Chapter 2: create and destroy objects. Item1: consider replacing the constructor with the static factory Method

In addition to using the constructor to obtain an instance of the class, you can also use the static factory method ).

The following method converts the basic Boolean Type to a Boolean object reference:

1 public final class Boolean implements Java. io. serializable, 2 comparable <Boolean> 3 {4 // matches 5 public static final Boolean true = new Boolean (true) with the basic type boolean = true ); 6 7 // match with the basic type boolean = False 8 public static final Boolean false = new Boolean (false); 9 10 // if you do not need a new Boolean instance, this method should take precedence over the constructor 11 // because this method has a significant increase in time and space. 12 public static Boolean valueof (Boolean B) {13 return (B? True: false); 14} 15}

Static factory advantages:

1. They have names. : When a class requires multiple constructors with the same signature, the static factory method is used instead of the constructor,Select a name with caution.

2. You do not have to create a new object every time you call them.This allows the immutable class to use pre-built instances, or cache the built instances for reuse to avoid unnecessary repeated objects. The above code illustrates this.

　　3. They can return any child type objects of the original return type. Java. util. enumset in jdk1.5 does not have a common constructor and only has a static factory method.

　　

/* Create an empty Enum set. Return instances of different types based on the number of enumerated elements. */Public static <E extends Enum <E> enumset <E> noneof (class <E> elementtype) {Enum [] universe = getuniverse (elementtype ); if (universe = NULL) throw new classcastexception (elementtype + "not an Enum"); If (universe. length <= 64) return New regularenumset <> (elementtype, universe); else return New jumboenumset <> (elementtype, universe );}

4. When creating parameterized instances, they make the code more concise.

　　

public static <K,V> HashMap<K,V> newInstance() {            return new HashMap<K,V>() ;        }

Disadvantages of static Factory:

1. If the class does not include a public or protected constructor, the class cannot be classified as a quilt.

2. They are no different from other static methods.

Some common names: valueof, of, getinstance, newinstance, GetType, newtype.

 

Chapter 2: create and destroy objects. Item1: consider replacing the constructor with the static factory Method