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846abc4d9a
* Fix word order in class documentation * Change access level modifier of age variable The comment inside eat() mentions that the variable is private. Furthermore, there would be no error if the variable were used inside Dog given the current access level.
32 KiB
32 KiB
| language | contributors | filename | ||||||||||||||||||||||||||||||
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| java |
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LearnJava.java |
Java is a general-purpose, concurrent, class-based, object-oriented computer programming language. Read more here.
// Single-line comments start with //
/*
Multi-line comments look like this.
*/
/**
* JavaDoc comments look like this. Used to describe the Class or various
* attributes of a Class.
* Main attributes:
*
* @author Name (and contact information such as email) of author(s).
* @version Current version of the program.
* @since When this part of the program was first added.
* @param For describing the different parameters for a method.
* @return For describing what the method returns.
* @deprecated For showing the code is outdated or shouldn't be used.
* @see Links to another part of documentation.
*/
// Import ArrayList class inside of the java.util package
import java.util.ArrayList;
// Import all classes inside of java.security package
import java.security.*;
// Each .java file contains one outer-level public class, with the same name
// as the file.
public class LearnJava {
// In order to run a java program, it must have a main method as an entry
// point.
public static void main (String[] args) {
///////////////////////////////////////
// Input/Output
///////////////////////////////////////
/*
* Ouput
*/
// Use System.out.println() to print lines.
System.out.println("Hello World!");
System.out.println(
"Integer: " + 10 +
" Double: " + 3.14 +
" Boolean: " + true);
// To print without a newline, use System.out.print().
System.out.print("Hello ");
System.out.print("World");
// Use System.out.printf() for easy formatted printing.
System.out.printf("pi = %.5f", Math.PI); // => pi = 3.14159
/*
* Input
*/
// use Scanner to read input
// must import java.util.Scanner;
Scanner scanner = new Scanner(System.in);
// read string input
String name = scanner.next();
// read byte input
byte numByte = scanner.nextByte();
// read int input
int numInt = scanner.nextInt();
// read long input
float numFloat = scanner.nextFloat();
// read double input
double numDouble = scanner.nextDouble();
// read boolean input
boolean bool = scanner.nextBoolean();
///////////////////////////////////////
// Variables
///////////////////////////////////////
/*
* Variable Declaration
*/
// Declare a variable using <type> <name>
int fooInt;
// Declare multiple variables of the same
// type <type> <name1>, <name2>, <name3>
int fooInt1, fooInt2, fooInt3;
/*
* Variable Initialization
*/
// Initialize a variable using <type> <name> = <val>
int barInt = 1;
// Initialize multiple variables of same type with same
// value <type> <name1>, <name2>, <name3> = <val>
int barInt1, barInt2, barInt3;
barInt1 = barInt2 = barInt3 = 1;
/*
* Variable types
*/
// Byte - 8-bit signed two's complement integer
// (-128 <= byte <= 127)
byte fooByte = 100;
// If you would like to interpret a byte as an unsigned integer
// then this simple operation can help
int unsignedIntLessThan256 = 0xff & fooByte;
// this contrasts a cast which can be negative.
int signedInt = (int) fooByte;
// Short - 16-bit signed two's complement integer
// (-32,768 <= short <= 32,767)
short fooShort = 10000;
// Integer - 32-bit signed two's complement integer
// (-2,147,483,648 <= int <= 2,147,483,647)
int bazInt = 1;
// Long - 64-bit signed two's complement integer
// (-9,223,372,036,854,775,808 <= long <= 9,223,372,036,854,775,807)
long fooLong = 100000L;
// L is used to denote that this variable value is of type Long;
// anything without is treated as integer by default.
// Note: byte, short, int and long are signed. They can have positive and negative values.
// There are no unsigned variants.
// char, however, is 16-bit unsigned.
// Float - Single-precision 32-bit IEEE 754 Floating Point
// 2^-149 <= float <= (2-2^-23) * 2^127
float fooFloat = 234.5f;
// f or F is used to denote that this variable value is of type float;
// otherwise it is treated as double.
