Month: May 2021

http://blog.cleancoder.com/uncle-bob/2014/11/24/FPvsOO.html (OO versus FP)

Java Lambda 기본 함수형 인터페이스 java.util.function 패키지에 정의되어 있음.

• • Functions

    public interface Function<T, R> {
      
      R apply(T t);
    }

• • Suppliers

    public interface Supplier<T> {
      T get();
    }

  • Consumers

    public interface Consumer<T> {
    
      void accept(T t); 
    }

  • Predicates

    public interface Predicate<T> {
    
      boolean test(T t); 
    }

  • Operators

    public interface UnaryOperator<T> extends Function<T, T> {
      static <T> UnaryOperator<T> identity() {
        return t -> t;
      }
    }
    

    public interface BinaryOperator<T> extends BiFunction<T,T,T> {
    
      public static <T> BinaryOperator<T> minBy(Comparator<? super T> comparator) {
        Objects.requireNonNull(comparator);
        return (a, b) -> comparator.compare(a, b) <= 0 ? a : b;
      }
    
      public static <T> BinaryOperator<T> maxBy(Comparator<? super T> comparator) {
        Objects.requireNonNull(comparator);
        return (a, b) -> comparator.compare(a, b) >= 0 ? a : b;
      }
    }

Java9 Immutable Collection
https://dzone.com/articles/immutable-collections-in-java-9
https://docs.oracle.com/javase/9/core/creating-immutable-lists-sets-and-maps.htm#JSCOR-GUID-202D195E-6E18-41F6-90C0-7423B2C9B381

List list3 = List.of("One", "Two", "Three");
Set set3 = Set.of("One", "Two", "Three");
Map map = Map.of("One", "1", "Two", "2", "Three", "3");

Creating Immutable Collection (Java8 or lower)

• Collections.unmodifiableList(list) 사용
• Arrays.asList( … ) 사용
• stream.of(….).collect(collectingAndThen(toList(), Collections::unmodifiableList)) 사용
• Guava 라이브러리 사용

Java Collections: List, Set, Map (http://web.mit.edu/6.031/www/sp17/classes/02-basic-java/#java_collections)

A List contains an ordered collection of zero or more objects, where the same object might appear multiple times.

A Set is an unordered collection of zero or more unique objects.

A Map is similar to a dictionary (key, value).

PersonStreamTest
PersonStreamTest

PersonClassInterfaceCollectionTest
PersonClassInterfaceCollectionTest

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) {
///////////////////////////////////////////////////////////////////////
        Person jason1 = new Person("Jason", 10);
        Person jason2 = new Person("Jason", 10);
        Person jason3 = jason1;
        Person jason4 = new Person("Jason", 20);

        if (jason1 == jason2)
            System.out.println("jason1 == jason2");
        else
            System.out.println("jason1 != jason2"); // 동일한 reference를 가리키지 않으므로 jason1 != jason2

        if (jason1 == jason3)
            System.out.println("jason1 == jason3"); // 동일한 reference이므로 jason1 == jason3
        else
            System.out.println("jason1 != jason3");

        if (jason1.equals(jason4))
            System.out.println("jason1 == jason4");
        else
            System.out.println("jason1 != jason4"); // 동일한 reference를 가리키지 않으므로 jason1 != jason4

        if (jason1.equals(jason2))
            System.out.println("jason1 equals to jason2"); // 내용이 같으므로 jason1 equals to jason2
        else
            System.out.println("jason1 is not equal to jason2");

        if (jason1.equals(jason3))
            System.out.println("jason1 equals to jason3"); // 내용이 같으므로 jason1 equals to jason3
        else
            System.out.println("jason1 is not equal to jason3");

        if (jason1.equals(jason4))
            System.out.println("jason1 equals jason4");
        else
            System.out.println("jason1 is not equal to jason4"); // 내용이 다르므로 jason1 is not equal to jason4

///////////////////////////////////////////////////////////////////////
        Student john1 = new Student("John", 10, 100);
        Student john2 = new Student("John", 10, 100);
        Student john3 = john1;
        Student john4 = new Student("John", 20, 100);

        if (john1.equals(john2))
            System.out.println("john1 equals to john2"); // 내용이 같으므로 john1 equals to john2
        else
            System.out.println("john1 is not equal to john2");

        if (john1.equals(john3))
            System.out.println("john1 equals to john3"); // 내용이 같으므로 john1 equals to john3
        else
            System.out.println("john1 is not equal to john3");

        if (john1.equals(john4))
            System.out.println("john1 equals to john4");
        else
            System.out.println("john1 is not equal to john4"); // 내용이 다르므로 john1 is not equal to john4

