Author: kpark

import java.util.*;

class Person implements Comparable<Person> {
    private static int count = 0; // static (class) field
    protected String name; // instance field
    protected int age; // instance field

    public Person() {
        //System.out.println("Person Constructor"); // this("", 0); error: call to this must be first statemenht in constructor
        this("", 0);
    }

    public Person(String name, int age) {
        count++;
        this.name = name;
        this.age = age;
    }

    public Person(Person other) {
        this(other.name, other.age);
    }

    public String getName() {
        return name;
    }

    public int getAge() {
        return age;
    }

    public void setName(String name) {
        this.name = name;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public void set(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public void set(Person other) {
        this.name = other.name;
        this.age = other.age;
    }

    public Person clone() {
        Person p = new Person(this.name, this.age);
        return p;
    }

    @Override
    public boolean equals(Object other) { // Object.equals overriding
        if (other instanceof Person) {
            Person that = (Person) other;
            return that.canEqual(this) && this.getName().equals(that.getName()) && this.getAge() == that.getAge();
        }
        return false;
    }

    @Override
    public int hashCode() {
        return (41 * getName().hashCode() + getAge());
    }

    public boolean canEqual(Object other) {
        return (other instanceof Person);
    }

    @Override
    public String toString() { // Object.toString() overriding
        return "Person Name: " + name + " Age: " + age;
    }

    public void print() { // instance methods
        System.out.println("Person Name: " + name + " Age: " + age);
    }

    public static void printCount() { // static (class) methods
        System.out.println("Person Count: " + count);
    }

    public static int getCount() {  // static (class) methods
        return count; 
    }

    public static void setCount(int value) { // static (class) methods
        count = value; 
    } 

    public int compareTo(Person other) {
        String thisName = this.getName().toUpperCase();
        String otherName = ((Person)other).getName().toUpperCase();
        //ascending order
        return thisName.compareTo(otherName);
        //descending order
        //return otherName.compareTo(thisName);
    }

    public static Comparator<Person> AgeComparator = new Comparator<Person>() {
        public int compare(Person p1, Person p2) {
            int age1 = p1.getAge();
            int age2 = p2.getAge();
            //ascending order
            return age1 – age2;
            //descending order
            //return age2 – age1;
        }
    };
}

///////////////////////////////////////////////////////////////////////////////////////
class Student extends Person {
    private static int count = 0; // static (class) field
    protected int id;

    public Student() {
        id = 5208;
    }

    public Student(String name, int age, int id) {
        super(name, age);
        this.id = id;
        count++;
    }

    public int getID() {
        return id;
    }

    public void setID(int id) {
        this.id = id;
    }

    public void set(String name, int age, int id) {
        super.set(name, age);
        this.id = id;
    }

    public void set(String name, int age) {
        super.set(name, age);
    }

    public void set(Student other) {
        this.set(other.name, other.age, other.id);
    }

    public void set(Person other) {
        if (other instanceof Person)
            super.set(other);
        else
            this.set((Student)other);
    }

    public Student clone() {
        Student s = new Student(this.name, this.age, this.id);
        return s;
    }

    @Override
    public boolean equals(Object other) { // Object.equals overriding
        if (other instanceof Student) {
            Student that = (Student) other;
            return that.canEqual(this) && this.getName().equals(that.getName()) && this.getAge() == that.getAge() && this.getID() == that.getID();
        }
        return false;
    }

    @Override
    public int hashCode() {
        return (41 * super.hashCode() + getID());
    }

    public boolean canEqual(Object other) {
        return (other instanceof Student);
    }

    @Override
    public String toString() { // Object.toString() overriding
        return "Student Name: " + name + " Age: " + age + " ID: " + id;
    }

    public void superPrint() {
        super.print();
    }

    public void print() { // Person.print() method overriding
        System.out.println("Student Name: " + name + " Age: " + age + " ID: " + id);
    }

    public static void printCount() { // static (class) methods
        System.out.println("Student Count: " + count);
    }

    public static int getCount() { // static (class) methods
        return count; 
    } 
    public static void setCount(int value) { // static (class) methods
        count = value; 
    }

    public int compareTo(Student other) {
        String thisName = this.getName().toUpperCase();
        String otherName = ((Student)other).getName().toUpperCase();
        //ascending order
        return thisName.compareTo(otherName);
        //descending order
        //return otherName.compareTo(thisName);
    }

    public static Comparator<Student> AgeComparator = new Comparator<Student>() {
        public int compare(Student p1, Student p2) {
            int age1 = p1.getAge();
            int age2 = p2.getAge();
            //ascending order
            return age1 – age2;
            //descending order
            //return age2 – age1;
        }
    };

    public static Comparator<Student> IDComparator = new Comparator<Student>() {
        public int compare(Student p1, Student p2) {
            int id1 = p1.getID();
            int id2 = p2.getID();
            //ascending order
            return id1 – id2;
            //descending order
            //return id2 – id1;
        }
    };
}

class PersonStudentTest {

    public static void print(Object[] array) {
        for(Object o : array) {
            System.out.println(o);
        }
    }

    public static void main(String[] args) {
///////////////////////////////////////////////////////////////////////
        Student[] sList = new Student[3];
        sList[0] = new Student("Kevin", 0, 222);
        sList[1] = new Student("Jason", 1, 333);
        sList[2] = new Student("John", 2, 111);

