Methods Common to All Objects

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10. Obey the General Contract when Overriding Equals

Don’t override equals if:

1. Each instance of the class is inherently unique
2. There is no need for the class to have equality
3. A superclass has overridden equals, and it is suitable
4. The class is private or package-private, and its equals will not be invoked

Equals must follow these properties:

1. Reflexivity: for any non-null reference, x.equals(x) returns true.
2. Symmetric: x.equals(y) should equal y.equals(x) if they are both non-null
3. Transitive: if x.equals(y) and y.equals(z) returns true, x.equals(z) should be true for non-null references
4. Consistent: x.equals(y) must always return the same result as long as x or y have not changed which is used in the equals method.
5. Non-nullability: For any non-null reference, x, x.equals(null) must be false

Also, always override hashCode when you override equals.

Note that record classes auto generate equals, hashCode, and toSTring.

11. Always Override hashCode when You Override Equals

hashcode allows an object to be placed in HashSet or HashMap, TreeSet, TreeMap.

It can look something like this (or use lombok to autogenerate it)

@Override public int hashCode() {
	int result = Short.hashCode(areaCode);
	result = 31 * result + Short.hashCode(prefix);
	result = 31 * result + Short.hashCode(lineNum);
	return result;
}

12. Always Override toString

toString makes your class more pleasant to use and debug. You should also include all the interesting information in the object:

/**
* Returns the string representation of this phone number.
* The string consists of twelve characters whose format is
* "XXX-YYY-ZZZZ", where XXX is the area code, YYY is the
* prefix, and ZZZZ is the line number. Each of the capital
* letters represents a single decimal digit.
*
* If any of the three parts of this phone number is too small
* to fill up its field, the field is padded with leading zeros.
* For example, if the value of the line number is 123, the last
* four characters of the string representation will be "0123".
*/
@Override public String toString() {
	return String.format("%03d-%03d-%04d", areaCode, prefix, lineNum);
}

13. Override clone Judiciously

Clone is a mixin that advertises that an object can be cloned, which is a cheap field by field copy of the object.

However, clone itself doesn’t require any methods to be implemented.

// Clone method for class with no references to mutable state
@Override public PhoneNumber clone() {
	try {
		return (PhoneNumber) super.clone();
	} catch (CloneNotSupportedException e) {
		throw new AssertionError(); // Can't happen
	}
}

14. Consider Implementing Comparable

compareTo is the method that implements sorting, and it allows for a natural ordering. To do this, implement the Comparable interface:

public interface Comparable<T> {
	int compareTo(T t);
}

Also, avoid using < and > operators — use compare itself when comparing fields for implementing Comparable

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