Parsing, Tokenizing and Formatting

Parsing

This section is intended to review all the approches to find, token and format stuff.
Let's see the first example:

	package br.com.ocjp.regex;
	import java.util.regex.Matcher;
	import java.util.regex.Pattern;
	public class RegexFirst {
	public static void main(String[] args) {
	Pattern p = Pattern.compile("aba");
	Matcher m = p.matcher("abababa"); // index starts from 0
	while ( m.find() ) {
	System.out.print(m.start() + " "); // Prints 0 4
	}
	}
	}

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As you can see the mehtod matcher from the class Matcher gets a source and the class Pattern uses the method compile to handle a pattern that you want to search.

!Important:

  a b a b a b a
  0 1 2 3 4 5 6

Why the little program showed above didn't print 0 2 4?
The reason is the regex engine does not consider the index 2 because it was consumed, and cannot be reused, but there are expections for this rule and will be shown sooner.

  a b a
  0 1 2

Using Metacharacters

\d A digit
\s A whitespace character
\w A word character (letters, digits, or "_" (underscore))
 . Any character

	package br.com.ocjp.regex;
	import java.util.regex.Matcher;
	import java.util.regex.Pattern;
	public class RegexMeta {
	public static void main(String[] args) {
	Pattern[] p = new Pattern[7];
	p[0] = Pattern.compile("\\d");
	p[1] = Pattern.compile("\\s");
	p[2] = Pattern.compile("\\w");
	p[3] = Pattern.compile(".");
	p[4] = Pattern.compile("[abc]");
	p[5] = Pattern.compile("[a-f]");
	p[6] = Pattern.compile("[a-fA-F]");
	Matcher[] matchers = new Matcher[7];
	for ( int i = 0; i < matchers.length ; i++ ) {
	matchers[i] = p[i].matcher("abcfd5 9erARqf C1#");
	// abcfd5 9erARqf C1#
	// 012345678901234567
	System.out.println("Matcher ==> " + i);
	while ( matchers[i].find() ) {
	System.out.print(matchers[i].start() + " ");
	}
	System.out.println("");
	}
	}
	}

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Output
Matcher ==> 0
5 7 16
Matcher ==> 1
6 14
Matcher ==> 2
0 1 2 3 4 5 7 8 9 10 11 12 13 15 16
Matcher ==> 3
0 1 2
Matcher ==> 4
0 1 2 3 4 8 13
Matcher ==> 5
0 1 2 3 4 8 10 13 15

Using Quantifiers

+ One or more occurrences
* Zero or more occurrences
? Zero or one occurrence

	package br.com.ocjp.regex;
	import java.util.regex.Matcher;
	import java.util.regex.Pattern;
	public class RegexQuantifiers {
	public static void main(String[] args) {
	Pattern[] p = new Pattern[3];
	p[0] = Pattern.compile("0[xX]([0-9a-fA-F])+");
	p[1] = Pattern.compile("proj1([^,])*");
	p[2] = Pattern.compile("\\d\\d\\d([-\\s])?\\d\\d\\d\\d"); // ? which means Zero or one occurrence
	Matcher[] matchers = new Matcher[3];
	for ( int i = 0; i < matchers.length ; i++ ) {
	if ( i == 0 ) {
	matchers[i] = p[i].matcher("12 0x 0x12 0Xf 0xg");
	} else if ( i == 1) {
	matchers[i] = p[i].matcher("proj3.txt,proj1sched.pdf,proj1,proj2,proj1.java");
	} else {
	matchers[i] = p[i].matcher("1234567,123-4567,123 4567");
	}
	System.out.println("Matcher ==> " + i);
	while ( matchers[i].find() ) {
	System.out.println(matchers[i].start() + " " + matchers[i].group());
	}
	System.out.println("");
	}
	}
	}

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Greedy Quantifiers

	package br.com.ocjp.regex;
	import java.util.regex.Matcher;
	import java.util.regex.Pattern;
	public class RegexGreedy {
	public static void main(String[] args) {
	Pattern p = Pattern.compile(".*xx"); // Greedy *
	Matcher m = p.matcher("yyxxxyxx");
	while ( m.find() ) {
	System.out.println( m.start() + " " + m.group());
	}
	System.out.println("#########################");
	Pattern p1 = Pattern.compile(".*?xx"); // Reluctant quantifier
	Matcher m1 = p1.matcher("yyxxxyxx");
	while ( m1.find() ) {
	System.out.println( m1.start() + " " + m1.group());
	}
	}
	}

