What will be the output of the following dart code? implements vs extends — difference between using it in class constructors and methods

The Code

dartCopy
main() {
  B b = B();
  D d = D();
}

class A {
  A() {
    print("Class A");
  }
  sampleMethodOfClassA() {
    print('Sample Method Of Class A');
  }
}

class B extends A {
  B() {
    print("Class B");
  }
}

class C {
  C() {
    print("Class C");
  }
  sampleMethodOfClassC() {
    print('Sample Method Of Class C');
  }
}

class D implements C {
  D() {
    print("Class D");
  }
}

Answer

This code will not compile. Class

D

implements C

D

C

sampleMethodOfClassC()

D

Why Does

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B

Work But

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D

Doesn't?

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extends

(Inheritance) — Class B

When

B extends A

• B inherits all methods from A (including
  textCopy

  sampleMethodOfClassA()

  )
• The parent constructor is called automatically before the child constructor
• B does not need to override inherited methods

dartCopy
B b = B();
// Output:
// Class A    ← Parent constructor called first
// Class B    ← Then child constructor

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implements

(Interface Contract) — Class D

When

D implements C

• D treats C as an interface — it promises to provide the same API
• D must override every single method from C (even concrete ones)
• The parent constructor is NOT called
• D gets no inherited implementation — only the contract

dartCopy
// ❌ Compile Error: D has no implementation of sampleMethodOfClassC()
class D implements C {
  D() {
    print("Class D");
  }
}

How to Fix Class D

dartCopy
class D implements C {
  D() {
    print("Class D");
  }

  @override
  sampleMethodOfClassC() {
    print('Sample Method Of Class C from D');
  }
}

Output After Fix

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Class A
Class B
Class D

Note:

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"Class C"

is never printed because

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implements

does not invoke the parent constructor.

Key Differences:

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extends

vs

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implements

extends

implements

Common Interview Follow-Up

dartCopy
// extends → inherits body, only override what you want
class B extends A {
  // sampleMethodOfClassA() is inherited — no need to override
}

// implements → must provide ALL methods yourself
class D implements C {
  @override
  sampleMethodOfClassC() {
    // Must write your own implementation
  }
}

Key Takeaway: Use

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extends

when you want to reuse a parent class's code. Use

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implements

when you want to guarantee a class follows a specific API contract without inheriting any implementation.