Mobile Application Development ( IT 100 ) Assignment - I

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1 1. a) Explain various control structures available in Java. (6M ) Various control structures available in Java language are: 1. if else 2. switch 3. while 4. do while 5. for 6. break 7. continue 8. return 1. if else statement: This statement helps to select one out of two possibilities based on the given conditions. This statement is also called conditional branch statement. General form of the if statement is : if(condition) statement1; else statement2; here each statement may be a single statement or compound statement enclosed in a curly braces. The condition is any expression that returns a boolean value. The else clause is optional. If the condition is true, then statement1 is executed. Otherwise, statement2 ( if exists ) is executed. Ex: int a=10, b=20; if(a<b) System.out.println( a is smaller ); else System.out.println( b is smaller ); nested if else statement : Nested if else statement is made by placing one if-else in another if-else statement. Nested if-else statement helps to select one out of many choices. The general form of the nested if-else statement is: if(condition1) if(condition2) if(condition3) statement4 else statement3 else statement2 else statement1 In the nested if-else statement, the outermost if is evaluated first. If the condition1 tested is false, the statement1 in the outermost else is evaluated and if-else ends. If the condition1 results in true, the control goes to execute the next inner if statement. If condition2 is false, statement2 is executed. Otherwise, condition3 is evaluated. If condition3 is false, statement3 is executed, otherwise statement4 is executed. 2. switch statement: The switch statement is Java s multi way branching statement. It helps to select one out of many choices. The general form of switch statement is: switch(expression) case val1: statement1; break; case val2: statement2; break; case val3: statement3; break;... case valn: statement; break; 3. while statement: The while statement is used for looping or iterating a block of statements while the given condition is true. Department of Information Technology, BEC, Bapatla Page 1

2 The general form of the while statement is : While(condition) statements; The condition, when evaluated, must result in the boolean value true or false. As long as the condition gives true, the statement block will be executed. When the condition becomes false, the control leaves the block statement. Ex : int n=10; while(n>0) System.out.println(n); n--; 4. do while statement: The do while control statement is used for looping a block of statements while the given condition is true. In this structure, the condition is tested at the end of the block. This is in contrast to the while statement, where the condition is tested at the start of the block. The general form of the do while statement is: do statements; while(condition); The condition tested can be any expression that will yield a boolean value. As long as the condition tested is true, the whole block will be executed. Once the condition tested becomes false, the control leaves the block. It is to be noted that the block is executed once before the condition is tested. Therefore, even if the condition tested is false at the first instance itself, the block statements are executed once. Ex: int n=10; int sum=0; do sum+=n--; while(n>0); System.out.println( sum of numbers from 1 to 10 is : +sum); 5. for statement : The general form of the for loop is : for(initialization;condition;iteration) //body The for loop operates as follows. When the loop first starts, the initialization portion of the loop is executed. Next, condition is evaluated. This must be a boolean expression. This tests the loop control variable against a target value. If this expression is true, then the body of the loop is executed. If it is false, the loop terminates. Next, the iteration portion of the loop is executed. Iteration portion increments or decrements the loop control variable. The loop then iterates, first evaluating the conditional expression, then executing the body of the loop, and then executing the iteration expression with each pass. This process repeates until the controlling expression is false. Ex: for(int i=1; i<=10; i++) System.out.println(i); 6. break statement : break statement is used in for, while and do..while statements to exit the loop. It is also used in switch statement to terminate the statement sequence. Ex: i=0; while(i<10) if(i==5) break; else System.out.print(i+ );i++; 7. continue statement: The continue statement is used inside control blocks. When the continue statement is executed, the control skips the remaining portion of the loop and goes to the beginning of the loop and continue. Ex: int i=0; While(i<10) If(i%2==0) continue; System.out.println(i); i++; Department of Information Technology, BEC, Bapatla Page 2