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Tokenizing

Tokenizing is the process of taking big pieces of source data, breaking them into
little pieces, and storing the little pieces in variables

Tokenizing with Scanner

	package br.com.ocjp.regex;
	import java.util.Scanner;
	public class AppScanner {
	public static void main(String [] args) {
	boolean b2, b;
	int i;
	String s, hits = " ";
	Scanner s1 = new Scanner(args[0]);
	Scanner s2 = new Scanner(args[0]);
	while(b = s1.hasNext()) {
	s = s1.next(); hits += "s";
	}
	while(b = s2.hasNext()) {
	if (s2.hasNextInt()) {
	i = s2.nextInt(); hits += "i";
	} else if (s2.hasNextBoolean()) {
	b2 = s2.nextBoolean(); hits += "b";
	} else {
	s2.next(); hits += "s2";
	}
	}
	System.out.println("hits " + hits);
	}
	}

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Formatting with printf() and format()

Both methods have exactly the same behaviour which means anything we say about one of these methods is applicable to both.

Let's see how formatting works:

%[arg_index$][flags][width][.precision]conversion char

The values within [ ] are optional.

1. arg_index - An integer followed directly by a $, this indicates which argument should be printed in this position.

2. flags - While many flags are available, for the exam you'll need to know:
¦ "-" Left justify this argument
¦ "+" Include a sign (+ or -) with this argument
¦ "0" Pad this argument with zeroes
¦ "," Use locale-specific grouping separators (i.e., the comma in 123,456)
¦ "(" Enclose negative numbers in parentheses


3. width -  This value indicates the minimum number of characters to print. (If you
want nice even columns, you'll use this value extensively.)

4. precision - For the exam you'll only need this when formatting a floating-point
number, and in the case of floating point numbers, precision indicates the number of
digits to print after the decimal point.

5. conversion - The type of argument you'll be formatting. You'll need to know:
¦ b boolean
¦ c char
¦ d integer
¦ f floating point
¦ s string

	package br.com.ocjp.regex;
	public class AppPrintf {
	public static void main(String[] args) {
	boolean b = true;
	double num = 565.99;
	long num1 = 6565415;
	System.out.printf("%1$010d \n",num1);
	System.out.printf("%1$,.5f \n",num);
	System.out.printf("%1$010d , %2$,.5f \n ",num1,num);
	System.out.printf("---> %1$b",b);
	}
	}

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Working with Dates/Numbers and Currencies

In this section you'll learn the most common methods to format Dates, Numbers and Currencies as well as the relationship between the main classes responsible to do that.

Let's start with the classes that you have to learn.

java.util.Date;
java.util.Calendar;
java.text.DateFormat;
java.text.NumberFormat;
java.util.Locale;

	package br.com.ocjp.formatting;
	import java.util.Date;
	public class AppDate {
	public static void main(String[] args) {
	Date d = new Date();
	System.out.println(d.toString());
	Date d2 = new Date(1000000L);
	System.out.println(d2.toString());
	Date d3 = new Date();
	System.out.println(d3.getTime());
	Date d4 = new Date(d3.getTime());
	System.out.println(d4.toString());
	Date d5 = new Date();
	d5.setTime(10000054700104L);
	System.out.println(d5.toString());
	}
	}

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Console:

Wed Jul 18 10:18:42 AMT 2012

Wed Dec 31 19:16:40 AMT 1969

1342621122734

Wed Jul 18 10:18:42 AMT 2012

Sun Nov 21 04:58:20 AMT 2286

The class java.util.Date is hard to work with,if you look at the preceding example to set a specific date we have to use a number representation(long), this class is normally used to create a date that represents "now" as well as working with java.util.Calendar.

