22: JAVA Set

1. 로또 프로그램을 작성하시오.(Set 으로)

//
//code
//
import java.util.*;
import java.util.concurrent.ThreadLocalRandom;
class Const {
    static final int LOTTO_COUNT = 6;
    static final int LOTTO_MIN = 1;
    static final int LOTTO_MAX = 45;
}
class Print {
    private static StringBuilder print = new StringBuilder();
    private static void printlnAndReset() {
        System.out.println(print);
        print.setLength(0);
    }
    static void printlnObject(Object obj) {
        print.append(obj);
        printlnAndReset();
    }
}
class Functions {
    private static Set<Integer> lottoNums = new HashSet<Integer>();
    private static ThreadLocalRandom random;
    static void run() {
        while(true) {
            try {
                lotto();
                break;
            } catch(Exception e) {
                e.printStackTrace();
            };
        };
    }
    private static void lotto() {
        while(lottoNums.size() < Const.LOTTO_COUNT) {
            lottoNums.add(random.current().nextInt(Const.LOTTO_MIN, Const.LOTTO_MAX + 1));
        }
        Print.printlnObject(lottoNums);
    }
}
class LottoMain {
    public static void main(String[] args) {
        Functions.run();
    }
}
/*print
[16, 4, 27, 13, 45, 14]
*/

2. Set에 대하여 설명하시오.

• It is similar with the set in math.
• It doesnt care elements order.
• During .add(), it categorizes the elements by hashCode() method. Then it compares the elements by equals() method. If there is same elements from the processes of hashCode() and equals(), then it removes old one and adds new one in the set.
• Overriding hashCode() and equals() can make easier to distinguish the set elements in detail.

3. 아래의 출력이 나오도록 만드시오.

//문제
public class Test1 {
//
	public static void main(String[] args) {
//
		HashSet<Person> hSet = new HashSet<Person>();
//		
		hSet.add(new Person("LEE", 10));
		hSet.add(new Person("LEE", 10));
		hSet.add(new Person("PARK", 35));
		hSet.add(new Person("PARK", 35));
//
		System.out.println("저장된 데이터 수: " + hSet.size());
		System.out.println(hSet);
//
	}
//
}
//
/*
저장된 데이터 수: 2
[LEE(10세), PARK(35세)]
*/

//
//code
//
class Person {
    private String name;
    private int age;
    Person(String name, int age) {
        this.name = name;
        this.age = age;
    }
    @Override
    public int hashCode() {
        return (this.age);
    }
    @Override
    public boolean equals(Object obj) {
        boolean equal = false;
        if (obj instanceof Person) {
            Person person = (Person)obj;
            if (this.name == person.name) {
                equal = true;
            };
        };
        return equal;
    }
    @Override
    public String toString() {
        return (this.name.concat("(").concat(String.valueOf(this.age)).concat("세)"));
    }
}
/*print
저장된 데이터 수: 2
[PARK(35세), LEE(10세)]
*/

4. 아래와 같이 출력이 나오도록 프로그래밍을 하시오.(개별진척도1번)

//문제
public class Test2 {
	public static void main(String[] args) {
//		
        HashSet<Num> set = new HashSet<>();
        set.add(new Num(7799));
        set.add(new Num(9955));
        set.add(new Num(7799));
//
        System.out.println("인스턴스 수: " + set.size());
//
        for(Num n : set)
            System.out.print(n.toString() + '\t');
//
        System.out.println();
	}
}
/*
====출력
인스턴스 수: 2
7799 9955
*/

//
//code
//
class Num {
    private int number;
    Num(int number) {
        this.number = number;
    }
    @Override
    public int hashCode() {
        return (this.number);
    }
    @Override
    public boolean equals(Object obj) {
        boolean equal = false;
        if (obj instanceof Num) {
            equal = true;
        };
        return equal;
    }
    @Override
    public String toString() {
        return(String.valueOf(this.number));
    }
}
/*print
인스턴스 수: 2
9955    7799
*/

5.TreeSet 과 HashSet 의 차이는?

• TreeSet has order of elements but HashSet doesnt have.
• Elements of TreeSet are connected like tree.
• Elements of HashSet are categorized by hashcode.

6. Set 호출되는 원리와 순서를 설명하시오.

• There are sets in a Set.
• Call the set of element first.
• Find the element from the set and unbox it.