// Double - Double-precision 64-bit IEEE 754 Floating Point
// 2^-1074 <= x <= (2-2^-52) * 2^1023
double fooDouble = 123.4;
// Boolean - true & false
boolean fooBoolean = true;
boolean barBoolean = false;
// Char - A single 16-bit Unicode character
char fooChar = 'A';
// final variables can't be reassigned to another object,
final int HOURS_I_WORK_PER_WEEK = 9001;
// but they can be initialized later.
final double E;
E = 2.71828;
// BigInteger - Immutable arbitrary-precision integers
//
// BigInteger is a data type that allows programmers to manipulate
// integers longer than 64-bits. Integers are stored as an array of
// of bytes and are manipulated using functions built into BigInteger
//
// BigInteger can be initialized using an array of bytes or a string.
BigInteger fooBigInteger = new BigInteger(fooByteArray);
// BigDecimal - Immutable, arbitrary-precision signed decimal number
//
// A BigDecimal takes two parts: an arbitrary precision integer
// unscaled value and a 32-bit integer scale
//
// BigDecimal allows the programmer complete control over decimal
// rounding. It is recommended to use BigDecimal with currency values
// and where exact decimal precision is required.
//
// BigDecimal can be initialized with an int, long, double or String
// or by initializing the unscaled value (BigInteger) and scale (int).
BigDecimal fooBigDecimal = new BigDecimal(fooBigInteger, fooInt);
// Be wary of the constructor that takes a float or double as
// the inaccuracy of the float/double will be copied in BigDecimal.
// Prefer the String constructor when you need an exact value.
BigDecimal tenCents = new BigDecimal("0.1");
// Strings
String fooString = "My String Is Here!";
// \n is an escaped character that starts a new line
String barString = "Printing on a new line?\nNo Problem!";
// \t is an escaped character that adds a tab character
String bazString = "Do you want to add a tab?\tNo Problem!";
System.out.println(fooString);
System.out.println(barString);
System.out.println(bazString);
// String Building
// #1 - with plus operator
// That's the basic way to do it (optimized under the hood)
String plusConcatenated = "Strings can " + "be concatenated " + "via + operator.";
System.out.println(plusConcatenated);
// Output: Strings can be concatenated via + operator.
// #2 - with StringBuilder
// This way doesn't create any intermediate strings. It just stores the string pieces, and ties them together
// when toString() is called.
// Hint: This class is not thread safe. A thread-safe alternative (with some impact on performance) is StringBuffer.
StringBuilder builderConcatenated = new StringBuilder();
builderConcatenated.append("You ");
builderConcatenated.append("can use ");
builderConcatenated.append("the StringBuilder class.");
System.out.println(builderConcatenated.toString()); // only now is the string built
// Output: You can use the StringBuilder class.
// StringBuilder is efficient when the fully constructed String is not required until the end of some processing.
StringBuilder stringBuilder = new StringBuilder();
String inefficientString = "";
for(int i = 0 ; i < 10; i++){
stringBuilder.append(i).append(" ");
inefficientString += i + " ";
}
System.out.println(inefficientString);
System.out.println(stringBuilder.toString());
// inefficientString requires a lot more work to produce, as it generates a String on every loop iteration.
// Simple concatenation with + is compiled to a StringBuilder and toString()
// Avoid string concatenation in loops.
// #3 - with String formatter
// Another alternative way to create strings. Fast and readable.
String.format("%s may prefer %s.", "Or you", "String.format()");
// Output: Or you may prefer String.format().
// Arrays
// The array size must be decided upon instantiation
// The following formats work for declaring an array
// <datatype>[] <var name> = new <datatype>[<array size>];
// <datatype> <var name>[] = new <datatype>[<array size>];
int[] intArray = new int[10];
String[] stringArray = new String[1];
boolean boolArray[] = new boolean[100];
// Another way to declare & initialize an array
int[] y = {9000, 1000, 1337};
String names[] = {"Bob", "John", "Fred", "Juan Pedro"};
boolean bools[] = {true, false, false};
// Indexing an array - Accessing an element
System.out.println("intArray @ 0: " + intArray[0]);
// Arrays are zero-indexed and mutable.
intArray[1] = 1;
System.out.println("intArray @ 1: " + intArray[1]); // => 1
// Other data types worth checking out
// ArrayLists - Like arrays except more functionality is offered, and
// the size is mutable.