3 8. return statement : The return statement is used to explicitly return from a method. That is, it causes program control to transfer back to the caller of the method. void m() int i=10; if(i>5) return; else System.out.println(i); 1(b). Explain various access specifiers available in Java. ( 4M ) Using Encapsulation, you can control what parts of a program can access the members of a class. By controlling access, you can prevent misuse. How a member of a class can be accessed is determined by the access specifier that modifies its declaration. Java supports the 4 types of access specifiers: 1. public 2. private 3. protected 4. no access specifier ( default ) When a member of a class is specified as public, that member can be accessed by any other code. When a member of a class is specified as private, that member can only be accessed by other members of its class. When a member of a class is declared as protected, then that member is accessible in the whole package ( package is a collection of classes ) and in the subclass of another package. When a member of a class is declared without any access specifier, then that member is accessible in the whole package only. In other packages it is not accessible. 2(a). Explain about Nested and Inner classes in Java. ( 4M ) A class defined within another class is known as nested class. A nested class has access to the members, including private members, of the class in which it is nested. However, the enclosing class does not have access to the members of the nested class. A nested class that is declared directly within its enclosing class scope is a member of its enclosing class. There are two types of nested classes: static and non-static. A static nested class is one that has the static modifier applied. An inner class is a non-static nested class. A nested class is a member of its enclosing class. Non-static nested classes (inner classes) have access to other members of the enclosing class, even if they are declared private. // Demonstrate an inner class. class Outer int outer_x = 100; void test() Inner inner = new Inner(); inner.display(); // this is an inner class class Inner void display() System.out.println("display: outer_x = " + outer_x); Department of Information Technology, BEC, Bapatla Page 3

4 class InnerClassDemo public static void main(string args[]) Outer outer = new Outer(); outer.test(); o/p: display: outer_x = 100 In the program, an inner class named Inner is defined within the scope of class Outer. Therefore, any code in class Inner can directly access the variable outer_x. An instance method named display( ) is defined inside Inner. This method displays outer_x on the standard output stream. The main( ) method of InnerClassDemo creates an instance of class Outer and invokes its test( ) method. That method creates an instance of class Inner and the display( ) method is called. 2(b). Explain the buzzwords of Java programming language. ( 6M ) The following are the list of Java buzzwords. Simple Secure Portable Object-oriented Robust Multithreaded Architecture-neutral Interpreted High performance Distributed Dynamic Simple: Because Java inherits the C/C++ syntax and many of the object oriented features of C++, it is easy to learn and use the Java Programming Language. The syntax for Java is, indeed, a cleaned-up version of the syntax for C++. There is no need for header files, pointer arithmetic (or even a pointer syntax), structures, unions, operator overloading, virtual base classes, and so on. If you know C++, you will find the transition to the Java syntax easy. Secure: Java is intended to be used in networked/distributed environments. Toward that end, a lot of emphasis has been placed on security. Java enables the construction of virus-free, tamper-free systems. Java s security feature allows Java program from doing overrunning the runtime stack, corrupting memory outside its own process space. Java has digitally signed classes. With a signed class, you can be sure of who wrote it. Any time you trust the author of the class, the class can be allowed more privileges on your machine. Object Oriented: object-oriented design is a technique for programming that focuses on the data (= objects) and on the interfaces to that object. To make an analogy with carpentry, an object-oriented carpenter would be mostly concerned with the chair he was building, and secondarily with the tools used to make it; a non-object-oriented carpenter would think primarily of his tools. The object-oriented facilities of Java are essentially those of C++.Java language is Object Oriented language. The Object model of Java is simple and easy to extend. Robust: Java programs are more robust means that programs run without crashes. Java provides automatic memory management and mechanism for handling of exceptional conditions (runtime errors). In a Java program all runtime errors can and should be managed by your program. Java force you to find mistakes early in program development. As Java is strictly typed language, it checks your code at both at compile time and runtime for type safety. Multithreaded: Java allows you to create and run multiple threads in a program at a time. This allows you to write programs that do many things simultaneously. Architecture-neutral: The compiler generates an architecture-neutral object file format the compiled code is executable on many processors, given the presence of the Java run time system. The Java compiler Department of Information Technology, BEC, Bapatla Page 4

