Item 1: Consider static factory methods instead of constructors
Advantage:
One advantage of static factory methods is that, unlike constructors, they have names.
A second advantage of static factory methods is that, unlike constructors, they are not required to create a new object each time they’re invoked.
A third advantage of static factory methods is that, unlike constructors, they can return an object of any subtype of their return type.
A fourth advantage of static factory methods is that they reduce the verbosity(冗长) of creating parameterized type instances.
Disadvantage:
The main disadvantage of providing only static factory methods is that classes without public or protected constructors cannot be subclassed.
A second disadvantage of static factory methods is that they are not readily distinguishable from other static methods.
Item 2: Consider a builder when faced with many constructor parameters
the telescoping constructor pattern works, but it is hard to write client code when there are many parameters, and harder still to read it.
a JavaBean may be in an inconsistent state partway through its construction.
the JavaBeans pattern precludes the possibility of making a class immutable.
Advantage:
The Builder pattern simulates named optional parameters.
Class.newInstance breaks compile-time exception checking.
the Builder pattern is a good choice when designing classes whose constructors or static factories would have more than a handful of parameters.
Item 3: Enforce the singleton property with a private constructor or an enum type
Making a class a singleton can make it difficult to test its clients.
a single-element enum type is the best way to implement a singleton.
#1. Singleton with public final field
public class Elvis {
public static final Elvis INSTANCE = new Elvis();
private Elvis() { ... }
public void leaveTheBuilding() { ... }
}
#2. Singleton with static factory
public class Elvis {
private static final Elvis INSTANCE = new Elvis();
private Elvis() { ... }
public static Elvis getInstance() { return INSTANCE; }
public void leaveTheBuilding() { ... }
}
#3. readResolve method to preserve singleton property
private Object readResolve() {
// Return the one true Elvis and let the garbage collector
// take care of the Elvis impersonator.
return INSTANCE;
}
#4. Enum singleton - the preferred approach
public enum Elvis {
INSTANCE;
public void leaveTheBuilding() { ... }
}
Item 4: Enforce noninstantiability with a private constructor
Attempting to enforce noninstantiability by making a class abstract does not work.
a class can be made noninstantiable by including a private constructor.
public class UtilityClass {
// Suppress default constructor for noninstantiability
private UtilityClass() {
throw new AssertionError();
}
... // Remainder omitted
}