Using packages

Data Types and Exceptions in Java

Jim White

Java Developer

What are Java packages?

Packages organize Java code
- A package acts like a file folder or directory
- Combining related Java code (classes, interfaces, enums, etc.)
Like file folders
- Each package has a name

Java packages are like file folders and filing cabinet for organizing code

¹ Image from https://commons.wikimedia.org/wiki/File:File_Cabinet.jpg

Types of packages

Built-in packages
- Part of Java
- Name begins with "java" or "javax"
User defined packages
- Packages we define
- Packages we get from 3rd parties

Package names

Package names follow a convention
- Written in all lowercase
- Use periods (.) to delimit name parts
Built-in packages
- Start with java or javax
- Rest of the name suggests purpose
User defined package names
- Start with the reverse of the organization's domain
- Rest of the name suggests functionality

Example built-in packages
```
java.security
java.time
```

Example user-defined package names

com.mycompany.myproject
com.mycompany.myproject.account
com.mycompany.myproject.controller
com.mycompany.myproject.ui

Built-in packages

Some commonly used built-in packages

Package	Contains/Provides
`java.lang`	Base language support classes
`java.io`	Input / output operations
`java.util.logging`	Logging framework
`java.math`	Precision integer and decimal arithmetic
`java.net`	Networking operations
`java.util`	Date / time and data structures like Linked List, Dictionary and support
`java.security`	Security framework

java.math

java.math provides classes for arithmetic
- Used in cryptographic, scientific, and currency/money applications
BigInteger for representing large integers
- Integers larger than int or long can handle
- To work with integers of almost unlimited number of digits.
BigDecimal for representing very large or small floating point number
- Deals with rounding errors that can occur with float or double

Using packages

Use import + package name at the top

import java.math.BigInteger;
public class HelloWorld {
  BigInteger acct = new BigInteger("123");
}

Packages can contain many types.
- Use * to import all types in the package

import java.math.*;
public class HelloWorld {
  BigInteger acct = new BigInteger("123");
  BigDecimal pi = new BigDecimal("3.14");
}

BigInteger & BigDecimal from java.math

BigInteger and BigDecimal act as wrappers for big numbers
Construct them using a String or numeric
Come with add, subtract, multiply, and divide methods
Have additional methods like pow for power

// Imports go at the top of the class
import java.math.BigInteger;
import java.math.BigDecimal;

// Create BigInteger or BigDecimal with String
BigInteger big = new BigInteger("1000");
BigInteger ten = new BigInteger("10");
BigDecimal pi = new BigDecimal("3.14");
// Using a primitive to create BigDecimal
BigDecimal one = new BigDecimal(1.0);

BigInteger x = big.add(ten); // = 1010
BigDecimal y = pi.add(one); // = 4.14
BigInteger bigSqr = big.pow(2); // = 1000000
BigDecimal piCubed = pi.pow(3); // = 30.959144

BigInteger and BigDecimal methods

Method	Description
abs()	Absolute value of the integer
add(x)	Add x to the integer or decimal
divide(x)	Divide the integer or decimal by x
multiply(x)	Multiply the integer or decimal by x
negate()	Negate the integer or decimal
pow(int x)	The integer or decimal to the power of x
subtract(x)	Subtract x from the integer or decimal

¹ See https://docs.oracle.com/javase/7/docs/api/java/math/BigInteger.html and https://docs.oracle.com/javase/7/docs/api/java/math/BigDecimal.html

Import not required

Importantly - java.lang package is imported automatically.
- When using anything from java.lang it does not require an import
java.lang includes System, String, the wrapper classes and Exception.
- Explains why we can learn and use the base language elements without seeing an import.

Let's practice!

Data Types and Exceptions in Java