Controlling Instantiation – Intermediate Programming

A constructor provides a way to create an instance of a class. Sometimes we may want to prohibit creating instances of the class or provide alternate ways to create instances of the class.

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Non-instantiable Classes

Some classes should not be instantiated. For example, classes that have only static fields and methods should not be instantiated. In such a case, we should prohibit instantiation by making the default constructor private.

public class Foo {
    public static final int MAXSIZE = 10;
    private Foo() {}
    ...
}

Singleton Classes

If a class allows only one instance of it in existence at any point in time, we say the class implements the singleton pattern. The preferred method of creating a singleton class to use an enumerated class. Users are then not allowed to create additional instances of the class.

public enum Foo {     
    INSTANCE;     

    public void method1() {...} 
    ....
}

Access to the single instance is through the enumerated class.

Foo singleton = Foo.INSTANCE;
singleton.method1();

Static Final Fields

Another way to create a singleton class is to create a class and

make the default constructor private
create a static final field that holds a reference to the single instance

public class Foo {
    public static final Foo INSTANCE = new Foo();

    private Foo() { }
    ...
    public void method1() {...}
}

Access to the single instance is through the static field INSTANCE.

Foo singleton = Foo.INSTANCE;
singleton.method1();

Static Factory Method with Static Final Fields

Another approach is to make the static field private and to provide a static method that returns the singleton instances.

public class Foo {
    private static final Foo INSTANCE = new Foo();
    private Foo() { }
    
    public static Foo getInstance() { return INSTANCE; }
    ...
    public void method1() {...}
}

Access to the singleton instance is through the static factory method getInstance.

Foo singleton = Foo.getInstance();
singleton.method1();

A disadvantage to using an enumerated class or a class without a public or protected constructor is that they can not be extended.

Static Methods with a Cache

There are times when we need to manage a bounded set of instances of a class. Static factory methods are not required to return a new instance of a class like constructors do. This allows us to create a cache of instances. The static factory method can return a shared instance, an unused instance in the cache, or no instance at all (null).

Non-instantiable Classes

Singleton Classes

Static Final Fields

Static Factory Method with Static Final Fields

Static Methods with a Cache

More on Static Factory Methods