Chapter 3: Using Classes and Objects

Overview

Chapter 2: basic Java language → Java libraries
Chapter 3: libraries in java.lang package
- String and Math classes
- Enumerated types, wrapper classes
- Finish with (peripheral) graphics classes
Java has many libraries
Exams will emphasize core Java:
- Language
- java.lang and some of java.util

DrJava

Will use Dr Java because of its interactive capabilities
Download and install from DrJava
- No admin privileges needed

(3.1) Creating Objects

The new operator
- Creates an object
- Initializes it by calling its constructor
- Returns a "reference" to the object.
- Example:
```
Thing myThing = new Thing("left", 21);
```
References are pointers and we may represent them as arrows.
- Memory addresses
- "Reference types"
- null

Calling Methods of Objects

Dot operator (.) accesses methods of an object.

Examples

myThing.setWeight(12);

String myString = new String("home sweet home");
int len = myString.length();

What happens if the variable is null?

Deleting Objects

Garbage collection

(3.2) String methods

See Figure 3.1. Examples—evaluate:

String s = "Mississippi";
s.charAt(0)
s.charAt(2)
s.compareTo("Alabama")
"cat".concat("dog")
"cat".equals("dog")
"cat".equalsIgnoreCase("Cat")
s.length()
s.replace('s', 't')
s.replace('s', 't').replace('p', 'q').replace('i', 'j').replace('M', 'N')
s // unchanged
s.substring(0, 4)
s.substring(4, 8)
s.toLowerCase()
s.toUpperCase()

Discussion

Compare Java and Python string operations

Example

Listing 3.1 StringMutation (but it does not really mutate the string!)
Modify this program to print phrase again, to show that it really is unchanged.

(3.3) Packages

A class library is a group of related classes.
- Usually grouped into packages.
- Some important standard Java packages:
  - java.lang
  - java.util
  - java.awt
  - javax.swing
  - More in Fig. 3.2, p. 123
- Reasons for packages:
  - Better organization.
  - Avoid name conflicts.

The Unnamed Package

When you're writing your program, and you don't make a package declaration, by default you're writing for the unnamed package.

The import Statement

Import a package to use its classes
```
import java.util.*;
```
Or import a single class:
```
import java.util.Scanner;
```
After import, the classes can be named without their package names
- Scanner
- java.util.Scanner
java.lang does not need to be imported
The unnamed package is not imported

APIs

An Application Programming Interface (API) is a collection of related classes, which may belong to one or more packages.
Example: Java graphics API includes the packages java.awt and javax.swing.
To use an API or library, we need a "reference manual" or some notes.
- Visit Oracle/Sun Java; find and navigate the documentation

(3.4) Random and its methods

Random = java.util.Random

Examples:

import java.util.Random;
Random r = new Random();
r.nextFloat()
r.nextInt()
r.nextInt(100)  // overloaded

Details: Fig. 3.4

Example

Listing 3.2: RandomNumber

(3.5) Math and its methods

Math = java.lang.Math
Provides mathematical functions
- Static methods: no instance needed
- To call a static method, write class name, dot, method name
- Example: Math.sqrt(16.2)

Math methods

Examples:

Math.abs(-7)
Math.floor(7.9)
Math.floor(-7.9)
Math.ceil(7.1)
Math.ceil(-7.1)
Math.pow(2, 8)
Math.exp(8)  // e to the 8th power
Math.sqrt(2.0)
Math.cos(0);

Details: Fig. 3.5

Example

Review
- To solve quadratic equations
  
  ax² + bx + c = 0
- Use the quadratic formula:
  
  x = [−b ± sqrt(b² − 4 ac)] / (2 a)
- It has two solutions.
Listing 3.3: Quadratic

(3.6) Formatting Output

Class java.text.NumberFormat
- Getting an instance
- Methods
- Examples
Locales
Class java.text.DoubleFormat

The NumberFormat Class

Do not use new to create an instance
Use one of the static methods to create an instance
Use the format method with the instance

Examples

import java.text.NumberFormat;
NumberFormat nf;
nf = NumberFormat.getCurrencyInstance();
nf.format(12345.6789)
nf = NumberFormat.getPercentInstance();
nf.format(12345.6789)
nf = NumberFormat.getNumberInstance();
  // or getInstance()
nf.format(12345.6789)

Why Not "new"?

