learnxinyminutes-docs/java.md at 4f8cffda2ebda8621b6cdeb18c3dd6881bbc151f

// 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 * @deprecated  For showing the code is outdated or shouldn't be used.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.*;
// Java to illustrate calling of static members and methods without calling classname
import static java.lang.Math.*;
import static java.lang.System.*;

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
    ///////////////////////////////////////

        /*
        * Output
        */

        // 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
        long numLong = scanner.nextLong();

        // read float 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>
        // <name1> = <name2> = <name3> = <val>
        int barInt1, barInt2, barInt3;
        barInt1 = barInt2 = barInt3 = 1;
        // Shorthand for multiple declarations
        int barInt4 = 1, barInt5 = 2; 


        /*
        *  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,
        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
        // 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");

        // Type inference with 'var'
        var x = 100; // int
        var y = 1.90; // double
        var z = 'a'; // char
        var p = "tanu"; // String
        var q = false; // boolean

        // Strings
        String fooString = "My String Is Here!";

        // Text blocks
        var textBlock = """
                        This is a <Text Block> in Java 
                        """;

        // \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 mapping of key Objects to value Objects. 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-amortized even for large sets.
        // TreeMap - A Map that is sorted by its keys. Each modification
        //           maintains the sorting defined by either a Comparator
        //           supplied at instantiation, or comparisons of each Object
        //           if they implement the Comparable interface.
        //           Failure of keys to implement Comparable combined with failure to
        //           supply a Comparator will throw ClassCastExceptions.
        //           Insertion and removal operations take O(log(n)) time
        //           so avoid using this data structure unless you are taking
        //           advantage of the sorting.

        ///////////////////////////////////////
        // Operators
        ///////////////////////////////////////
        System.out.println("\n->Operators");

        int i1 = 1, i2 = 2;

        // 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("1/2.0 = " + (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 100 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.
        // Starting in Java 7 and above, we can also use the String type.
        // Note: Do remember that, not adding "break" at end any particular case ends up in
        // executing the very next case(given it satisfies the condition provided) as well.
        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);


        // 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 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 (IOException | SQLException ex) {
            // Java 7+ Multi catch block handle both exceptions
        } 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 : " + bar); // Prints "bar : A", because the
        // statement is true.
        // Or simply
        System.out.println("bar : " + (foo < 10 ? "A" : "B"));


        ////////////////////////////////////////
        // Converting Data Types
        ////////////////////////////////////////

        // 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

        ///////////////////////////////////////
        // 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());
    } // End main method

    private static class TestInitialization {
        // Double Brace Initialization
        // Before Java 11, the Java Language had no syntax for how to create
        // static Collections in an easy way. Usually you end up like this:
        private static final Set<String> COUNTRIES = new HashSet<String>();
        static {
           COUNTRIES.add("DENMARK");
           COUNTRIES.add("SWEDEN");
           COUNTRIES.add("FINLAND");
        }

        // There's a nifty way to achieve the same thing, 
        // by using something that is called Double Brace Initialization.
        private static final Set<String> COUNTRIES_DOUBLE_BRACE = 
        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.


        // Another option was to initialize the Collection from an array,
        // using Arrays.asList() method:
        private static final List<String> COUNTRIES_AS_LIST = 
                        Arrays.asList("SWEDEN", "DENMARK", "NORWAY");
        // This has one catch: the list we get is internally backed by the array,
        // and since arrays can't change their size, the list backed by the array
        // is not resizeable, which means we can't add new elements to it: 
        public static void main(String[] args) {
            COUNTRIES.add("FINLAND"); // throws UnsupportedOperationException!
            // However, we can replace elements by index, just like in array: 
            COUNTRIES.set(1, "FINLAND");
            System.out.println(COUNTRIES); // prints [SWEDEN, FINLAND, NORWAY]
        }
        // The resizing problem can be circumvented 
        // by creating another Collection from the List:
         private static final Set<String> COUNTRIES_SET = 
                new HashSet<>(Arrays.asList("SWEDEN", "DENMARK", "NORWAY"));
        // It's perfectly fine to add anything to the Set of COUNTRIES now. 
    } // End TestInitialization class

    private static class TestJava11Initialization {
        // Since Java 11, there is a convenient option to initialize Collections:
        // Set.of() and List.of() methods. 
        private static final Set<String> COUNTRIES = 
                Set.of("SWEDEN", "DENMARK", "NORWAY");
        // There is a massive catch, though: Lists and Sets initialized like this 
        // 1) are immutable 
        // 2) can't contain null elements (even check for null elements fails)!
        public static void main(String[] args) {
            COUNTRIES.add("FINLAND"); // throws UnsupportedOperationException
            COUNTRIES.remove("NORWAY"); // throws UnsupportedOperationException 
            COUNTRIES.contains(null); // throws NullPointerException
        }
        private static final Set<String> COUNTRIES_WITH_NULL = 
                    Set.of("SWEDEN", null, "NORWAY"); // throws NullPointerException

    } // End TestJava11Initialization class
} // 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;
    }
}

// Object casting
// Since the PennyFarthing class is extending the Bicycle class, we can say
// a PennyFarthing is a Bicycle and write :
// Bicycle bicycle = new PennyFarthing();
// This is called object casting where an object is taken for another one. There
// are lots of details and deals with some more intermediate concepts here:
// https://docs.oracle.com/javase/tutorial/java/IandI/subclasses.html

