Effective Java

// Public class with exposed immutable fields - questionable
public final class Time {
private static final int HOURS_PER_DAY = 24;
private static final int MINUTES_PER_HOUR = 60;
public final int hour;
public final int minute;
public Time(int hour, int minute) {
if (hour < 0 || hour >= HOURS_PER_DAY)
throw new IllegalArgumentException("Hour: " + hour);
if (minute < 0 || minute >= MINUTES_PER_HOUR)
throw new IllegalArgumentException("Min: " + minute);
this.hour = hour;
this.minute = minute;
}
... // Remainder omitted
}
In summary, public classes should never expose mutable fields. It is less
harmful, though still questionable, for public classes to expose immutable fields.
It is, however, sometimes desirable for package-private or private nested classes to
expose fields, whether mutable or immutable.

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CHAPTER 4
CLASSES AND INTERFACES
80
Item 17: Minimize mutability
An immutable class is simply a class whose instances cannot be modified. All of
the information contained in each instance is fixed for the lifetime of the object, so
no changes can ever be observed. The Java platform libraries contain many
immutable classes, including 
String
, the boxed primitive classes, and
BigInteger
and 
BigDecimal
. There are many good reasons for this: Immutable
classes are easier to design, implement, and use than mutable classes. They are
less prone to error and are more secure.
To make a class immutable, follow these five rules:
1.
Don’t provide methods that modify the object’s state 
(known as 
mutators
).
2.
Ensure that the class can’t be extended.
This prevents careless or malicious
subclasses from compromising the immutable behavior of the class by
behaving as if the object’s state has changed. Preventing subclassing is
generally accomplished by making the class final, but there is an alternative
that we’ll discuss later.
3.