省去有无抽象关键字的区别?

用 abstract 您可以省略方法和getter/setter的实现

// with
abstract class A {
  int foo();
  String get bar;
  set baz(String value);
}

var a = A(); // error about instantiating abstract class

class B extends A {
  // error about missing implementations
}

var b = B(); // ok

// without
class A {
  int foo(); // error about missing implementation
  String get bar; // error about missing implementation
  set baz(String value); // error about missing implementation
}

class B extends A {}