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public class Complex {

    private final double re;
    private final double im;

    private Complex(double re, double im) {
        this.re = re;
        this.im = im;
    }

    public static Complex valueOf(double re, double im) {
        return new Complex(re, im);
    }
}


public class BigInteger extends Number implements Comparable<BigInteger> {
    ~~~
}

// 재정의 후 불변객체가 아닌데 불변객체라 생각하고 사용하면 문제가 생긴다.
class BiBiBigInteger extends BigInteger {
    ~~~~
}

// 따라서 진짜 불변 클래스인지 확인 후 사용을 해야한다.
public static BigInteger safeInstance(BigInteger val) {
    return val.getClass() == BigInteger.class ?
        val : new BigInteger(val.toByteArray());    
}


public final class Person {

    private final String name;
    private final PhoneNumber phoneNumber;
    
}


public class PhoneNumber {

    private String phoneNum;
    
    // 생성자, 게터, 세터 ...
}


public class Person {

    private final String name;
    private final PhoneNumber phoneNumber;

    public Person(String name, PhoneNumber phoneNumber) {
        this.name = name;
        this.phoneNumber = phoneNumber;
    }

    // 게터, 세터 ...
}

void immutable_test() {
    PhoneNumber phoneNumber = new PhoneNumber("010-1111-1111");

    Person person = new Person("형욱", phoneNumber);
    String beforeNumber = person.getPhoneNumber().getPhoneNum().toString();

    phoneNumber.setPhoneNum("010-2222-2222");
    String afterNumber = person.getPhoneNumber().getPhoneNum().toString();

    System.out.println(beforeNumber); // 010-1111-1111
    System.out.println(afterNumber); // 010-2222-2222
}

// 보완하기
public class Person {

    private final String name;
    private final PhoneNumber phoneNumber;

    public Person(String name, PhoneNumber phoneNumber) {
        this.name = name;
        this.phoneNumber = new PhoneNumber(phoneNumber.getPhoneNum());
    }

    // 게터, 세터 ...
}


public static BigDecimal valueOf(long val) {
    if (val >= 0 && val < ZERO_THROUGH_TEN.length)
    return ZERO_THROUGH_TEN[(int)val];
    else if (val != INFLATED)
    return new BigDecimal(null, val, 0, 0);
    return new BigDecimal(INFLATED_BIGINT, val, 0, 0);
    }
    
    private static final BigDecimal ZERO_THROUGH_TEN[] = {
        new BigDecimal(BigInteger.ZERO,       0,  0, 1),
        new BigDecimal(BigInteger.ONE,        1,  0, 1),
        new BigDecimal(BigInteger.TWO,        2,  0, 1),
        new BigDecimal(BigInteger.valueOf(3), 3,  0, 1),
        new BigDecimal(BigInteger.valueOf(4), 4,  0, 1),
        new BigDecimal(BigInteger.valueOf(5), 5,  0, 1),
        new BigDecimal(BigInteger.valueOf(6), 6,  0, 1),
        new BigDecimal(BigInteger.valueOf(7), 7,  0, 1),
        new BigDecimal(BigInteger.valueOf(8), 8,  0, 1),
        new BigDecimal(BigInteger.valueOf(9), 9,  0, 1),
        new BigDecimal(BigInteger.TEN,        10, 0, 2),
    }


public class Person {

    private final String name;
    private final PhoneNumber[] phoneNumbers;

    public PhoneNumber[] getPhoneNumbers() {
        return phoneNumbers.clone();
    }
}