// 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();
    // Since Java 8, interfaces can have default method.
    public default void defaultMethod() {
        System.out.println("Hi from default method ...");
    }
}

// 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 class <abstract-class-name> extends
// <super-abstract-classes> {
//     // Constants and variables
//     // Method declarations
// }

// Abstract Classes cannot be instantiated.
// Abstract classes may define abstract methods.
// Abstract methods have no body and are marked abstract
// Non-abstract child classes must @Override all abstract methods
// from their super-classes.
// Abstract classes can be useful when combining repetitive logic
// with customised behavior, but as Abstract classes require
// inheritance, they violate "Composition over inheritance"
// so consider other approaches using composition.
// https://en.wikipedia.org/wiki/Composition_over_inheritance

public abstract class Animal
{
    private int age;

    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;
    }

    public void printAge()
    {
        System.out.println(age);
    }

    // Abstract classes can have main method.
    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;
    }
}

// Java Records are a concise way to define immutable data carrier classes, automatically
// generating boilerplate code like constructors, equals(), hashCode()and toString().
// This automatically creates an immutable class Person with fields name and age.
public record Person(String name, int age) {}
Person p = new Person("Alice", 30);

// 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

// Getting Started with Lambda Expressions
//
// New to Java version 8 are lambda expressions. Lambdas are more commonly found
// in functional programming languages, which means they are methods which can
// be created without belonging to a class, passed around as if it were itself
// an object, and executed on demand.
//
// Final note, lambdas must implement a functional interface. A functional
// interface is one which has only a single abstract method declared. It can
// have any number of default methods. Lambda expressions can be used as an
// instance of that functional interface. Any interface meeting the requirements
// is treated as a functional interface. You can read more about interfaces
// above.
//
import java.util.Map;
import java.util.HashMap;
import java.util.function.*;
import java.security.SecureRandom;

public class Lambdas {
    public static void main(String[] args) {
        // Lambda declaration syntax:
        // <zero or more parameters> -> <expression body or statement block>

        // We will use this hashmap in our examples below.
        Map<String, String> planets = new HashMap<>();
            planets.put("Mercury", "87.969");
            planets.put("Venus", "224.7");
            planets.put("Earth", "365.2564");
            planets.put("Mars", "687");
            planets.put("Jupiter", "4,332.59");
            planets.put("Saturn", "10,759");
            planets.put("Uranus", "30,688.5");
            planets.put("Neptune", "60,182");

        // Lambda with zero parameters using the Supplier functional interface
        // from java.util.function.Supplier. The actual lambda expression is
        // what comes after numPlanets =.
        Supplier<String> numPlanets = () -> Integer.toString(planets.size());
        System.out.format("Number of Planets: %s\n\n", numPlanets.get());

        // Lambda with one parameter and using the Consumer functional interface
        // from java.util.function.Consumer. This is because planets is a Map,
        // which implements both Collection and Iterable. The forEach used here,
        // found in Iterable, applies the lambda expression to each member of
        // the Collection. The default implementation of forEach behaves as if:
        /*
            for (T t : this)
                action.accept(t);
        */

        // The actual lambda expression is the parameter passed to forEach.
        planets.keySet().forEach((p) -> System.out.format("%s\n", p));

        // If you are only passing a single argument, then the above can also be
        // written as (note absent parentheses around p):
        planets.keySet().forEach(p -> System.out.format("%s\n", p));

        // Tracing the above, we see that planets is a HashMap, keySet() returns
        // a Set of its keys, forEach applies each element as the lambda
        // expression of: (parameter p) -> System.out.format("%s\n", p). Each
        // time, the element is said to be "consumed" and the statement(s)
        // referred to in the lambda body is applied. Remember the lambda body
        // is what comes after the ->.

        // The above without use of lambdas would look more traditionally like:
        for (String planet : planets.keySet()) {
            System.out.format("%s\n", planet);
        }

        // This example differs from the above in that a different forEach
        // implementation is used: the forEach found in the HashMap class
        // implementing the Map interface. This forEach accepts a BiConsumer,
        // which generically speaking is a fancy way of saying it handles
        // the Set of each Key -> Value pairs. This default implementation
        // behaves as if:
        /*
            for (Map.Entry<K, V> entry : map.entrySet())
                action.accept(entry.getKey(), entry.getValue());
        */

        // The actual lambda expression is the parameter passed to forEach.
        String orbits = "%s orbits the Sun in %s Earth days.\n";
        planets.forEach((K, V) -> System.out.format(orbits, K, V));

        // The above without use of lambdas would look more traditionally like:
        for (String planet : planets.keySet()) {
            System.out.format(orbits, planet, planets.get(planet));
        }

        // Or, if following more closely the specification provided by the
        // default implementation:
        for (Map.Entry<String, String> planet : planets.entrySet()) {
            System.out.format(orbits, planet.getKey(), planet.getValue());
        }

        // These examples cover only the very basic use of lambdas. It might not
        // seem like much or even very useful, but remember that a lambda can be
        // created as an object that can later be passed as parameters to other
        // methods.
    }
}
40 KiB

Raw Blame History

Further Reading

Official Oracle Guides

Online Practice and Tutorials

Books

40 KiB Raw Blame History

Further Reading

Official Oracle Guides

Online Practice and Tutorials

Books

40 KiB

Raw Blame History
