Chapter 5 Revision Tour 3

CHAPTER-5

JAVA GUI PROGRAMMING REVISION TOUR - III [Methods etc.]

Brief Summary of the Chapter:

In this chapter concept related with Class, Objects, Constructors and methods are discussed. In Java method or function   is a sequence of some declaration and executable statements.
In Java, which is strictly Object-oriented, any action can take place through methods and methods have to be exist as a part of the class.

Key points:

·   Methods is a sequence of statements that carry out specific tasks. ·   Methods returns a value through return statement.
·   Class is a blue print for creating objects of a certain charactersticks. ·   Class contains fields and methods.

·   Classes created through keyword class.

·   Object is instance of a class created through new operator.

·     Constructor is method with the same name as of that class it is used to initialized object of
      class.

·   Constructor can either be parameterized or non-parameterized. ·   The “this” keyword is used to refer to current object.

SOLVED QUESTIONS

1. In java, methods reside in __________.

(a) Function (b) Library (c) Classes (d) Object Ans: (c) Classes

2. The number and type of arguments of a method are known as _____________.

(a) Parameter list (b) Calling (c) Definition (d)None to these.

Ans: (a) Parameter list

3. The first line of method definition that tells about the type of return value along with number and type of arguments is called_____________.

(a) Class (b) Object (c) Prototype (d) Datatype Ans: (c) Prototype

4. A member method having the same name as that of its class is called______method.

(a) Destructor (b) Constructor (c) Object (d) Variable

Ans: (b) Constructor

5. A constructor method has__________return type.

(a) float (b) void (c) no (d) int

Ans: (c) no

6.   A_________constructor takes no arguments.

(a) Copy constructor (b) Non-Parameterized constructor (c) Parameterized constructor Ans: (b) Non-Parameterized constructor

7. A_________constructor creates objects through values passed to it.

(a) Copy constructor (b) Default constructor (c) Parameterized constructor Ans: (c) Parameterized constructor

8. The keyword_________refers to current object.

(a) void (b) goto (c) this (d) null

Ans: (c) this

9. Define a method. What is method prototype and signature?

Ans:A message to an object is a call to the object’s method requesting that it performs some specified action.

int absval(int a) {

return(a<0?-a:a);

}

The first line of the method definition is the prototype of the method i.e. the prototypes of method defined above is:

int absval(int a)

10. How are following passed in Java: (i) primitive types (ii) reference types? Ans: (i) By Value (ii) By reference

11. The String objects being reference types are passed by reference but changes, if any, are not reflected back to them. Why?

Ans: The String objects are immutable in Java, which   means once they are created, the cannot
changed. That is why, even though Strings are passed by reference, they cannot be changed.

12.At what time is the constructor method automatically invoked?

Ans: Every time an object is created, the constructor method is automatically invoked.

13. What are Composite and user defined data types?

Ans: The data types that are based on fundamental or primitive data types, are known as Composite Datatypes. Since these data types are created by users, these are also known as User Defined
Datatypes.

14. Can you refer to a class as a composite type/ user-defined type?
Ans: Yes, class is referred to as   a composite type/user defined type.

15.How is a constructor invoked?

Ans: A constructor is automatically called with a new operator in order to create a new object.

16. Which method of a class is invoked just once for an object? When? Ans: The constructor method.

It is invoked for initializing values of the object at the time of its creation.

17. Passing the address means call by value or call by reference? Ans: Call by reference.

18. What’s wrong with the following constructor definition for the class PlayInfo?
            public void PlayInfo( int sticks)

{

nsticks = sticks;

}

Ans: A constructor cannot have a return type, not even void.

19. How many values can be returned from a method?

Ans: Only one value can be returned from a   method though a method can have multiple return statements but only one gets executed which is reached first and thus returns the value.

20. What do you understand by Class and Object?

Ans: The basic unit of OOP is the Class. It can be described as a blue print of Objects. In other words,
an Object is an instance of a class. A JAVA program may have various class definitions.
An Object is an entity having a unique Identity, characteristics (Properties) and Behavior (Methods).

22. What is the difference between instance and static variable?

Ans: Instance Variable- These data member are created for every object of the class i.e. replicated with objects.

Class variable (static)- These data members that is declared once for each class and all objects share these members. Only a single copy is maintained in

the memory. These are declared with static keyword.

23. What do you understand by constructor in OOP?

Ans: A Constructor is a member method of a class, used to initialize an Object, when it is created (instantiated).

24.What are the properties of Constructor?

Ans:   There are some properties of constructor:

·   A Constructor must have the same name as the class name and provides initial values to its
      data members.

·   A constructor have no return type not even void.

·   JAVA automatically creates a constructor method, if it is not defined with default values.

25.What do you understand by methods? What are the advantages of methods?

Ans: Definition: A Method or function is sequence of statement which are written to perform a

specific job in the application. In Object Oriented Programming, Method represents the behavior of the object. A message can be thought as a call to an object’s method.

The following three advantages/reasons describes that why we use methods.

To cope with complexity:

When programs become more complex and big in size, it is best technique to fol

conquer” i.e. a complex problem is broken in to smaller and easier task, so that we can make it manageable. Some times it is also called Modularization.

Hiding Details:

Once a method is defined, it works like a Black-box and can be used when required, without concerning that “How it Works?”

Reusability of code:

Once a method is implemented, it can be invoked or called from anywhere in the program when needed i.e. Method can be reused. Even a packaged method may be used in

multiple applications. This saves our time and effort. Most of the method like Math.sqrt() is available as ready to use which can be used anywhere in the application.

26. How to define a method?

Ans: A method must be defined before its use. The method always exist in a class. A Java Program must contain a main() method from where program execution starts. The general form of defining method is as-

[Access specifier]<return_type> <method_name>(<parameter(s)>) {……………. ;

body of the method i.e. statement (s);
}

Access Specifier:

It specified the access type and may be public or protected or private.

Return Type:

Specifies the return data type like int, float etc. Void is used when nothing is to be returned.

Method Name:

Specified the name of method and must be a valid Java identifier.

Parameters List:

It is list of variable(s), also called Formal Parameter or Argument, which are used to catch the values when method is invoked. Also a method may have no parameters.

27.What are the way to pass values to methods in Java?

Ans: You can pass arguments (Actual parameters) to method (Formal Parameters) using valid data
types like int, float, byte, char, double, boolean etc. or Reference data type like Object and
Arrays.

A method can called in two ways -

Call by Value: In this method, the values of Actual parameters are copied to Formal parameters, so
any changes made with Formal parameters in Method’s body, will not reflected back in the calling
function.

The original value of Actual parameters is unchanged because the changes are made on copied value.

Call by Reference:

In Reference method, the changes made on the formal parameters are reflected back in the Actual

parameters of calling function because instead of values, a Reference (Address of Memory location) is passed.

In general, all primitive data types are passed by Value and all
Reference types (Object, Array) are passed by Reference..

28. Differentiate between constructor and method.

Ans: Though Constructor are member method of the class like other methods, but they are different from other method members-

Constructor creates (initializes) an Object where a method is a group of statements which are packaged to perform a specific job.

Constructor has no return type, even void also. Whereas method may have any return type including
void.

The Constructor has the same name as Class, but method may have any name except Class name.
It is called at the time of object creation, but a method can be called any time when required.

29. What is “this” keyword?

Ans: As you are aware that static data and method members of a class is kept in the memory in a

single copy only. All the object are created by their instance variables but shares the class variables (static) and member methods.