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public class ClassName implements Comparable<ClassName> { ... }


public class Main {
    public static void main(String[] args) {
        Integer a = 1;
        Integer b = 2;
        System.out.println(a.compareTo(b)); // -1
    }
}


public class Main {
    public static void main(String[] args) {
        int a = 1;
        int b = 2;
        System.out.println(a.compareTo(b));
				// error: int cannot be dereferenced
    }
}


public PriorityQueue(Collection<? extends E> c) {
    if (c instanceof SortedSet<?>) {
        SortedSet<? extends E> ss = (SortedSet<? extends E>) c;
        this.comparator = (Comparator<? super E>) ss.comparator();
        initElementsFromCollection(ss);
    }
    else if (c instanceof PriorityQueue<?>) {
        PriorityQueue<? extends E> pq = (PriorityQueue<? extends E>) c;
        this.comparator = (Comparator<? super E>) pq.comparator();
        initFromPriorityQueue(pq);
    }
    else {
        this.comparator = null;
        initFromCollection(c);
    }
}


public class Main {
    public static void main(String[] args) {
        Integer iA = 1; Integer iB = 2;
        Float fA = 1F; Float fB = 2F;
        Double dA = 1.0; Double dB = 2.0;
        
        System.out.println(iA.compareTo(iB)); // -1
        System.out.println(fA.compareTo(fB)); // -1
        System.out.println(dA.compareTo(dB)); // -1
    }
}


public final class Integer extends Number
        implements Comparable<Integer>, Constable, ConstantDesc { ... }


public abstract class Enum<E> extends Enum<E>>
        implements Constable, Comparable<E>, Serializable { ... }


public class Main {
    public static void main(String[] args) {
        BigDecimal a = new BigDecimal("1.0");
        BigDecimal b = new BigDecimal("1.00");

        System.out.println(a.compareTo(b)); // 0
        System.out.println(a.equals(b)); // false

				HashSet<Object> A = new HashSet<>(); // 동치 비교에 equals 사용
        TreeSet<Object> B = new TreeSet<>(); // 동치 비교에 compareTo 사용

        A.add(a); A.add(b);
        B.add(a); B.add(b);

        System.out.println(A.size()); // 2
        System.out.println(B.size()); // 1
    }
}


public int compareTo(PhoneNumber pn) {
    int result = Short.compare(areaCode, pn.areaCode); // 가장 중요한 필드

    if (result == 0) {
        result = Short.compare(prefix, pn.prefix); // 두번째로 중요한 필드

				// 세번째로 중요한 필드
        if (result == 0) result = Short.compare(lineNum, pn.lineNum);
    }
    return result;
}


@Override
public int compare(A,B){
    if (A > B) { return 1; }
		else if (A < B) { return -1; }
		else { return 0; } 
}


private static final Comparator<PhoneNumber> COMPARATOR =
        Comparator.comparingInt((PhoneNumber pn) -> pn.areaCode)
                .thenComparingInt(pn -> pn.prefix)
                .thenComparingInt(pn -> pn.lineNum);

public int compareTo(PhoneNumber pn) {
    return COMPARATOR.compare(this, pn);
}