        System.out.println("STUDENT SORT BY NAME (DEFAULT)!!!");
        Arrays.sort(sList);
        print(sList);

        System.out.println("STUDENT SORT by AGE!!!");
        Arrays.sort(sList, Student.AgeComparator);
        print(sList);

        System.out.println("STUDENT SORT by ID!!!");
        Arrays.sort(sList, Student.IDComparator);
        print(sList);

///////////////////////////////////////////////////////////////////////
        Student[] sList2 = new Student[3];
        sList2[0] = new Student("Kevin", 0, 222);
        sList2[1] = new Student("Jason", 1, 333);
        sList2[2] = new Student("John", 2, 111);

        System.out.println("STUDENT SORT BY NAME (anonymous method)!!!");
        Arrays.sort(sList2, new Comparator<Student>() {
            public int compare(Student s1, Student s2) {
                return s1.getName().toUpperCase().compareTo(s2.getName().toUpperCase());
            }
        });
        print(sList2);

        System.out.println("STUDENT SORT by AGE (anonymous method)!!!");
        Arrays.sort(sList2, new Comparator<Student>() {
            public int compare(Student s1, Student s2) {
                return s1.getAge() – s2.getAge();
            }
        });
        print(sList2);

        System.out.println("STUDENT SORT by ID (anonymous method)!!!");
        Arrays.sort(sList2, new Comparator<Student>() {
            public int compare(Student s1, Student s2) {
                return s1.getID() – s2.getID();
            }
        });
        print(sList2);

///////////////////////////////////////////////////////////////////////
        Student[] sList3 = sList2;

        System.out.println("STUDENT SORT BY NAME (lambda)!!!");
        Arrays.sort(sList3, (Student ss1, Student ss2) ->
                            ss1.getName().compareTo(ss2.getName())
        );
        Arrays.stream(sList3).forEach((s) -> System.out.println(s));

        System.out.println("STUDENT SORT by AGE (lambda)!!!");
        Arrays.sort(sList3, (Student ss1, Student ss2) ->
                            Integer.compare(ss1.getAge(), ss2.getAge())
        );
        Arrays.stream(sList3).forEach((s) -> System.out.println(s));

        System.out.println("STUDENT SORT by ID (lambda)!!!");
        Arrays.sort(sList3, (Student ss1, Student ss2) ->
                            Integer.compare(ss1.getID(), ss2.getID())
        );
        Arrays.stream(sList3).forEach((s) -> System.out.println(s));

///////////////////////////////////////////////////////////////////////
        List<Student> sList4 = new ArrayList<Student>();
        sList4.add(new Student("Kevin", 0, 222));
        sList4.add(new Student("Jason", 1, 333));
        sList4.add(new Student("John", 2, 111));

        System.out.println("STUDENTLIST SORT BY NAME (lambda)!!!");
        sList4.sort((Student ss1, Student ss2) ->
                    ss1.getName().compareTo(ss2.getName())
        );
        sList4.forEach((s) -> System.out.println(s));

        System.out.println("STUDENTLIST SORT by AGE (lambda)!!!");
        sList4.sort((Student ss1, Student ss2) ->
                    ss1.getAge() – ss2.getAge()
        );
        sList4.forEach((s) -> System.out.println(s));

        System.out.println("STUDENTLIST SORT by ID (lambda)!!!");
        sList4.sort((Student ss1, Student ss2) ->
                    ss1.getID() – ss2.getID()
        );
        sList4.forEach((s) -> System.out.println(s));
    }

}

Shape 추상클래스 has-a ShapeType, ShapeColor, ShapeRect

Shape 추상클래스를 상속받은 Triangle, Rectangle, Square, Circle 클래스는 추상메소드인 void area(), void perimeter()를 반드시 구현하였다.

Shape 추상클래스는 Moveable, Scalable 인터페이스를 상속하여 추상메소드인 Moveable 인터페이스의 void move(int, int)와 Scalable 인터페이스의 void scale(int)를 구현하였다.

ShapeTest-Interface-src

https://stackoverflow.com/questions/2223386/why-doesnt-java-allow-overriding-of-static-methods

Yes. Practically Java allows overriding static method, and No theoretically if you Override a static method in Java then it will compile and run smoothly but it will lose Polymorphism which is the basic property of Java. You will Read Everywhere that it is not possible to try yourself compiling and running. you will get your answer. e.g. If you Have Class Animal and a static method eat() and you Override that static method in its Subclass lets called it Dog. Then when wherever you Assign a Dog object to an Animal Reference and call eat() according to Java Dog’s eat() should have been called but in static Overriding Animals’ eat() will Be Called.

class Animal {
public static void eat() {
System.out.println(“Animal Eating”);
}
}

class Dog extends Animal{
public static void eat() {
System.out.println(“Dog Eating”);
}
}

class Test {
public static void main(String args[]) {
Animal obj= new Dog();//Dog object in animal
obj.eat(); //should call dog’s eat but it didn’t
}
}

Output Animal Eating

According to Polymorphism Principle of Java, the Output Should be Dog Eating.
But the result was different because to support Polymorphism Java uses Late Binding that means methods are called only at the run-time but not in the case of static methods. In static methods compiler calls methods at the compile time rather than the run-time, so we get methods according to the reference and not according to the object a reference a containing that’s why You can say Practically it supports static overring but theoretically, it doesn’t.

http://www.programmerinterview.com/index.php/java-questions/method-overriding-vs-overloading/

Method Overloading: 동일한 함수명에 매개변수가 다른 함수를 둘 이상 정의하는 것으로, 동일한 함수 기능을 수행하지만 다른 매개변수의 경우를 처리할 때 사용

//compiler error – can’t overload based on the
//type returned (one method returns int, the other returns a float):
//int changeDate(int Year) ;
//float changeDate (int Year);

//compiler error – can’t overload by changing just
//the name of the parameter (from Year to Month):
//int changeDate(int Year);
//int changeDate(int Month) ;

//valid case of overloading,
//since the methods have different number of parameters:
int changeDate(int Year, int Month) ;
int changeDate(int Year);

//also a valid case of overloading,
//since the parameters are of different types:
int changeDate(float Year) ;
int changeDate(int Year);

Method Overriding: 상속받은 파생 클래스에서 동일한 함수명에 동일한 매개변수로 정의하여 함수를 재정의하는 것으로 상속되어진 함수의 기능을 변경해서 재사용하고 싶을 때 사용

public class Parent {
    public int someMethod() {
        return 3;
    }
}

public class Child extends Parent{

    // this is method overriding:
    public int someMethod() {
        return 4;
    }
}

공용 데이터를 저장하여 사용하고자 할 때, singleton design pattern이나 inner static class를 사용한다.

싱글톤 패턴 (Singleton design pattern) -싱글톤 패턴이란 single instance object(해당 클래스의 인스턴스 하나)가 만들어지고, 어디서든지 그 싱글톤에 접근할 수 있도록 하기 위한 패턴

public final class SiteStructureSingletonPattern {
    private Object[] data = new Object[10];

    private static SiteStructureSingletonPattern instance = new SiteStructureSingletonPattern(); // private static member인 instance
    public static SiteStructureSingletonPattern getInstance() { // getInstance() 메소드를 사용하여 instance 객체 사용
        return instance;
    }
    private SiteStructureSingletonPattern() {
        System.out.println("SiteStructureSingletonPattern constructor");
        for(int i=0; i<10; i++) {
            data[i] = i+1;
        }
    }
    public void print() {
        System.out.print("SiteStructureSingletonPattern data=");
        for(int i=0; i<10; i++) {
            System.out.print(" " + data[i]);
        }
        System.out.println();
    }
}

public class SiteStructureStaticClass {
    private Object[] data = new Object[10];

    private static class SiteStructureStatic { // inner static class
        private static final SiteStructureStaticClass instance = new SiteStructureStaticClass();
    }
    public static SiteStructureStaticClass getInstance() {
        return SiteStructureStatic.instance;
    }
    private SiteStructureStaticClass() {
        System.out.println("SiteStructureStaticClass constructor");
        for(int i=0; i<10; i++) {
            data[i] = i+1;
        }
    }

    public void print() {
        System.out.print("SiteStructureStaticClass data=");
        for(int i=0; i<10; i++) {
            System.out.print(" " + data[i]);
        }
        System.out.println();
    }
}

public class StaticClassSingletonPatternTest {

    public static void main(String[] args) {
        SiteStructureSingletonPattern instance = SiteStructureSingletonPattern.getInstance();
        instance.print();
        instance = SiteStructureSingletonPattern.getInstance();
        instance.print();

        SiteStructureStaticClass instance2 = SiteStructureStaticClass.getInstance();
        instance2.print();
        instance2 = SiteStructureStaticClass.getInstance();
        instance2.print();
    }
}

PhoneNumber – String workPhone, homePhone, cellPhone
Person – String name, PhoneNumber phone, String email

PersonContactTest (Person has-a PhoneNumber)

// private constructor는 정적 메소드와 필드 (static method & field)만 있는 경우 사용함.

class Counter {
    private Counter() { }
    public static int currentCount;
    public static int IncrementCount() { return ++currentCount; }
}

class TestCounter {
    public static void main(String[] args) {
        // If you uncomment the following statement, it will generate
        // an error because the constructor is inaccessible:
        //Counter aCounter = new Counter(); // Error
        Counter.currentCount = 100;
        Counter.IncrementCount();
        System.out.println("count=" + Counter.currentCount); // count=101
    }
}

 

// protected constructor는 추상클래스 (abstract class)에서 사용을 권고함. 추상 클래스를 상속받는 파생클래스에서 파생 클래스 생성자가 부모 클래스 즉, 추상 클래스를 초기화 하기 위해 추상 클래스 생성자를 호출 할 수 있도록 지원함.

public abstract class Shape {
    protected Shape(String name) { 
        this.name = name;
    }
    private String name;
    public void print() { System.out.print(this.name); }
}

public class Rectangle extends Shape {
    private double width, height;
    public Rectangle(String name) { 
        super(name); 
        this.width = 2; 
        this.height = 3; 
    }
    @Override
    public void print() {
        super.print();
        System.out.printf(" 가로: %f 세로: %f\n", this.width, this.height);
    }
}

public class Triangle extends Shape {
    private double bottom, height;
    public Triangle(String name) { 
        super(name); 
        this.bottom = 1; 
        this.height = 1; 
    }
    @Override
    public void print() {
        super.print();
        System.out.printf(" 밑변: %f 높이: %f\n", this.bottom, this.height);
    }
}

class ShapeTest {
    public static void main(String[] args) {
        //Shape s1 = new Shape("도형"); // Error; Shape is abstract; cannot be instantiated
        Shape s = new Triangle("삼각형");
        s.print(); // 삼각형 밑변: 1 높이: 1
        s = new Rectangle("직사각형");
        s.print(); // 직사각형 가로: 2 세로: 3
    }
}

Static Initializer Block

• class 로딩 시 호출
• instance variable이나 method에 접근 못함
• static variable 초기화에 사용

public class StaticIntializerBlockTest {
    private static int id = 5;
    static {
        if (id <10) id = 20;
        else id = 100;
    }

    public static int getId() {
        return id;
    }

    public static void print() {
        System.out.println("StaticIntializerBlockTest.id=" + getId());
    }
}

public class StaticInstanceInitializerBlockTest {

    public static void main(String[] args) {
        StaticIntializerBlockTest.print();
    }
}

StaticIntializerBlockTest.id=20 // static block 이 호출되면서 20으로 셋팅

Instance Initializer Block

• 객체 생성시 호출
• super 생성자 이후에 실행하고, 생성자보다 먼저 실행
• instance variable이나 method에 접근 가능
• 모든 생성자의 공통 부분을 instance initializer block에 넣어줌

class InstanceInitializerBlockSuper {
    public InstanceInitializerBlockSuper() {
        System.out.println("InstanceInitializerBlockSuper");
    }
}

public class InstanceInitializerBlockTest extends InstanceInitializerBlockSuper {
    private int id = 5;
    {
        if (id <10) id = 20;
        else id = 100;
    }

    public InstanceInitializerBlockTest() {
        System.out.println("InstanceInitializerBlockTest.id=" + this.id);
    }

    public InstanceInitializerBlockTest(int id) {
        System.out.println(“InstanceInitializerBlockTest.id=" + this.id);
        this.id = id;
    }

    public int getId() {
        return id;
    }

    public void print() {
        System.out.println("StaticIntializerBlockTest.id=" + getId());
    }
}

public class StaticInstanceInitializerBlockTest {
    public static void main(String[] args) {
        InstanceInitializerBlockTest i = new InstanceInitializerBlockTest();
        i.print();
        i = new InstanceInitializerBlockTest(30);
        i.print();
    }
}

InstanceInitializerBlockSuper // super 생성자 이후에 실행
InstanceInitializerBlockTest.id=20 // instance block 호출되면서 20으로 셋팅
id=20
InstanceInitializerBlockSuper // super 생성자 이후에 실행
InstanceInitializerBlockTest.id=20 // instance block 호출되면서 20으로 셋팅된후 this.id = id를 통해서 30으로 셋팅
id=30

https://stackoverflow.com/questions/12550135/static-block-vs-initializer-block-in-java