	package br.com.ocjp.formatting;
	import java.util.Calendar;
	import java.util.Date;
	public class AppCalendarDate {
	public static void main(String[] args) {
	Date d1 = new Date("09/15/2012");
	System.out.println(d1.toString());
	Date d = new Date();
	System.out.println("Today is " + d.toString());
	Calendar cal = Calendar.getInstance();
	cal.setTime(d);
	System.out.println("Day of the week " + cal.get(Calendar.DAY_OF_WEEK));
	System.out.println("Day of the year " + cal.get(Calendar.DAY_OF_YEAR));
	Calendar c1 = Calendar.getInstance();
	c1.setTime(d1);
	System.out.println("Last day of the year " + c1.get(Calendar.DAY_OF_YEAR));
	long current = cal.get(Calendar.DAY_OF_YEAR);
	long last = c1.get(Calendar.DAY_OF_YEAR);
	long remaining = last - current;
	System.out.println("Remaining Days ==> " + remaining );
	cal.add(Calendar.YEAR, 2);
	d = cal.getTime();
	System.out.println(d.toString());
	cal.roll(Calendar.MONTH, 10);
	d = cal.getTime();
	System.out.println(d.toString());
	}
	}

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Console:

Sat Sep 15 00:00:00 AMT 2012
Today is Wed Jul 18 10:37:26 AMT 2012
Day of the week 4
Day of the year 200
Last day of the year 259
Remaining Days ==> 59
Fri Jul 18 10:37:26 AMT 2014
Sun May 18 10:37:26 AMT 2014


Watch out, the class Calendar is abstract you cannot create an instance of it(look at line 16), also it provides friendly methods to manipulate dates, where you can add hours, days, weeks, and so on.
Notice the class java.util.Calendar work together java.util.Date, instead of setting a date using number(long) you can use a Date object.
Finally the method roll is similar to add, the difference is it does not change into next month(if you're adding days), years(if you're adding months) and so on.

	package br.com.ocjp.formatting;
	import java.text.DateFormat;
	import java.util.Calendar;
	import java.util.Date;
	import java.util.Locale;
	public class AppDateLocale {
	public static void main(String[] args) {
	Calendar c = Calendar.getInstance();
	c.set(2012, 07, 17);
	Date d = c.getTime();
	Locale us = new Locale("en");
	Locale pt = new Locale("pt","br");
	Locale fr = new Locale("fr");
	Locale sp = new Locale("es");
	Locale ja = new Locale("ja");
	DateFormat df = DateFormat.getDateInstance(DateFormat.FULL, us);
	DateFormat df1 = DateFormat.getDateInstance(DateFormat.FULL, pt);
	DateFormat df2 = DateFormat.getDateInstance(DateFormat.FULL, fr);
	DateFormat df3 = DateFormat.getDateInstance(DateFormat.FULL, sp);
	DateFormat df4 = DateFormat.getDateInstance(DateFormat.FULL, ja);
	System.out.println(df.format(d));
	System.out.println(df1.format(d));
	System.out.println(df2.format(d));
	System.out.println(df3.format(d));
	System.out.println(df4.format(d));
	try {
	String date = df.format(d);
	Date parseDate = df.parse(date);
	System.out.println("Parse ==> " + parseDate);
	} catch ( ParseException e){}
	}
	}

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Console:

Friday, August 17, 2012
Sexta-feira, 17 de Agosto de 2012
vendredi 17 août 2012
viernes 17 de agosto de 2012
2012?8?17?
Parse ==> Fri Aug 17 00:00:00 AMT 2012


The class java.text.DateFormat is an abstract class that provides us formatted dates using or not pré-defined styles as well as Locales.
You can get a DateFormat class by invoking the methods getInstance() and getDateInstance().
Finally we can parse a String to Date by using the method parse() which must be enclosed in a try-catch block.

	package br.com.ocjp.formatting;
	import java.util.Locale;
	public class AppLocale {
	public static void main(String[] args) {
	Locale[] loc = new Locale[4];
	loc[0] = new Locale("pt");
	loc[1] = new Locale("pt","br");
	loc[2] = new Locale("fr");
	loc[3] = new Locale("en","us");
	for ( Locale l : loc) {
	System.out.print(l.getLanguage() + " -> ");
	System.out.println(l.getDisplayCountry(l));
	}
	}
	}

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Console:

pt ->
pt -> Brasil
fr ->
en -> United States


Let's take a briefly look at java.util.Locale.

The first argument is related to the language and the second is a country.
The most important methods are getLanguage() and getDisplayCountry().
There are not setters to define language and country at java.util.Locale.

	package br.com.ocjp.formatting;
	import java.text.NumberFormat;
	import java.text.ParseException;
	import java.util.Locale;
	public class AppNumberFormat {
	public static void main(String[] args) {
	float f = 1100.56845f;
	Locale pt = new Locale("pt");
	Locale us = new Locale("en","us");
	NumberFormat nf[] = new NumberFormat[6];
	nf[0] = NumberFormat.getNumberInstance();
	nf[0].setMaximumFractionDigits(5);
	nf[1] = NumberFormat.getNumberInstance(pt);
	nf[2] = NumberFormat.getNumberInstance(us);
	nf[3] = NumberFormat.getCurrencyInstance();
	nf[4] = NumberFormat.getCurrencyInstance(pt);
	nf[5] = NumberFormat.getCurrencyInstance(us);
	for ( NumberFormat nfs : nf ) {
	System.out.println(nfs.format(f));
	}
	System.out.println(nf[0].getMaximumFractionDigits());
	try {
	NumberFormat nf1 = NumberFormat.getInstance();
	System.out.println(nf1.parse("1234.567"));
	nf1.setParseIntegerOnly(true);
	System.out.println(nf1.parse("1234.567"));
	} catch (ParseException pe) {
	System.out.println("parse exc");
	}
	}
	}

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Console:

1.100,56848
1.100,568
1,100.568
R$ 1.100,57
¤ 1.100,57
$1,100.57
5
1234567
1234567


Similar to java.text.DateFormat the class java.text.NumberFormat works with numbers(currency, percent, and so on), where there are many ways to get a concrete class since NumberFormat is abstract.

getInstance(),
getNumberInstance(),
getCurrencyInstance(),
.. and so on

The method parse() is present as well where you can get a Number object.

I/O - Getting better understanding of API

Let's start with the class File

	package br.com.ocjp.io;
	import java.io.File;
	import java.io.IOException;
	public class ClassFile {
	public static void main(String[] args) {
	File f = new File("C:\\temp\\file.txt"); // There is no file yet.
	System.out.println("Does the file exist? " + f.exists());
	try {
	boolean flag = f.createNewFile(); // May Create a real file
	System.out.println("File has been created " + flag);
	System.out.println("Does the file exist? " + f.exists());
	} catch (IOException e) {}
	}
	}

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Notice that the file has not been created until the line 13 be reached, to memorize it think of when you create a File(line 10) it doesn't throw any exception but the method createNewFile does.

Serialization

Serialization is the mechanism that allows you save the state of objects.
Find below the keys to make a class serializable.

	package br.com.ocjp.serializable;
	import java.io.FileInputStream;
	import java.io.FileOutputStream;
	import java.io.IOException;
	import java.io.ObjectInputStream;
	import java.io.ObjectOutputStream;
	import java.io.Serializable;
	import java.util.Date;
	class Request implements Serializable {
	public int number;
	public Date create;
	public String description;
	}
	public class Serialization {
	public static void main(String[] args) throws IOException, ClassNotFoundException {
	Request request = new Request();
	request.number = 100;
	request.create = new Date();
	request.description = "@@ version 1.3 @@";
	FileOutputStream output = new FileOutputStream("C:\\temp\\request.ser");
	ObjectOutputStream os = new ObjectOutputStream(output);
	os.writeObject(request);
	os.close();
	FileInputStream file = new FileInputStream("C:\\Temp\\request.ser");
	ObjectInputStream io = new ObjectInputStream(file);
	Request r = (Request) io.readObject();
	System.out.println(r.number);
	System.out.println(r.create);
	System.out.println(r.description);
	}
	}

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First of all we have to implement the interface serializable( look at line 11), also to save the state of object we need the classes FileOutputStream and ObjectOutputStream, the writeObject(object) method is responsible for it and to retrieve the serializable object from the file we need FileInputStream and ObjectInputStream the readObject() is responsible for reading it.

!Import

- If you don't want to save a member of object you must mark it as transient
- Whether a super class implements Serializable its subclasses are indirectly Serializable
- Whether a subclass implements Serializable and a superclass doesn't the constructor will be invoked as well as the instance variable will get their default values

Let's see an example:

	package br.com.ocjp.serializable;
	import java.io.FileOutputStream;
	import java.io.IOException;
	import java.io.ObjectOutputStream;
	import java.io.Serializable;
	class Something {
	Something() {
	System.out.println("Something constructor");
	}
	}
	class Animal extends Something {
	Animal() {
	System.out.println("Animal constructor");
	}
	String name = "something";
	int age;
	}
	class Dog extends Animal implements Serializable {
	Dog() {
	System.out.println("Dog constructor");
	}
	String kind;
	String owner;
	}
	public class SerializableExamples {
	public static void main(String[] args) throws IOException, ClassNotFoundException {
	FileOutputStream out = new FileOutputStream("C:\\temp\\animal.ser");
	ObjectOutputStream st = new ObjectOutputStream(out);
	Dog a1 = new Dog();
	a1.age = 5;
	a1.name = "Dog";
	a1.kind = "Pet";
	a1.owner = "Somebody";
	st.writeObject(a1);
	}
	}

view raw gistfile1.java hosted with ❤ by GitHub

Let's see what's been saved.

	package br.com.ocjp.serializable;
	import java.io.FileInputStream;
	import java.io.IOException;
	import java.io.ObjectInputStream;
	public class Input {
	public static void main(String[] args) throws IOException, ClassNotFoundException {
	FileInputStream f = new FileInputStream("C:\\temp\\animal.ser");
	ObjectInputStream in = new ObjectInputStream(f);
	Dog a2 = (Dog)in.readObject();
	System.out.println("Age ==> " + a2.age);
	System.out.println("Name ==> " + a2.name);
	System.out.println("Kind ==> " + a2.kind);
	System.out.println("Owner ==> " + a2.owner);
	}
	}

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- if you wanna save a state of object that has a reference to another object which does not implement Serializable you have to write the methods writeObject() and readObject().

	package br.com.ocjp.serializable;
	import java.io.FileInputStream;
	import java.io.FileOutputStream;
	import java.io.IOException;
	import java.io.ObjectInputStream;
	import java.io.ObjectOutputStream;
	import java.io.Serializable;
	class Monitor {
	String model;
	}
	class PC implements Serializable {
	String system;
	int version;
	public transient Monitor monitor;
	private void writeObject(ObjectOutputStream o) {
	try {
	o.defaultWriteObject();
	o.writeObject(monitor.model);
	} catch (IOException e) {
	e.printStackTrace();
	}
	}
	private void readObject(ObjectInputStream in) {
	try {
	in.defaultReadObject();
	Monitor m = new Monitor();
	m.model = (String)in.readObject();
	this.monitor = m;
	} catch (ClassNotFoundException e) {
	e.printStackTrace();
	} catch( IOException e) {}
	}
	}
	public class WriteReadObject {
	public static void main(String[] args) {
	PC m = new PC();
	m.system = "iOS";
	m.version = 1;
	Monitor d = new Monitor();
	d.model = "SX465";
	m.monitor = d;
	try {
	FileOutputStream f = new FileOutputStream("C:\\temp\\pc.ser");
	ObjectOutputStream out = new ObjectOutputStream(f);
	out.writeObject(m);
	FileInputStream f1 = new FileInputStream("C:\\temp\\pc.ser");
	ObjectInputStream in = new ObjectInputStream(f1);
	PC pc = (PC)in.readObject();
	System.out.println("PC Monitor ==> " + pc.monitor.model);
	} catch (IOException e) {
	System.out.println("Cannot save it");
	} catch (ClassNotFoundException e) {
	e.printStackTrace();
	}
	}
	}

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- Finally we can use serialization with static variables but as you've seen serialization is useful for objects

Print this post

Working with Strings and StringBuffer/StringBuilder

In this section we'll discuss about String as well as its creation, polemic things like "Strings are immutable" and see wthat's going on behind the scenes also StringBuffer and StringBuilder will be on focus.

Let's take a look at how the string objects are created and understand the immutability.

	package br.com.ocjp.string;
	public class StringImmutable {
	public static void main(String[] args) {
	String st = new String("OCJP");
	String st2 = st;
	st = st.concat(" - Java");
	System.out.println("st variable ==> " + st);
	System.out.println("st2 variable ==> " + st2);
	}
	}

view raw gistfile1.java hosted with ❤ by GitHub

	package br.com.ocjp.string;
	public class StringImmutable {
	public static void main(String[] args) {
	String book = new String("Book");
	book.concat(" - Java");
	book.upperCase();
	System.out.println("book variable ==> " + book); // it prints "Book"
	}
	}

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As you can see it prints "Book" because we don't refer the new String created to "Java - Book", so the object is in the heap however nobody refers to it.

Commom methods
Let's see common methods in action.

	package br.com.ocjp.string;
	public class StringCommon {
	public static void main(String[] args) {
	String example = "Java Example ";
	System.out.println("charAt() ==> " + example.charAt(0)); // it prints J
	System.out.println("concat() ==> " + example.concat(" -1")); // it prints Java Example - 1
	System.out.println("equalsIgnoreCase() ==> " + example.equalsIgnoreCase("java example ")); // it prints true
	System.out.println("length() ==> " + example.length()); // it prints 14
	System.out.println("replace() ==> " + example.replace("a","@")); // it prints J@v@ Ex@mple
	System.out.println("substring() ==> " + example.substring(0)); // it prints Java Example
	System.out.println("substring() ==> " + example.substring(0,4)); // it prints Java
	System.out.println("toLowerCase() ==> " + example.toLowerCase()); // it prints java example
	System.out.println("toString() ==> " + example.toString()); // it prints Java Example
	System.out.println("toUpperCase() ==> " + example.toUpperCase()); // it prints JAVA EXAMPLE
	System.out.println("trim() ==> " + example.trim()); // no space at the end... it prints Java Example
	}
	}

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Pool of String

	package br.com.ocjp.string;
	public class StringPool {
	public static void main(String[] args) {
	String t = "java"; // Creates an object in a heap and puts it in a pool of String
	String t1 = new String("java"); //Creates a new object in a heap and puts it in a pool of String
	System.out.println(t == t1); // prints false
	String s = "ocjp";
	String s1 = "ocjp"; // Retrieve the string ocjp from the pool
	System.out.println(s == s1); // prints true
	}
	}

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Also you can use the method intern() (String class) this way we can get the string from the pool if the string pass using the method equals().


StringBuffer and StringBuilder friends of memory

Both classes are intented to help you handle Strings without wasting memory, as we mentioned Strings are immutable and if we have to handle lots and lots of them StringBuffer and StringBuilder will help you.
The unique difference between StringBuffer and StringBuilder is StringBuilder is not thread-safe which means its methods aren't synchronized, as you can imagine StringBuilder is faster than StringBuffer.

	package br.com.ocjp.string;
	public class StringBufferBuilder {
	public static void main(String[] args) {
	StringBuffer sb = new StringBuffer();
	sb.append("Java Test - ");
	sb.append("OCJP");
	System.out.println(sb); // Prints Java Test - OCJP
	StringBuffer sd = new StringBuffer("0123456789");
	sd.delete(4, 6);
	System.out.println(sd); //Prints 01236789
	StringBuffer si = new StringBuffer("0123456789");
	si.insert(4, "----");
	System.out.println(si); //Prints 0123----46789
	StringBuffer sr = new StringBuffer("0123456789");
	sr.reverse();
	System.out.println(sr); //Prints 987654321
	StringBuilder builder = new StringBuilder("abcdef");
	builder.append("ghij").reverse().delete(5,20);
	System.out.println(builder); // Prints jihgf
	}
	}

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The most import thing here is StringBuffer and StringBuilder are not immutable like String and the StringBuffer contains the synchronized methods;

Unusual things in Exceptions

I called this post "Unusual things in Exceptions" because you're going to see something that isn't usually used in a real word as well as staffs that you have to memorize for the OCJP certification.

Let's start with the class hierarchy:

	package br.com.ocjp.exception;
	import java.io.IOException;
	public class ExceptionOrder {
	public static void doSomething() {
	try {
	String s = "Exception example";
	} catch (IOException e) { // does not compile
	}
	}
	}

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The code above does not compile you cannot handle an exception if the block try does not "throw" one.

