Objects and Classes

Lecture-7

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Objects

• An object represents an entity in the real world

that can be distinctly identified. For example, a

student, a desk, a circle, a button, and even a

loan can all be viewed as objects. An object has

a unique identity, state, and behaviour.

• The state of an object (also known as its

properties or attributes) is represented by data

fields with their current values. A circle object, for

example, has a data field radius, which is the

property that characterizes a circle.

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Objects

• The behaviour of an object (also known as its

actions) is defined by methods. To invoke a

method on an object is to ask the object to

perform an action. For example, you may define

a method named getArea() for circle objects. A

circle object may invoke getArea() to return its

area.

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Classes

• A class is a template, blueprint, or contract

that defines what an object’s data fields

and methods will be. An object is an

instance of a class. You can create many

instances of a class. Creating an instance

is referred to as instantiation.

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A class is a template for creating

objects

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A class is a construct that defines

objects of the same type.

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Constructors

• A class provides methods of a special type,

known as constructors, which are invoked to

create a new object. A constructor can perform

any action, but constructors are designed to

perform initializing actions, such as initializing

the data fields of objects.

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UML (Unified Modelling Language) class

diagram, or simply a class diagram

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Example: Defining Classes and

Creating Objects

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Example: Defining Classes and

Creating Objects

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Example: Defining Classes and

Creating Objects

• Outputs

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Strings

• Constructing a String

– String message = new String("Welcome to Java");

or

– String message = "Welcome to Java";

• You can also create a string from an array of

characters.

– char[] charArray = {'G', 'o', 'o', 'd', ' ', 'D', 'a', 'y'};

String message = new String(charArray);

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String Comparisons

• Returns true if this string is equal to string

s1.

– +equals(s1: String): boolean

• Returns true if this string is equal to string

s1 case

– insensitive.+equalsIgnoreCase(s1: String):

boolean

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String Comparisons

String s1 = new String("Welcome to Java");

String s2 = "Welcome to Java";

if (s1.equals(s2))

System.out.println("string1 and string2 have the same contents");

else

System.out.println("string1 and string2 are not equal");

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The following statements display

true and then false

String s1 = new String("Welcome to Java");

String s2 = "Welcome to Java";

String s3 = "Welcome to C++";

System.out.println(s1.equals(s2)); // true

System.out.println(s1.equals(s3)); // false

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Concatenation

• You can use the concat method to concatenate two strings. The statement shown below, for example, concatenates strings s1 and s2 into s3:

String s1 = new String("Welcome to ");

String s2 = "Java";

String s3 = s1.concat(s2); s3 = " Welcome to Java";

• You can use the plus (+) sign to concatenate two or more strings. So the abovestatement is equivalent to

String s3 = s1 + s2;

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Obtaining Substrings

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Obtaining Substrings

• For example,

• String message = "Welcome to

Java".substring(0, 11) + "HTML";

• The string message now becomes "Welcome

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replaceAll

• For example, the following statement returns a

new string that replaces $, +, or # in "a+b$#c"

with the string NNN.

String s = "a+b$#c".replaceAll("[$+#]", "NNN");

System.out.println(s);

• Here the regular expression [$+#] specifies a

pattern that matches $, +, or #. So, the output is

aNNNbNNNNNNc.

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Problem: Checking Palindromes

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END

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