Number format is an abstract class
new won't work
So use "factory" static methods instead
Also, the factory methods will return an instance suitable for our locale

Locales

Locale summarizes one's language and other local characteristics
Unix shell variable LANG

Two ways to run gedit in French:

$ env LANG=fr_FR gedit
$ export LANG=fr_FR
$ gedit

Other locales: fr_CA, es_ES, en_UK, …
locale -a gives a list of available codes
Not all programs are ready for this

Example

Listing 3.4 Purchase

The DecimalFormat Class

Use to format double data as decimal numbers
Non-abstract class; use new
Constructor takes a pattern string
- See online API documentation for details

Examples

import java.text.DecimalFormat;
DecimalFormat df = new DecimalFormat("0.###");
df.format(123.4)
df.format(123.456789)

Example

Listing 3.5 CircleStats

(3.7) Enumerated Types

New Java language feature in 1.5
Useful for giving names to a small set of symbolic constants
- Each symbolic value is associated with an integer 0, 1, 2, ..., its ordinal value
- Type-safe: you cannot store out of range values
Is actually a special kind of class definition.

Declaring

Example:
```
enum Size {Small, Medium, Large};
```
Within another class:
```
enum TYPENAME {VALUE1, ..., VALUEN};
```

Standalone:

public enum TYPENAME {VALUE1, ..., VALUEN};

Methods of Enumerated Types

toString () → String
name () (like toString)
ordinal () → int
Examples

Example

Listing 3.6 IceCream

enum declaration within IceCream class
- Inner class

(3.8) Wrapper Classes

Recall primitive and reference types
Primitive types are efficient
But sometimes we need an object
Wrapper classes bridge the gap
- Stores a primitive type
- Encloses it in an object
- Like putting letter into envelope

The Nine Wrapper Classes

Byte
Short
Integer // int
Long
Float
Double
Character // char
Boolean
Void // what's this?

Example

Integer w = new Integer(67);
int i = w.intValue(); // 67

Wrapper Utilities

Conversion methods to and from String

Examples: Integer.parseInt, Integer.toString

Examples using base ≠ 10

Autoboxing and Unboxing

New in Java 1.5
Makes the compiler do the boring part:
```
Integer w = 67;
int i = w;
```

Data Conversion

Wrapper classes also provide conversion between primitive types and Strings

Examples

w.toString(); // "26"
int i = Integer.parseInt("26"); // 26
Integer.toString(37) // "37"

Graphics

(3.9) Components and Containers
(3.10) Nested Panels
(3.11) Images

(3.9) Components and Containers

Real-world graphics (unlike Turtle)
Components (widgets) are elements of a GUI such as buttons and labels.
Containers are components that may contain other components.

Two container classes

JFrame
- Heavyweight component with multiple panes
- Its content pane contains all its visible components.
JPanel.
- Lightweight
- No content pane

Another Component

JLabel represents a label
Can have text and/or an image.

Layout Managers

Each container has a layout manager
The L.M. controls the arrangement of its components
May rearrange them when the window is resized.
Programmer does not have to specify numerical coordinates

Example

Listing 3.7 Authority

(3.10) Nested Panels

Using panels within a panel for better control of grouping.
Listing 3.8 NestedPanels

(3.11) Images

A picture loaded from a file or URL
Creating an Image
Rendering an Image

Creating Images

import javax.swing.ImageIcon;

ImageIcon icon = new ImageIcon("IMAGEFILE"); 
                 // gif, jpeg, png

Rendering Images

With Graphics.drawImage
In a JLabel

Images in JLabels (1)

new JLabel(TEXT, ICON, POSITION)

where

TEXT is a String,
ICON is an ImageIcon
HPOSITION describes the label's position in its container; SwingConstants.LEFT|CENTER|BOTTOM.
See API documentation for other options

Images in JLabels (2)

Setting position of text in label:

label.setHorizontalTextPosition(HPOSITION);
label.setVerticalTextPosition(VPOSITION);

where VPOSITION allows SwingConstants.TOP|CENTER|BOTTOM.

Example

Listing 3.9 LabelDemo

Lab

Overview of Lab 3