Tag Archives: dart

[Solved] Error: The superclass, ‘Animal‘, has no unnamed constructor that takes no arguments.

Error: The superclass, ‘Animal’, has no unnamed constructor that takes no arguments.

Problem Description:

Because the constructor cannot inherit
an error is reported during inheritance, which prompts that the constructor in the parent class is composed of parameters. You need to write the constructor in the subclass and pass the constructor of the parent class to parameters

class Animal {
  String name;
  int age;
  Animal(this.name, this.age);

  void printInfo() {
    print('$name is $age years old');
  }
}

//Inherit Animal class by extends keyword
class Cat extends Animal {
 
}

void main(List<String> args) {
  Cat cat = new Cat();
 print( cat.name);
}

Solution:

Super keyword

Try declaring a zero argument constructor in ‘Animal’, or declaring a constructor in Cat that explicitly invokes a constructor in ‘Animal’.dart(no_default_super_constructor)

Try declaring a zero parameter constructor in “animal” or a constructor that explicitly calls the constructor in “animal” in cat.dart (no default super constructor)

class Animal {
  String name;
  int age;
  Animal(this.name, this.age);

  void printInfo() {
    print('$name is $age years old');
  }
}

//Inherit Animal class by extends keyword
class Cat extends Animal {
  Cat(String name, int age) : super(name, age);

}

void main(List<String> args) {
  Cat cat = new Cat("Huahua",3);
 print( cat.name);
}

Common attributes and methods of list and map in Dar

list common properties and methods

common property

1,length gets the length of list 

main() {
  var list = ['red','yellow','pink'];
  print(list.length);
}

2, judge whether it is empty 

main() {
  var list = ['red','yellow','pink'];
  print(list.isEmpty);
}

3, not null 

main() {
  var list = ['red','yellow','pink'];
  print(list.isNotEmpty);
}

4, the array flips 

main() {
  var list = ['red','yellow','pink'];
  print(list.reversed.toList());
}
(pink, yellow, red)

common method

1, add data 

main() {
  var list = ['red','yellow','pink'];
  list.add("green");
  print(list);
}
[red, yellow, pink, green]

2, find data indexOf, find return index, can’t find return -1

main() {
  var list = ['red','yellow','pink'];
  var res = list.indexOf('yellow');
  print(res);

}  

1

3, removed data (‘ value ‘),removeAt(index)

main() {
  var list = ['red','yellow','pink'];
  list.remove('yellow');
  list.removeAt(1);

  print(list);

}  
[red]

4, modify data fillRange(start,end,value) without end

main() {
  var list = ['red','yellow','pink'];
  list.fillRange(1, 2,'green');
  print(list);

}

[red, green, pink]

5, insert data insert(index,value) to the specified position. Insert

from index

main() {
  var list = ['red','yellow','pink'];
  list.insert(1, 'green');
  print(list);
}

[red, green, yellow, pink] 

main() {
  var list = ['red','yellow','pink'];
  list.insertAll(1, ['green','white']);
  print(list);
  
} 
[red, green, white, yellow, pink]

6, convert to string join(‘, ‘), instead of the original list

main() {
  var list = ['red','yellow','pink'];
  var str = list.join(',');
  print(str);

}
 
red,yellow,pink

7, convert string to array split(‘ – ‘), do not change the original string 

main() {
  var str = 'red-yellow-pink';
  var list = str.split('-');
  print(list);
  print(str);
  
}  
[red, yellow, pink]
red-yellow-pink

8, for(var item in list) loop list

main() {
  var list = ['red','yellow','pink'];
  for(var item in list) {
    print(item);
  }
}

red
yellow
pink

9,map loop list

main() {
  var list = [1,2,3,4];
  var newList = list.map((value) => value * 2);
  print(newList.toList());
}

10, see if the set satisfies some condition any

main() {
  var list = ['red','yellow','pink'];
  var f = list.any((value) => value == 'red');
  print(f);
  print(list);
  
}

11, through the set can not add repeated collection, so can not get

through the index

main() {
  var set = new Set();
  set.add('red');
  set.add('yellow');
  set.add('pink');
  set.add('red');
  print(set.toList());
}  

[red, yellow, pink]

maps type common properties and methods

1, get all keys 

main() {
  var person = {
    'name':'张三',
    'age':20,
  };
  print(person.keys.toList());
}

[name, age]

2, get all values 

main() {
  var person = {
    'name':'张三',
    'age':20,
  };
  print(person.values.toList());
}

3, determine whether the attribute isEmpty isEmpty

main() {
  var person = {
    'name':'张三',
    'age':20,
  };
  print(person.isEmpty);
  
}
false
common method

1, remove the specified key data (key)

main() {
  var person = {
    'name':'张三',
    'age':20,
  };
  person.remove('name');
  print(person);
}
{age: 20}

2, addAll({‘ key ‘:’ value ‘})

main() {
  var person = {
    'name':'张三',
    'age':20,
  };
  person.addAll({
    'sex':'男',
  });
  print(person);
}

{name: 张三, age: 20, sex: 男}

3, check to see if the fields are in the map containsKey(key), return true/false does not change the original pair like 

main() {
  var person = {
    'name':'张三',
    'age':20,
  };
  var boo = person.containsKey('name');
  print(boo);

}

true

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