// LinkedLists - Implementation of doubly-linked list. All of the
// operations perform as could be expected for a
// doubly-linked list.
// Maps - A set of objects that map keys to values. Map is
// an interface and therefore cannot be instantiated.
// The type of keys and values contained in a Map must
// be specified upon instantiation of the implementing
// class. Each key may map to only one corresponding value,
// and each key may appear only once (no duplicates).
// HashMaps - This class uses a hashtable to implement the Map
// interface. This allows the execution time of basic
// operations, such as get and insert element, to remain
// constant even for large sets.
// TreeMap - This class is a sorted tree structure. It implements a red
// black tree and sorts the entries based on the key value or
// the comparator provided while creating the object
///////////////////////////////////////
// Operators
///////////////////////////////////////
System.out.println("\n->Operators");
int i1 = 1, i2 = 2; // Shorthand for multiple declarations
// Arithmetic is straightforward
System.out.println("1+2 = " + (i1 + i2)); // => 3
System.out.println("2-1 = " + (i2 - i1)); // => 1
System.out.println("2*1 = " + (i2 * i1)); // => 2
System.out.println("1/2 = " + (i1 / i2)); // => 0 (int/int returns int)
System.out.println("1/2 = " + (i1 / (double)i2)); // => 0.5
// Modulo
System.out.println("11%3 = "+(11 % 3)); // => 2
// Comparison operators
System.out.println("3 == 2? " + (3 == 2)); // => false
System.out.println("3 != 2? " + (3 != 2)); // => true
System.out.println("3 > 2? " + (3 > 2)); // => true
System.out.println("3 < 2? " + (3 < 2)); // => false
System.out.println("2 <= 2? " + (2 <= 2)); // => true
System.out.println("2 >= 2? " + (2 >= 2)); // => true
// Boolean operators
System.out.println("3 > 2 && 2 > 3? " + ((3 > 2) && (2 > 3))); // => false
System.out.println("3 > 2 || 2 > 3? " + ((3 > 2) || (2 > 3))); // => true
System.out.println("!(3 == 2)? " + (!(3 == 2))); // => true
// Bitwise operators!
/*
~ Unary bitwise complement
<< Signed left shift
>> Signed/Arithmetic right shift
>>> Unsigned/Logical right shift
& Bitwise AND
^ Bitwise exclusive OR
| Bitwise inclusive OR
*/
// Increment operators
int i = 0;
System.out.println("\n->Inc/Dec-rementation");
// The ++ and -- operators increment and decrement by 1 respectively.
// If they are placed before the variable, they increment then return;
// after the variable they return then increment.
System.out.println(i++); // i = 1, prints 0 (post-increment)
System.out.println(++i); // i = 2, prints 2 (pre-increment)
System.out.println(i--); // i = 1, prints 2 (post-decrement)
System.out.println(--i); // i = 0, prints 0 (pre-decrement)
///////////////////////////////////////
// Control Structures
///////////////////////////////////////
System.out.println("\n->Control Structures");
// If statements are c-like
int j = 10;
if (j == 10) {
System.out.println("I get printed");
} else if (j > 10) {
System.out.println("I don't");
} else {
System.out.println("I also don't");
}
// While loop
int fooWhile = 0;
while(fooWhile < 100) {
System.out.println(fooWhile);
// Increment the counter
// Iterated 100 times, fooWhile 0,1,2...99
fooWhile++;
}
System.out.println("fooWhile Value: " + fooWhile);
// Do While Loop
int fooDoWhile = 0;
do {
System.out.println(fooDoWhile);
// Increment the counter
// Iterated 99 times, fooDoWhile 0->99
fooDoWhile++;
} while(fooDoWhile < 100);
System.out.println("fooDoWhile Value: " + fooDoWhile);
// For Loop
// for loop structure => for(<start_statement>; <conditional>; <step>)
for (int fooFor = 0; fooFor < 10; fooFor++) {
System.out.println(fooFor);
// Iterated 10 times, fooFor 0->9
}
System.out.println("fooFor Value: " + fooFor);
// Nested For Loop Exit with Label
outer:
for (int i = 0; i < 10; i++) {
for (int j = 0; j < 10; j++) {
if (i == 5 && j ==5) {
break outer;
// breaks out of outer loop instead of only the inner one
}
}
}
// For Each Loop
// The for loop is also able to iterate over arrays as well as objects
// that implement the Iterable interface.
