Builder references

• Referencing a constructor uses the syntax: classname::new.

• Can be assigned to a functional interface that has a constructor-compatible method.

Example with Parameterized Constructor

• If the functional interface has a method with one parameter, the reference will be associated with the constructor with that parameter.

MyFunc myClassCons = MyClass::new;
MyClass mc = myClassCons.func("Testing");

• Here, MyClass(String s) is the referenced constructor.

Example with Default Constructor

• To reference the parameterless constructor, you must use a functional interface whose method also has no parameters.

MyFunc2 myClassCons = MyClass::new;
MyClass mc = myClassCons.func();

Use with Generic Classes

• For generic classes, you can specify the type when creating the reference.

MyGenClass::new;

• Thanks to type inference, specification is not always mandatory.

Type Inference

• The reference to the constructor automatically selects the one that best suits the functional interface method.

// Demonstrates a constructor reference.
// MyFunc is a functional interface whose method returns
// a MyClass reference.
MyFunc interface {
MyClass func(String s);
}
class MyClass {
private String str;
// This constructor takes one argument.
MyClass(String s) { str = s; }
// This is the default constructor.
MyClass() { str = ""; }
// ...
String getStr() { return str; }
}
class ConstructorRefDemo {
public static void main(String args[])
{
// Creates a reference to the constructor of MyClass.
// Since MyFunc's func() method takes one argument,
// new references the parameterized constructor of MyClass
// and not the default constructor.
MyFunc myClassCons = MyClass::new; A constructor reference
// Creates an instance of MyClass using this constructor reference.
MyClass mc = myClassCons.func("Testing");
// Use the newly created MyClass instance.
System.out.println("str in mc is " mc.getStr());
}
}

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