Module 3

Chapter Overview

• Title: Introduction to Program Design and Data Structures

• Author: John Lewis

• Edition: Fifth Edition

Chapter Scope

• Key Concepts:

  • Creating objects

  • Services of the String class

  • Java API class library

  • Random and Math classes

  • Formatting output

  • Introducing enumerated types

  • Wrapper classes

Creating Objects

• Object Variables:

  • A variable can hold a primitive type or an object reference.

  • Example: String title; (No object created, just a reference variable)

  • The object must be instantiated separately.

• Instantiation:

  • Use new operator: title = new String("James Gosling");

  • This calls the String constructor, which initializes the object.

Creating Strings

• String Instantiation:

  • Strings can be created without the new operator: title = "Java rocks!"

  • Each string literal represents a String object.

Invoking Methods

• Method Invocation:

  • Use the dot operator: count = title.length()

  • Methods may return values for assignments or expressions.

  • Invoking a method = asking an object to perform a service.

Object References

• Variable Types:

  • Primitive variable contains the actual value.

  • Object variable contains the reference (address) of the object.

  • Reference can be visualized graphically.

  • Example:

    • name1 -> "Steve Jobs"

    • num1 -> 38

Assignment Revisited

• Primitive Types Assignment:

  • Assignment copies values (e.g., num2 = num1;)

  • Changes in num1 will not affect num2.

• Object References Assignment:

  • Assignment copies the reference (address).

  • Example:

    • name2 = name1;

    • Both now reference the same object, affecting each other.

Aliases

• Definition:

  • Multiple references can refer to the same object.

  • Useful but requires careful management to avoid unintended changes.

• Impact:

  • Changing an object through one reference changes it for all aliases.

Garbage Collection

• Concept:

  • Objects without valid references are considered garbage.

  • Java performs automatic garbage collection, reclaiming memory.

  • Unlike other languages, Java handles this automatically.

The String Class

• Immutability:

  • Strings cannot be changed after creation (e.g., length and values are fixed).

  • Methods return new modified String objects.

• Accessing Characters:

  • Use numeric index to access characters in a string.

  • Example: In "Hello", 'H' is at index 0, 'o' is at index 4.

String Class Methods

• Common Methods:

  • Constructor: String(String str) - creates a new string object.

  • charAt(int index): returns character at specified index.

  • compareTo(String str): compares lexicographically with another string.

  • concat(String str): concatenates another string.

  • equals(String str): checks equality based on case sensitivity.

  • length(): returns the string's length.

  • replace(char oldChar, char newChar): replaces characters in the string.

  • substring(int offset, int endIndex): returns a subset of the string.

  • toLowerCase() and toUpperCase(): convert case of characters.

The Java API

• Definition:

  • A collection of classes available in Java for program development.

  • API stands for Application Programming Interface.

• Using Classes:

  • Classes like System, Scanner, and String are part of the Java API.

  • Custom class libraries can also be created or obtained from vendors.

Packages

• Organization:

  • Java API classes are organized into packages.

Import Declarations

• Using Classes:

  • Fully qualify class names (e.g., java.util.Scanner).

  • Import classes for easier access (e.g., import java.util.Scanner;).

  • Wildcard to import all classes in a package (e.g., import java.util.*;).

The java.lang Package

• Automatic Import:

  • Classes in this package are automatically imported.

  • This includes System and String.

  • Scanner must be imported explicitly as it's in java.util.

The Random Class

• Overview:

  • Part of the java.util package.

  • Generates pseudorandom numbers using methods.

• Common Methods:

  • Constructor: Random() - creates a number generator.

  • nextFloat(): returns random float between 0.0 and 1.0.

  • nextInt(): generates an integer value.

  • nextInt(int num): ranges between 0 and specified limit.

The Math Class

• Overview:

  • Part of the java.lang package.

  • Contains static methods for mathematical functions.

• Common Methods:

  • Math.abs(int num) - absolute value.

  • Math.sqrt(double num) - square root.

  • Math.pow(double num, double power) - exponentiation.

  • Math.random() - generates a random number.

Formatting Output

• Purpose:

  • Formats values for display.

  • Useful in presenting data clearly.

• Classes:

  • NumberFormat: formats as currency or percentages.

  • DecimalFormat: formats based on patterns.

Enumerated Types

• Definition:

  • Defines a type with specified values.

  • Example: enum Season {winter, spring, summer, fall};

• Usage:

  • Type-safe, only assigns valid values from enumeration.

  • Each enumeration value has an ordinal integer starting from zero.

Wrapper Classes

• Definition:

  • Classes in java.lang that correspond to primitive types (e.g., Integer, Double).

• Usage:

  • Can be used where objects are required instead of primitive types (e.g., collections).

  • Contains static methods for type management, such as parsing strings to numbers.

• Autoboxing:

  • Automatic conversion of a primitive type to a wrapper object (e.g., int to Integer).

  • The opposite process is called unboxing.