5 does this by generating bytecode instructions which have nothing to do with a particular computer architecture. Rather, they are designed to be both easy to interpret on any machine and easily translated into native machine code on the fly. Interpreted: The Java interpreter can execute Java byte codes directly on any machine to which the interpreter has been ported. High Performance: Java byte code was designed so that it easy to translate into native machine (machine code for the particular CPU the application is running on) code for very high performance by using JIT ( Just In-Time ) compiler. Distributed: Java is designed for developing applications in distributed environment using Java RMI ( Remote Method Invocation ). Dynamic: Java programs carry with them substantial amounts of run-time type information that is used to verify and resolve accesses to objects at run time. This makes it possible to dynamically link code in a safe and easy manner. 3 a) Define an interface and write a program for implementing multiple inheritance. (5M) Interface: Using the keyword interface, you can fully abstract a class interface from its implementation. That is, using interface, you can specify what a class must do, but not how it does it. Interfaces are syntactically similar to classes, but they lack instance variables, and their methods are declared without any body. the general form of an interface: access interface name return-type method-name1(parameter-list); return-type method-name2(parameter-list); type final-varname1 = value; type final-varname2 = value; //... return-type method-namen(parameter-list); type final-varnamen = value; Here, access is either public or not used. name is the name of the interface. Interfaces help to realize the functionality of multiple inheritance in Java. Program for implementing multiple inheritance: // multiple inheritance (MultipleInherit.java) interface Exam void percent_cal(); class Student String name; int roll_no,mark1,mark2; Student(String n, int r, int m1, int m2) name=n; roll_no=r; mark1=m1; mark2=m2; void display() Department of Information Technology, BEC, Bapatla Page 5

6 System.out.println ("Name of Student: "+name); System.out.println ("Roll No. of Student: "+roll_no); System.out.println ("Marks of Subject 1: "+mark1); System.out.println ("Marks of Subject 2: "+mark2); class Result extends Student implements Exam Result(String n, int r, int m1, int m2) super(n,r,m1,m2); public void percent_cal() int total=(mark1+mark2); float percent=total*100/200; System.out.println ("Percentage: "+percent+"%"); void display() super.display(); class MultipleInherit public static void main(string args[]) Result R = new Result("Ravi",12,93,84); R.display(); R.percent_cal(); o/p: Name of Student: Ravi Roll No. of Student: 12 Marks of Subject 1: 93 Marks of Subject 2: 84 Percentage: 88.0% 3. b) Explain about super keyword with an example program in Java. (5M) Whenever a subclass needs to refer to its immediate super class, it can access super class members by using super keyword. super keyword is used for two purposes. 1. To call the super class constructor 2. To access a member of a super class that has been hidden by a member of a subclass. 1. super( ) To call super class constructor : A subclass can call a constructor method defined by its superclass by use of the following form of super: super(parameter-list); Here, parameter-list specifies any parameters needed by the constructor in the superclass. super() must always be the first statement executed inside a subclass constructor. 2. second use of super keyword: super keyword is used to access the members of a superclass hidden by sub class members in the subclass. form: super.member Department of Information Technology, BEC, Bapatla Page 6

7 Here, member can be a method or an instance variable. Example Program: //usage of super keyword class Vehicle int speed=50; Vehicle() System.out.println("Vehicle is created"); class Bike extends Vehicle int speed=100; Bike() super(); System.out.println("Bike is created"); void display() System.out.println(super.speed); public static void main(string args[]) Bike b=new Bike(); b.display(); o/p: Vehicle is created Bike is created a) Explain method overriding with an example program. (5M) A method in a subclass has the same name, type of the variables and order of the variables as a method in its super class, then the method in the subclass is said to be override the method in the super class. The process is called Method Overriding. In such process the method defined by the super class will be hidden. When the method is called, the method defined in the super class has invoked and executed instead of the method in the super class. The super reference followed by the dot (.) operator may be used to access the original super class version of that method from the subclass. Note 1: Method Overriding occurs only when the name and type signature of the two methods are identical. Note 2: If method name is identical and the type signature is different, then the process is said to be Method Overloading. Example Program: class A int i,j; public A(int a,int b) i=a; j=b; void show() System.out.println("i="+i+"\n"+"j="+j); Department of Information Technology, BEC, Bapatla Page 7