int[] fooList = {1, 2, 3, 4, 5, 6, 7, 8, 9};
// for each loop structure => for (<object> : <iterable>)
// reads as: for each element in the iterable
// note: the object type must match the element type of the iterable.
for (int bar : fooList) {
System.out.println(bar);
//Iterates 9 times and prints 1-9 on new lines
}
// Switch Case
// A switch works with the byte, short, char, and int data types.
// It also works with enumerated types (discussed in Enum Types), the
// String class, and a few special classes that wrap primitive types:
// Character, Byte, Short, and Integer.
int month = 3;
String monthString;
switch (month) {
case 1: monthString = "January";
break;
case 2: monthString = "February";
break;
case 3: monthString = "March";
break;
default: monthString = "Some other month";
break;
}
System.out.println("Switch Case Result: " + monthString);
// Starting in Java 7 and above, switching Strings works like this:
String myAnswer = "maybe";
switch(myAnswer) {
case "yes":
System.out.println("You answered yes.");
break;
case "no":
System.out.println("You answered no.");
break;
case "maybe":
System.out.println("You answered maybe.");
break;
default:
System.out.println("You answered " + myAnswer);
break;
}
// Try-with-resources (Java 7+)
// Try-catch-finally statements work as expected in Java but in Java 7+
// the try-with-resources statement is also available. Try-with-resources
// simplifies try-catch-finally statements by closing resources
// automatically.
// In order to use a try-with-resources, include a an instance of a class
// in the try statement. The class must implement java.lang.AutoCloseable.
try(BufferedReader br = new BufferedReader(new FileReader("foo.txt"))) {
// You can attempt to do something that could throw an exception.
System.out.println(br.readLine());
// In Java 7, the resource will always be closed, even if it throws
// an Exception.
} catch (Exception ex) {
//The resource will be closed before the catch statement executes.
System.out.println("readLine() failed.");
}
// No need for a finally statement in this case, the BufferedReader is
// already closed. This can be used to avoid certain edge cases where
// a finally statement might not be called.
// To learn more:
// https://docs.oracle.com/javase/tutorial/essential/exceptions/tryResourceClose.html
// Conditional Shorthand
// You can use the '?' operator for quick assignments or logic forks.
// Reads as "If (statement) is true, use <first value>, otherwise, use
// <second value>"
int foo = 5;
String bar = (foo < 10) ? "A" : "B";
System.out.println(bar); // Prints A, because the statement is true
////////////////////////////////////////
// Converting Data Types And Typecasting
////////////////////////////////////////
// Converting data
// Convert String To Integer
Integer.parseInt("123");//returns an integer version of "123"
// Convert Integer To String
Integer.toString(123);//returns a string version of 123
// For other conversions check out the following classes:
// Double
// Long
// String
// Typecasting
// You can also cast Java objects, there's a lot of details and deals
// with some more intermediate concepts. Feel free to check it out here:
// https://docs.oracle.com/javase/tutorial/java/IandI/subclasses.html
///////////////////////////////////////
// Classes And Functions
///////////////////////////////////////
System.out.println("\n->Classes & Functions");
// (definition of the Bicycle class follows)
// Use new to instantiate a class
Bicycle trek = new Bicycle();
// Call object methods
trek.speedUp(3); // You should always use setter and getter methods
trek.setCadence(100);
// toString returns this Object's string representation.
