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What is the best way to set a default instance for a trailing private field with an optional parameter?

For a class with a trailing private field _bar:

class Foo() {

final Bar _bar;

    Foo({Bar bar}) : _bar = bar {

        //error: list cannot be used as setter, it is final
       _bar = new Bar();
    }

}

Trying to set it in the parameter list will result in an error

//error: default values of an object must be constant
Foo({Bar bar: new Bar()}) : _bar = bar ..

I would like to keep an optional parameter so that I can inject mocks during unit tests. What's the best approach to do this?

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2 answers 




class Foo {
  final Bar _bar;
  Foo({Bar bar}) : _bar = bar != null ? bar : new Bar();
}
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I recommend using static methods when the logic is complex.



class Foo {
  final Object _field1;

  Foo({Object value1}) : this._field1 = _setUpField1(value1);

  static Object _setUpField1(Object value) {
    // Some complex logic
    return value;
  }
}
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