8 class B extends A int k; B(int a,int b,int c) super(a,b); k=c; void show() System.out.println("k="+k); class override public static void main(string[] args) B obj=new B(1,2,3); obj.show(); o/p: k=3 4. b) Explain the abstract class with an example program. (5M) Classes from which objects cannot be instantiated with new operator are called Abstract Classes. Each abstract class contains one or more abstract methods. In a class if there exist any method with no method body is known as abstract method. An abstract method is declared as syntax: abstract type method-name (parameter-list); abstract is a keyword used for declaring abstract methods. To declare a class as abstract, use the abstract keyword in front of the class keyword at the beginning of the class declaration. No objects are created to an abstract class. For the abstract methods, the implementation code will be defined in its subclass. Example Program: abstract class figure double dim1,dim2; figure(double x,double y) dim1=x; dim2=y; abstract double area(); class rectangle extends figure rectangle(double a,double b) super(a,b); double area() System.out.println("Rectangle Area"); return dim1*dim2; Department of Information Technology, BEC, Bapatla Page 8

9 class triangle extends figure triangle(double x,double y) super(x,y); double area() System.out.println("Triangle Area"); return dim1*dim2/2; class abs public static void main(string[] args) //figure obj=new figure(10,10); //error rectangle obj1=new rectangle(9,5); System.out.println("Area="+obj1.area()); triangle obj2=new triangle(10,8); figure a; a=obj2; System.out.println("Area="+a.area()); o/p: Rectangle Area Area=45.0 Triangle Area Area=40.0 Note: We cannot declare any abstract constructors. Concrete (general) methods are also being inside any abstract class. Abstract classes are used for general purposes. An abstract class must be subclass and override it methods. Super class has only method name and signature end with semicolon. Abstract classes cannot be used to instantiate objects, but they can used to create object reference due to Java supports Run-time Polymorphism. 5. a) Explain about static keyword in Java. (5M) Generally each class contains instance variables and instance methods. Every time the class is instantiated, a new copy of each of them is created. They are accessed using the objects with the dot operator. If we define a member that is common to all the objects and accessed without using a particular object i.e., the member belong to the class as a whole rather than the objects created from the class. Such facility can be possible by using static keyword. The members declared with the static keyword are called static members. Example: static int x; static int show(int x,int y); Department of Information Technology, BEC, Bapatla Page 9

10 The important points about static class members are Static members can be accessible without the help of objects. It can be accessible by using the syntax: classname.members; A static method can access only static members (data + method) A static block will be executed first (before main). Static variables are initialized first A static member data store the same memory for all objects Static method cannot be referred to this or super in anyway. Example 1: class static1 static int c=0; void increment() c++; void display() System.out.println("Value="+c); class Mstatic public static void main(string[] args) static1 s=new static1(); static1 s1=new static1(); static1 s2=new static1(); s.increment(); s1.increment(); s2.increment(); s.display(); s1.display(); s2.display(); o/p: value=3 value=3 value=3 without static keyword s1 s2 s3 with static keyword 0 s1 s2 s3 Example 2: class static1 static int a=10; static int b=34; static void call() Department of Information Technology, BEC, Bapatla Page 10

11 System.out.println("Value1="+a); class Mstatic1 public static void main(string[] args) static1.call(); System.out.println("Value2="+static1.b); o/p: Example 3: Value1=10 Value2=34 class static2 static int a=10,b; static void method(int x) System.out.println("x="+x); System.out.println("a="+a); System.out.println("b="+b); static System.out.println("static block"); b=a*4; class Mstatic2 public static void main(string[] args) static2.method(35); o/p: static block x=35 a=10 b=40 5. b) What is runtime polymorphism. Write a program for implementing it. (5M) Method Overriding forms on the basis of Java concept Dynamic Method Dispatch. It is the mechanism by which a call to an overridden function is resolved at run time, rather than compile time. And this method is very useful to show for Java implements run-time polymorphism. Super class reference variable can refer to a subclass object is the principle to resolve calls to overridden methods at run time. If a super class contains a method that is overridden by a subclass, then when different types of objects are referred to through a super class reference variable, different version of the methods are executed and the determination is made at run time. // run-time polymorphism example program class Figure double dim1; double dim2; Department of Information Technology, BEC, Bapatla Page 11

12 Figure(double a, double b) dim1 = a; dim2 = b; double area() System.out.println("Area for Figure is undefined."); return 0; class Rectangle extends Figure Rectangle(double a, double b) super(a, b); // override area for rectangle double area() System.out.println("Inside Area for Rectangle."); return dim1 * dim2; class Triangle extends Figure Triangle(double a, double b) super(a, b); // override area for right triangle double area() System.out.println("Inside Area for Triangle."); return dim1 * dim2 / 2; class FindAreas public static void main(string args[]) Figure f = new Figure(10, 10); Rectangle r = new Rectangle(9, 5); Triangle t = new Triangle(10, 8); Figure figref; figref = r; System.out.println("Area is " + figref.area()); figref = t; System.out.println("Area is " + figref.area()); figref = f; System.out.println("Area is " + figref.area()); o/p: Inside Area for Rectangle. Area is 45.0 Inside Area for Triangle. Area is 40.0 Area for Figure is undefined. Area is a) Explain the final keyword with an example program. (5M) A final is a keyword used for three purposes. They are Department of Information Technology, BEC, Bapatla Page 12

13 a) final as constant: A variable can be declared as constant with the keyword final. It must be initialized while declaring. One we initialized the variable with final keyword it cannot be modified in the program. This is similar to the const in C/C++. Example: final int x = 10; x = 45 //error b) final to prevent overriding: A method that is declared as final cannot be overridden in a subclass method. It the methods that are declared as private are implicitly final. Example: class A final void show( ) System.out.println( Hello ); class B extends A void show( ) //error System.out.println( Hai ); c) final to prevent inheritance: The class that is declared as final implicitly declares all of its methods as final. The class cannot be extended to its subclass. Example: final class A void show( ) System.out.println( Hello ); class B extends A //error void show( ) System.out.println( Hai ); 6. b) Write about Interfaces. (5M) Interface is a collection of method declaration and constants that one or more classes of objects will use. Interface definition is same as class except that it consists of the methods that are declared have no method body. They end with a semicolon after the parameter list. Syntax for an interface is as follows. Syntax: <access specifier> interface <it-name> type varname1=value; type varname2=value;.. returntype method-name1(parameter-list); returntype method-name2(parameter-list);. Here, access specifier is public or not used. public access specifier indicates that the interface can be used by any class. Otherwise, the interface will accessible to class that are defined in the same package as in the interface. interface keyword is used to declare the class as an interface. it-name is the name of the interface and it is a valid identifier. Department of Information Technology, BEC, Bapatla Page 13

14 Variables declared inside the interface are implicitly final and static. They cannot be changed in the implementing class. All the methods in an interface are implicitly abstract methods. Method implementation is given in later stages. The main difference between a class and interface is class contains methods with method body and the interface contains only abstract methods. Example: public interface shape int radius=2; public void area(int a); Implementing Interfaces: Once an interface has been defined, one or more classes can implement that interface. implements keyword used for implementing the classes. The syntax of implements is as follows. Syntax: class class-name implements interface1, interface2,.. interfacen.. // class body.. If a class implements more than one interface, the interfaces are separated with a comma operator. The methods that implement an interface must be declared public. Also type signature of the implementing method must match exactly the type signature specified in the interface definition. Interface methods are similar to the abstract classes so, that it cannot be instantiated. Interface methods can also be accessed by the interface reference variable refer to the object of subclass. The method will be resolved at run time. This process is similar to the super class reference to access a subclass object. Example: interface it1 int x=10,y=20; public void add(int a,int b); public void sub(int a,int b); class it2 implements it1 public void add(int s,int w) System.out.println("Addition="+(s+w)); public void sub(int s,int w) System.out.println("Subtraction="+(s-w)); public static void main(string[] args) it2 obj=new it2(); it1 ref; ref=obj; System.out.println(ref.x+ref.y); o/p: Addition=7 Subtraction=3 30 Department of Information Technology, BEC, Bapatla Page 14