System.out.println("trek info: " + trek.toString());
// Double Brace Initialization
// The Java Language has no syntax for how to create static Collections
// in an easy way. Usually you end up in the following way:
private static final Set<String> COUNTRIES = new HashSet<String>();
static {
COUNTRIES.add("DENMARK");
COUNTRIES.add("SWEDEN");
COUNTRIES.add("FINLAND");
}
// But there's a nifty way to achieve the same thing in an
// easier way, by using something that is called Double Brace
// Initialization.
private static final Set<String> COUNTRIES = new HashSet<String>() {{
add("DENMARK");
add("SWEDEN");
add("FINLAND");
}}
// The first brace is creating a new AnonymousInnerClass and the
// second one declares an instance initializer block. This block
// is called when the anonymous inner class is created.
// This does not only work for Collections, it works for all
// non-final classes.
} // End main method
} // End LearnJava class
// You can include other, non-public outer-level classes in a .java file,
// but it is not good practice. Instead split classes into separate files.
// Class Declaration Syntax:
// <public/private/protected> class <class name> {
// // data fields, constructors, functions all inside.
// // functions are called as methods in Java.
// }
class Bicycle {
// Bicycle's Fields/Variables
public int cadence; // Public: Can be accessed from anywhere
private int speed; // Private: Only accessible from within the class
protected int gear; // Protected: Accessible from the class and subclasses
String name; // default: Only accessible from within this package
static String className; // Static class variable
// Static block
// Java has no implementation of static constructors, but
// has a static block that can be used to initialize class variables
// (static variables).
// This block will be called when the class is loaded.
static {
className = "Bicycle";
}
// Constructors are a way of creating classes
// This is a constructor
public Bicycle() {
// You can also call another constructor:
// this(1, 50, 5, "Bontrager");
gear = 1;
cadence = 50;
speed = 5;
name = "Bontrager";
}
// This is a constructor that takes arguments
public Bicycle(int startCadence, int startSpeed, int startGear,
String name) {
this.gear = startGear;
this.cadence = startCadence;
this.speed = startSpeed;
this.name = name;
}
// Method Syntax:
// <public/private/protected> <return type> <function name>(<args>)
// Java classes often implement getters and setters for their fields
// Method declaration syntax:
// <access modifier> <return type> <method name>(<args>)
public int getCadence() {
return cadence;
}
// void methods require no return statement
public void setCadence(int newValue) {
cadence = newValue;
}
public void setGear(int newValue) {
gear = newValue;
}
public void speedUp(int increment) {
speed += increment;
}
public void slowDown(int decrement) {
speed -= decrement;
}
public void setName(String newName) {
name = newName;
}
public String getName() {
return name;
}
//Method to display the attribute values of this Object.
@Override // Inherited from the Object class.
public String toString() {
return "gear: " + gear + " cadence: " + cadence + " speed: " + speed +
" name: " + name;
}
} // end class Bicycle
// PennyFarthing is a subclass of Bicycle
class PennyFarthing extends Bicycle {
// (Penny Farthings are those bicycles with the big front wheel.
// They have no gears.)
public PennyFarthing(int startCadence, int startSpeed) {
// Call the parent constructor with super
super(startCadence, startSpeed, 0, "PennyFarthing");
}
// You should mark a method you're overriding with an @annotation.
// To learn more about what annotations are and their purpose check this
// out: http://docs.oracle.com/javase/tutorial/java/annotations/
@Override
public void setGear(int gear) {
this.gear = 0;
}
}
// Interfaces
// Interface declaration syntax
// <access-level> interface <interface-name> extends <super-interfaces> {
// // Constants
// // Method declarations
// }
// Example - Food:
public interface Edible {
public void eat(); // Any class that implements this interface, must
// implement this method.
}
public interface Digestible {
public void digest();
// In Java 8, interfaces can have default method.
// public void digest() {
// System.out.println("digesting ...");
// }
}
// We can now create a class that implements both of these interfaces.
public class Fruit implements Edible, Digestible {
@Override
public void eat() {
// ...
}
@Override
public void digest() {
// ...