!Import
The block finally always is called, even if there is a return statement in the try/catch block, the finally block will be executed before the return stamement.

	package br.com.ocjp.exception;
	import java.io.File;
	import java.io.FileNotFoundException;
	import java.io.FileOutputStream;
	import java.io.IOException;
	public class ExceptionOrder {
	public static void doSomething() {
	try {
	new FileOutputStream( new File("C:\\file.txt"));
	} catch (IOException e) {
	} catch (FileNotFoundException e) { // does not compile
	}
	}
	}

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In this case the block try "throws" an exception throught the creation of FileOutputStream, but we still getting a compiler error, because the catch that handles FileNotFoundException will never be reached.

	package br.com.ocjp.exception;
	import java.io.IOException;
	public class ExceptionCatchDeclare {
	public void doSomething()/* throws IOException*/ {
	throw new IOException(); // does not compile you have to declare(throws)
	}
	public void doAnything() {
	doSomething(); // here you have to handle(try/catch) or declare(throws)
	}
	}

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That's the rule for checked exceptions if you throw an exception you must declare(throws) from the calling method or catch(try/catch).


Different from checked exception, every exception under RuntimeException/Error are not obligated to catch or declare, but if you did you would be able to catch it by using try/catch block.

	package br.com.ocjp.exception;
	public class ExceptionRunTime {
	public void doIt() throws ArithmeticException {
	throw new ArithmeticException();
	}
	public void doThat() {
	doIt();
	}
	}

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Common Exception

Let's take a look at the common exception, watch out where they come from

Autoboxing Overloading - Part 3

I've created a new topic for overloading because it involves Autoboxing and as you'll see there are some tricks that need to seen closely.
As you know to overload a method you have to change its arguments and the compiler will decide which method to invoke.
Let's see a simple example of overloaded method:

	public class Overloaded {
	public static void doSomething(byte b) {
	System.out.println("byte " + b);
	}
	public static void doSomething(double f) {
	System.out.println("double " + f);
	}
	public static void doSomething(int i) {
	System.out.println("int " + i);
	}
	public static void main(String[] args) {
	short s = 9;
	int i = 45;
	float f = 12.55f;
	doSomething(s) ; // Prints int 9
	doSomething(i) ; // Prints int 45
	doSomething(f) ; // Prints double 12.55
	}
	}

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Widening
Ok that's a easy one, the important thing here is the statement at line 19, it invokes the method doSomething(int i), despite we're passing a short value you know that int supports short because int is bigger than short, that's why the method doSomething(int i) was called instead of doSomething(byte b). Also if the method doSomething(int i) did not exist the method doSomething(double f) would be invoked.

Now take a look at the following image:

The image tells us that Widening beats Boxing and Var-args and Boxing beats Var-args, so to get a better understanding of it keep in your mind this rule.
Let's see it in action

	package br.com.ocjp.autoboxing;
	public class AutoBoxingBeats {
	public static void doIt(long value) {
	System.out.println("Primitive long");
	}
	public static void doIt(Integer value) {
	System.out.println("Integer");
	}
	public static void doIt(int... value) {
	System.out.println("var-args");
	}
	public static void doThat(Integer value) {
	System.out.println("Integer");
	}
	public static void doThat(int... value) {
	System.out.println("var-args");
	}
	public static void doSomething(Object s) {
	System.out.println("Object");
	}
	public static void doSomething(Number s) {
	System.out.println("Number");
	}
	public static void main(String[] args) {
	int n = 100;
	doIt(n); // Prints Primitive long - Widening
	doThat(n); // Prints Integer - Boxing
	doSomething(n); // Prints Number - Variable Reference Widening
	}
	}

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!Important

You cannot widen an Integer wrapper to a Long, so keep it in your mind it's not able to widen Wrapper to Wrapper.
Looking closely to the doSomething() method what happens first is a boxing int - Integer and then Integer is a Number.

Finally sometimes the compiler cannot tell apart which overloaded method is eligible to be invoke which results in a compiler error, for more details about this look it out. click here