///////////////////////////////////////////////////////////////////////
        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));
    }

}

CollectionEqualsHashcodeContainsTest (updated)

class CollectionEqualsHashcodeContainsTest{
    // getReference 
    public static String getReference(Object o){
        return o.getClass().getName() + "@" + Integer.toHexString(System.identityHashCode(o));
    }

    public static void main(String[] args)  {

	// primitive type (== 연산자는 값이 같으면 true)
	int i = 1000;
	int j = 1000;
	int k = i;
	System.out.println("i == j " + (i == j));
	System.out.println("i == k " + (i == k));
	System.out.println();

	// reference type (reference가 같으면 true)
	// WARNING: JVM tries to save memory, when the Integer falls in a range (from -128 to 127). Integer v1 = 100, v2 = 100 v1 == v2
	Object o1 = i; // boxing
	Object o2 = j; // boxing
	Object o3 = o1;
	System.out.println(getReference(o1) + " o1=" + o1);
	System.out.println(getReference(o2) + " o2=" + o2);
	System.out.println(getReference(o3) + " o3=" + o3);
	System.out.println("o1 == o2 " + (o1 == o2));
	System.out.println("o1 == o3 " + (o1 == o3));
	System.out.println("o1 equals o2 " + o1.equals(o2));
	System.out.println("o1 equals o3 " + o1.equals(o3));
	System.out.println("o1.hashCode() == o2.hashCode() " + (o1.hashCode() == o2.hashCode()));
	System.out.println("o1.hashCode() == o3.hashCode() " + (o1.hashCode() == o3.hashCode()));
	System.out.println();

	// reference type (== 연산자는 reference가 같으면 true)
	Person p1 = new Person("P",10);
	Person p2 = new Person("P",10);
	Person p3 = p1;
	System.out.println(getReference(p1) + " p1=" + p1);
	System.out.println(getReference(p2) + " p2=" + p2);
	System.out.println(getReference(p3) + " p3=" + p3);
	System.out.println("p1 == p2 " + (p1 == p2));
	System.out.println("p1 == p3 " + (p1 == p3));
	System.out.println("p1 equals p2 " + p1.equals(p2));
	System.out.println("p1 equals p3 " + p1.equals(p3));
	System.out.println("p1.hashCode() == p2.hashCode() " + (p1.hashCode() == p2.hashCode()));
	System.out.println("p1.hashCode() == p3.hashCode() " + (p1.hashCode() == p3.hashCode()));
	System.out.println();

	// String type (== 연산자는 reference가 같으면 true)
	String s1 = "PP"; // String literal을 사용할 경우, pool에서 관리
	String s2 = "PP"; // String literal을 사용할 경우, pool에서 관리
	String s3 = s1;
	String s4 = "P" + "P"; // String literal을 사용할 경우, pool에서 관리
	String s5 = new String("PP");
	String s6 = "PP" + ""; // String literal을 사용할 경우, pool에서 관리
	String s7 = s1 + ""; // String + String literal이라서 새로 생성
	System.out.println(getReference(s1) + " s1=" + s1);
	System.out.println(getReference(s2) + " s2=" + s2);
	System.out.println(getReference(s3) + " s3=" + s3);
	System.out.println(getReference(s4) + " s4=" + s4);
	System.out.println(getReference(s5) + " s5=" + s5);
	System.out.println(getReference(s6) + " s6=" + s6);
	System.out.println(getReference(s7) + " s7=" + s7);