}
}
// In Java, you can extend only one class, but you can implement many
// interfaces. For example:
public class ExampleClass extends ExampleClassParent implements InterfaceOne,
InterfaceTwo {
@Override
public void InterfaceOneMethod() {
}
@Override
public void InterfaceTwoMethod() {
}
}
// Abstract Classes
// Abstract Class declaration syntax
// <access-level> abstract <abstract-class-name> extends <super-abstract-classes> {
// // Constants and variables
// // Method declarations
// }
// Marking a class as abstract means that it contains abstract methods that
// must be defined in a child class. Similar to interfaces, abstract classes
// cannot be instantiated, but instead must be extended and the abstract
// methods defined. Different from interfaces, abstract classes can contain a
// mixture of concrete and abstract methods. Methods in an interface cannot
// have a body, unless the method is static, and variables are final by default,
// unlike an abstract class. Also abstract classes CAN have the "main" method.
public abstract class Animal
{
public abstract void makeSound();
// Method can have a body
public void eat()
{
System.out.println("I am an animal and I am Eating.");
// Note: We can access private variable here.
age = 30;
}
// No need to initialize, however in an interface
// a variable is implicitly final and hence has
// to be initialized.
private int age;
public void printAge()
{
System.out.println(age);
}
// Abstract classes can have main function.
public static void main(String[] args)
{
System.out.println("I am abstract");
}
}
class Dog extends Animal
{
// Note still have to override the abstract methods in the
// abstract class.
@Override
public void makeSound()
{
System.out.println("Bark");
// age = 30; ==> ERROR! age is private to Animal
}
// NOTE: You will get an error if you used the
// @Override annotation here, since java doesn't allow
// overriding of static methods.
// What is happening here is called METHOD HIDING.
// Check out this SO post: http://stackoverflow.com/questions/16313649/
public static void main(String[] args)
{
Dog pluto = new Dog();
pluto.makeSound();
pluto.eat();
pluto.printAge();
}
}
// Final Classes
// Final Class declaration syntax
// <access-level> final <final-class-name> {
// // Constants and variables
// // Method declarations
// }
// Final classes are classes that cannot be inherited from and are therefore a
// final child. In a way, final classes are the opposite of abstract classes
// because abstract classes must be extended, but final classes cannot be
// extended.
public final class SaberToothedCat extends Animal
{
// Note still have to override the abstract methods in the
// abstract class.
@Override
public void makeSound()
{
System.out.println("Roar");
}
}
// Final Methods
public abstract class Mammal()
{
// Final Method Syntax:
// <access modifier> final <return type> <function name>(<args>)
// Final methods, like, final classes cannot be overridden by a child
// class, and are therefore the final implementation of the method.
public final boolean isWarmBlooded()
{
return true;
}
}
// Enum Type
//
// An enum type is a special data type that enables for a variable to be a set
// of predefined constants. The variable must be equal to one of the values
// that have been predefined for it. Because they are constants, the names of
// an enum type's fields are in uppercase letters. In the Java programming
// language, you define an enum type by using the enum keyword. For example,
// you would specify a days-of-the-week enum type as:
public enum Day {
SUNDAY, MONDAY, TUESDAY, WEDNESDAY,
THURSDAY, FRIDAY, SATURDAY
}
// We can use our enum Day like that:
public class EnumTest {
// Variable Enum
Day day;
public EnumTest(Day day) {
this.day = day;
}
public void tellItLikeItIs() {
switch (day) {
case MONDAY:
System.out.println("Mondays are bad.");
break;
case FRIDAY:
System.out.println("Fridays are better.");
break;
case SATURDAY:
case SUNDAY:
System.out.println("Weekends are best.");
break;
default:
System.out.println("Midweek days are so-so.");
break;
}
}
public static void main(String[] args) {
EnumTest firstDay = new EnumTest(Day.MONDAY);
firstDay.tellItLikeItIs(); // => Mondays are bad.
EnumTest thirdDay = new EnumTest(Day.WEDNESDAY);
thirdDay.tellItLikeItIs(); // => Midweek days are so-so.
}
}
// Enum types are much more powerful than we show above.
// The enum body can include methods and other fields.
// You can see more at https://docs.oracle.com/javase/tutorial/java/javaOO/enum.html
Further Reading
The links provided here below are just to get an understanding of the topic, feel free to Google and find specific examples.
Official Oracle Guides:
-
New features in Java 8:
Online Practice and Tutorials
Books: