The document discusses objects, classes, and strings in Java. It defines an object as having a unique identity, state, and behavior, with state represented by data fields and behavior defined by methods. A class is a template that defines an object's data fields and methods, and multiple objects can be instances of the same class. The document provides examples of defining classes and creating objects, and discusses string operations like comparisons, concatenation, substrings, and replacement.

1. Objects and Classes
Lecture-7
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2. 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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3. 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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4. 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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5. A class is a template for creating
objects
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6. A class is a construct that defines
objects of the same type.
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7. 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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8. UML (Unified Modelling Language) class
diagram, or simply a class diagram
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9. Example: Defining Classes and
Creating Objects
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10. Example: Defining Classes and
Creating Objects
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11. Example: Defining Classes and
Creating Objects
• Outputs
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12. 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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13. 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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14. 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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15. 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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16. 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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17. Obtaining Substrings
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18. Obtaining Substrings
• For example,
• String message = "Welcome to
Java".substring(0, 11) + "HTML";
• The string message now becomes "Welcome
to HTML".
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21. 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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22. Problem: Checking Palindromes
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24. END
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