	System.out.println("s1 == s2 " + (s1 == s2));
	System.out.println("s1 == s3 " + (s1 == s3));
	System.out.println("s1 == s4 " + (s1 == s4));
	System.out.println("s1 == s5 " + (s1 == s5));
	System.out.println("s1 == s6 " + (s1 == s6));
	System.out.println("s1 == s7 " + (s1 == s7));
	System.out.println("s1 equals s2 " + s1.equals(s2));
	System.out.println("s1 equals s3 " + s1.equals(s3));
	System.out.println("s1 equals s4 " + s1.equals(s4));
	System.out.println("s1 equals s5 " + s1.equals(s5));
	System.out.println("s1 equals s6 " + s1.equals(s6));
	System.out.println("s1 equals s7 " + s1.equals(s7));
	System.out.println("s1.hashCode() == s2.hashCode() " + (s1.hashCode() == s2.hashCode()));
	System.out.println("s1.hashCode() == s3.hashCode() " + (s1.hashCode() == s3.hashCode()));
	System.out.println("s1.hashCode() == s4.hashCode() " + (s1.hashCode() == s4.hashCode()));
	System.out.println("s1.hashCode() == s5.hashCode() " + (s1.hashCode() == s5.hashCode()));
	System.out.println("s1.hashCode() == s6.hashCode() " + (s1.hashCode() == s6.hashCode()));
	System.out.println("s1.hashCode() == s7.hashCode() " + (s1.hashCode() == s7.hashCode()));
	System.out.println();

	// ArrayList
	System.out.println("pList");
	List<Person> pList = new ArrayList<Person>();
	pList.add(p1);
	pList.add(p2);
	pList.add(p3);
	pList.forEach((p) -> System.out.println(p));
	System.out.println("pList contains p1: " + pList.contains(p1));
	System.out.println("pList contains p2: " + pList.contains(p2));
	System.out.println("pList contains p3: " + pList.contains(p3));
	System.out.println("pList contains new Person: " + pList.contains(new Person("P",10)));
	System.out.println();

	// HashSet의 경우 hashCode가 일치하면 동일한 것으로 간주하여 replace함
	System.out.println("pSet");
	Set<Person> pSet = new HashSet();
	pSet.add(p1);
	pSet.add(p2);
	pSet.add(p3);
	pSet.forEach((p) -> System.out.println(p));
	System.out.println("pSet contains p1: " + pSet.contains(p1));
	System.out.println("pSet contains p2: " + pSet.contains(p2));
	System.out.println("pSet contains p3: " + pSet.contains(p3));
	System.out.println("pSet contains new Person: " + pSet.contains(new Person("P",10)));
	System.out.println();

	// HashMap의 경우 key 값은 hashCode가 일치하면 동일한 것으로 간주하여 replace함
	System.out.println("pMap");
	Map<Person, Integer> pMap = new HashMap<Person, Integer>();
	pMap.put(p1, 1);
	pMap.put(p2, 2);
	pMap.put(p3, 3);
	pMap.forEach((p, e) -> System.out.println(p + " " + e));
	System.out.println("pMap contains p1: " + pMap.containsKey(p1));
	System.out.println("pMap contains p2: " + pMap.containsKey(p2));
	System.out.println("pMap contains p3: " + pMap.containsKey(p3));
	System.out.println("pMap contains new Person: " + pMap.containsKey(new Person("P",10)));
	System.out.println();

	// Array
	System.out.println("pArray");
	Person[] pArray = new Person[3];
	pArray[0] = p1;
	pArray[1] = p2;
	pArray[2] = p3;
	Arrays.asList(pArray).forEach(p -> System.out.println(p));
	System.out.println("pArray contains p1: " + Arrays.asList(pArray).contains(p1));
	System.out.println("pArray contains p2: " + Arrays.asList(pArray).contains(p2));
	System.out.println("pArray contains p3: " + Arrays.asList(pArray).contains(p3));
	System.out.println("pArray contains new Person: " + Arrays.asList(pArray).contains(new Person("P",10)));
	System.out.println();
    }
}

https://medium.com/@govinda_raj/arraylist-vs-linkedlist-f8c5099153b5

ArrayArrayList

int[] integerArray = new int[3];

integerArray[0] = 1;

integerArray[1] = 2;

integerArray[2] = 3;

for (int i : integerArray) System.out.println(i);

for (int j=0; j<integerArray.length; j++) System.out.println(integerArray[ j ]);

int k = 0;

while (k < integerArray.length) System.out.println(integerArray[k++]);

 

ArrayList<Integer> integerList = new ArrayList<Integer>();

integerList.add(1); //cannot store primitive in ArrayList, instead autoboxing will convert int to Integer object

integerList.add(2); //cannot store primitive in ArrayList, instead autoboxing will convert int to Integer object

integerList.add(3); //cannot store primitive in ArrayList, instead autoboxing will convert int to Integer object

for (int m : integerList) System.out.println(m);

for (int n=0; n<integerList.size(); n++) System.out.println(integerList.get(n));

Iterator<Integer> itr = integerList.iterator();

while (itr.hasNext()) System.out.println(itr.next());