Java Error: The constructor is undefined

Question

In Java, Why am I getting this error:

Error: The constructor WeightIn() is undefined

Java Code:

public class WeightIn{
  pr         


        

Answer 1

In Java, Why am I getting this error:

    Error: The constructor WeightIn() is undefined

It's simply because you didn't have the matching constructor for your class:

public class WeightIn {
  ...

  public WeightIn (double weightIn, double heightIn){
    weight = weightIn;
    height = heightIn;
  }

  ...
}

you need to add the public WeightIn() {}.

In Java, the default constructor is automatically generated by the compiler if you didn't defined it. So, when you define the following class:

public class WeightIn {
  private double weight;
  private double height;

  // didn't define a constructor.
  public void setWeight(double weightIn){
    weight = weightIn;
  }
  public void setHeight(double heightIn){
    height = heightIn;
  }
}

compiler will generating a matching default constructor for you. So, your class is implicitly having a default constructor like this:

public class WeightIn {
  private double weight;
  private double height;

  // automatically generated by compiler
  public WeightIn() {
    // do nothing here.
  }

  // didn't define a constructor.
  public void setWeight(double weightIn){
    weight = weightIn;
  }
  public void setHeight(double heightIn){
    height = heightIn;
  }
}

when you instantiate the class with:

WeightIn weight = new WeightIn(); 

everything is alright.

But when you're adding a constructor by yourself, the default constructor will not generated by the compiler. So, when you're creating the class with:

public class WeightIn {
  ...

  // this won't automatically generated by compiler
  // public WeightIn() {
  //   nothing to do here.
  //}

  public WeightIn (double weightIn, double heightIn){
    weight = weightIn;
    height = heightIn;
  }

  ...
}

You won't have the default constructor (i.e public WeightIn(){}). And using the following will raise an error because you have no matching constructor:

 WeightIn weight = new WeightIn();