " + v.capacity()); v.addelement(new Integer(1)); v.addelement(new Integer(2)); v.addelement(new Integer(3)); v.addelement(new Integer(4)); System.out.println("Capacity after four additions: " + v.capacity()); Page 14

15 v.addelement(new Double(5.45)); System.out.println("Current capacity: " + v.capacity()); v.addelement(new Double(6.08)); v.addelement(new Integer(7)); System.out.println("Current capacity: " + v.capacity()); v.addelement(new Float(9.4)); v.addelement(new Integer(10)); System.out.println("Current capacity: " + v.capacity()); v.addelement(new Integer(11)); v.addelement(new Integer(12)); System.out.println("First element: " + (Integer)v.firstElement()); System.out.println("Last element: " + (Integer)v.lastElement()); if(v.contains(new Integer(3))) System.out.println("Vector contains 3."); // enumerate the elements in the vector. Enumeration venum = v.elements(); System.out.println("\nElements in vector:"); while(venum.hasmoreelements()) System.out.print(vEnum.nextElement() + " "); System.out.println(); } Page 15

16 } This will produce the following result Output Initial size: 0 Initial capacity: 3 Capacity after four additions: 5 Current capacity: 5 Current capacity: 7 Current capacity: 9 First element: 1 Last element: 12 Vector contains 3. Elements in vector: Page 16

17 Q2. Discuss the features of JAVA programming language. How JAVA programming language is better than previous programming languages? Features of Java The prime reason behind creation of Java was to bring portability and security feature into a computer language. Beside these two major features, there were many other features that played an important role in moulding out the final form of this outstanding language. Those features are : 1) Simple Java is easy to learn and its syntax is quite simple, clean and easy to understand.the confusing and ambiguous concepts of C++ are either left out in Java or they have been re-implemented in a cleaner way. Eg : Pointers and Operator Overloading are not there in java but were an important part of C++. 2) Object Oriented In java everything is Object which has some data and behaviour. Java can be easily extended as it is based on Object Model. 3) Robust Java makes an effort to eliminate error prone codes by emphasizing mainly on compile time error checking and runtime checking. But the main areas which Java improved were Memory Management and mishandled Exceptions by introducing automatic Garbage Collector and Exception Handling. 4) Platform Independent Unlike other programming languages such as C, C++ etc which are compiled into platform specific machines. Java is guaranteed to be write-once, run-anywhere language. On compilation Java program is compiled into bytecode. This bytecode is platform independent and can be run on any machine, plus this bytecode format also provide security. Any machine with Java Runtime Environment can run Java Programs. Page 17

18 5) Secure When it comes to security, Java is always the first choice. With java secure features it enable us to develop virus free, temper free system. Java program always runs in Java runtime environment with almost null interaction with system OS, hence it is more secure. 6) Multi Threading Java multithreading feature makes it possible to write program that can do many tasks simultaneously. Benefit of multithreading is that it utilizes same memory and other resources to execute multiple threads at the same time, like While typing, grammatical errors are checked along. 7) Architectural Neutral Compiler generates bytecodes, which have nothing to do with a particular computer architecture, hence a Java program is easy to intrepret on any machine. 8) Portable Java Byte code can be carried to any platform. No implementation dependent features. Everything related to storage is predefined, example: size of primitive data types Page 18

19 9) High Performance Java is an interpreted language, so it will never be as fast as a compiled language like C or C++. But, Java enables high performance with the use of just-in-time compiler. New Features of JAVA 8 Below mentioned are some of the core upgrades done as a part of Java 8 release. Just go through them quickly, we will explore them in details later. Enhanced Productivity by providing Optional Classes feature, Lamda Expressions, Streams etc. Ease of Use Improved Polyglot programming. A Polyglot is a program or script, written in a form which is valid in multiple programming languages and it performs the same operations in multiple programming languages. So Java now supports such type of programming technique. Improved Security and performance. Page 19

20 Java vs. Other Programming Languages: Java is, arguably, one of the most popular programming languages amongst developers and is used to create web applications, customized software and web portals, including ecommerce and m-commerce solutions. For many developers, programming languages begin and end with Java. While there is no doubt Java has been going strong over the years and therefore must be doing a whole lot of things right, it will be a mistake to think there is no other language as good as Java. The fact is, every language has strengths and weaknesses; yes even Java has a bunch of lacunae that get overlooked by programmers because of the truckload of benefits it brings to the table. As a programmer, it s important to compare Java with other programing languages so that you are able to choose the best language for a particular project. This article compares Java to some other commonly used languages and tries to find out whether Java comes out on top. (Note: We have not drawn comparisons with each and every feature offered by the languages covered in this article. We have identified certain key features offered by them and talk about how they compare with similar features in Java.) 1. Python Python is a high-level language which fully supports object-oriented programming. Java on the other hand is not a pure object-oriented language. Python is a powerful easy-to-use scripting language that excels as a glue language because it connects system components, whereas Java is characterized as a low-level implementation language. One of the key differences between the two is that Python programs are shorter as compared to Java programs. Let s for instance see the example of Hello World : Hello World in Java: public class example{ public static void main(string[] args) { System.out.println( hello world );} Page 20