Java Programming I 2025

Java :: 연산자 (Double Colon Operator)는 람다식을 대체하여 메서드 참조(method reference)로 사용된다. 즉, 람다식이 사용될 수 있는 Functional Interface Implementation에서만 사용 가능하다.

Using Lambda

Comparator<Person> c = (Person p1, Person p2) ->
                    p1.getName().compareTo(p2.getName());

Using Lambda with type interference

Comparator<Person> c = (p1, p2) -> p1.getName().CompareTo(p2.getName());

Using method reference ( :: operator)

Comparator<Person> c = Comparator.comparing(Person::getName);

Method reference ( :: operator)

Function<Person, String> getName = Person::getName; // Person->String
String name = getName.apply(person1);

Method reference ( :: operator)는 람다식과 동일한 처리를 하는 식이지만 메소드 본문을 제공하는 대신 기존 메소드를 이름으로 참조한다.

Function<Double, Double> sq = (Double x) -> x * x; // lambda 
double result = sq.apply(3); // 3 * 3 = 9
 
Function<Double, Double> sq2 = MyClass::square; // MyClass에 정의된 square 정적 메소드
double result2 = sq2.apply(3); // 3 * 3 = 9

BiFunction<Double, Double, Double> add = (x, y) -> x + y; // lambda
double result = add.apply(3.1, 3.2); // 3.1 + 3.2 = 6.3

BiFunction<Double, Double, Double> add2 = MyClass::sum; // MyClass에 정의된 sum 정적 메소드 
double result2 = add2.apply(3.1, 3.2); // 3.1 + 3.2 = 6.3

May 11, 2025May 14, 2025

Java Lambda Default Functional Interface

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;
  }
}

May 10, 2025May 14, 2025

Comparable & Comparator Interface

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));
    }

}

May 9, 2025May 14, 2025

Shape Polymorphism with Interface

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

May 8, 2025May 14, 2025

lab4

lab4 (due by 5/21)

java1-lab4

May 8, 2025May 14, 2025

lecture7

java1-lecture7-Interface

May 2, 2025May 14, 2025

Method Overloading vs Overriding

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;
    }
}

April 10, 2025April 4, 2025

Why doesn’t Java allow overriding of static methods

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.

April 10, 2025April 4, 2025

Abstract Class & Abstract Method & Polymorphism

Abstract Class (추상 클래스)는 하나 이상의 abstract method(추상메소드)를 가지거나, 추상메소드가 없지만 abstract으로 선언한 클래스. 추상클래스는 객체를 생성할 수 없다.

Abstract Method (추상 메소드)는 선언은 되어 있으나 구현 내용이 없는 메소드이다.

추상클래스를 상속받은 하위클래스는 반드시 추상 메소드를 재정의(method override) 구현해야한다.

Polymorphism (다형성)은 프로그램 실행(run-time execution)시 형(type)이 변화되는 객체.

다형성에 따라서, 클래스 상속관계(Inheritance)에서 메소드 재정의(Method overriding)된 것이 동적 바인딩(dynamic binding)으로 호출되었을 시, 해당 객체에 맞는 재정의된 메소드가 호출되는 것이다.

ShapePolymorphism

Shape 추상클래스 has-a ShapeType, ShapeColor, ShapeRect

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

https://www.cs.usfca.edu/~wolber/SoftwareDev/OO/abstractInterfacesIntro.htm

Posts

lecture8

Java Double Colon Operator