Get a compile time error when storing car of wrong class in workshop object | Together Java | Page 1

sly snow Feb 10, 2026, 8:09 AM

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I have a assignment where we are supposed to make some car-classes and a workshop that is able to store some types of cars (guess this should be specified when creating it), and which is meant to raise a compile time error when trying to add a car of the wrong class. So how can I achieve this?

jade stumpBOT Feb 10, 2026, 8:09 AM

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<@&987246399047479336> please have a look, thanks.

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TJ-Bot

I have a assignment where we are supposed to make some car-classes and a workshop that is able to store some types of cars (guess this should be specified when creating it), and which is meant to raise a compile time error when trying to add a car of the w

Your compile-time error likely comes from a type-safety issue with generics in your Workshop class. If your workshop is designed to store only specific car subclasses, it should be declared with a generic type parameter such as Workshop<T extends Car>. Then when you create an instance, specify the exact car class it can accept, e.g., Workshop<Sedan> workshop = new Workshop<>();. This ensures the compiler enforces that only Sedan or subclasses thereof can be stored.

If you try to add a different car type, the compiler will raise an error as expected. This is intentional and a benefit of generics because it catches type mismatches at compile time rather than runtime.

So, the key points are:

Use a generic type parameter with an upper bound (<T extends Car>) in your Workshop class.
Specify the exact subclass when creating a workshop instance.
The compile-time error is the expected safety feature of Java generics, not a bug.

If your current code doesn’t use generics or uses raw types, that could be why you see errors.

Useful links:

sly snow Feb 10, 2026, 8:10 AM

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Is there any way to have a parameter when creating a instance of the workshop that describes what types it is allowed to store?

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I have this code right now: ```
package org.example;

import java.util.Set;
import java.util.ArrayList;

public class Workshop {
private int maxCars;
private Set<?> allowedTypes;
private ArrayList<Car> storedCars = new ArrayList<>();

public Workshop(Set<?> types, int max) {
    allowedTypes = types;
    maxCars = max;
}

public <T extends Car> void storeCar(T car){
    if (storedCars.size() == maxCars) {return;}
    storedCars.add(car);
}

public Car retrieveCar() {
    return storedCars.removeLast();
}

}

jade stumpBOT Feb 10, 2026, 8:20 AM

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sly snow I have this code right now: ``` package org.example; import java.util.Set; impo...

Detected code, here are some useful tools:

Formatted code

package org.example;

import java.util.Set;
import java.util.ArrayList;

public class Workshop {
  private int maxCars;
  private Set<? > allowedTypes;
  private ArrayList<Car> storedCars = new ArrayList<>();
  public Workshop(Set<? > types, int max) {
    allowedTypes = types;
    maxCars = max;
  }
  public <T extends Car> void storeCar(T car) {
    if (storedCars.size() == maxCars) {
      return ;
    }
    storedCars.add(car);
  }
  public Car retrieveCar() {
    return storedCars.removeLast();
  }
}

storm goblet Feb 10, 2026, 8:24 AM

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yes, what u want is called generics

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drop that wildcard <?> and implement proper generics

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will look sth like this:

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public class Workshop<C extends Car> {
  ...
  private List<C> cars = new ArrayList<>();
  ...
  public void storeCar(C car) {
    ...
    cars.add(car);
  }

  public C retrieveCar() { ... }
}

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if ur confused by this, read a tutorial about generics first 🙂

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(or ask us concrete questions to it 👍)

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think about the class Workshop<C extends Car> like a parameter in a method. just that its type-parameter for the class

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people who create instances of the class can then choose the type they want

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new Workshop<Mercedes>()

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and so on

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its the same mechanism that arraylist uses as well

sly snow Feb 10, 2026, 8:29 AM

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now how can I get a list of classes? I tried this Class<? extends Vehicle> vehicles = new Class[] {Mercedes.class, Ford.class} but it didn't work, got the error that I can't convert Class[] to Class<? extends Vehicle>

storm goblet Feb 10, 2026, 8:34 AM

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ur overcomplicating it

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the user specifies the type on creation of the workshop

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new Workshop<Mercedes>()

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and that Mercedes is then ur C

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so u dont have to do anything anymore inside the class

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just what i wrote above

sly snow Feb 10, 2026, 8:35 AM

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yes but I want to have a list of the classes to make it be random for testing

storm goblet Feb 10, 2026, 8:35 AM

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then inside ur test just create different workshops

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new Workshop<Mercedes>(), new Workshop<Audi>(), ...

sly snow Feb 10, 2026, 8:37 AM

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I know I can do that, but isn't there any way to make it take a variable as the list of allowed types?

storm goblet Feb 10, 2026, 8:37 AM

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yes but why would u want to do that. u sacrifice compile time safety that way

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its not meaningful

sly snow Feb 10, 2026, 8:38 AM

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okay

storm goblet Feb 10, 2026, 8:38 AM

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if u want to have workshops that sell only mercedes and audi but not fiat, then u wont be able to get that with generics

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and not with compile-time safety

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in that case u would indeed take a list of class tokens

sly snow Feb 10, 2026, 8:39 AM

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can I use generics to make it be multiple classes allowed? for example workshop<Mercedes, Jaguar>?

storm goblet Feb 10, 2026, 8:42 AM

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class Workshop {
  private List<Car> cars = new ArrayList<>();
  private Set<Class<? extends Car>> tokens;

  public Workshop(List<Class<? extends Car>> tokens) {
    this.tokens = new HashSet<>(tokens);
  }

  void addCar(Car car) {
    tokens.findAny(t -> t.isAssignableFrom(car.getClass()))...
  }

  Car getCar() {
    cars...
  }
}

storm goblet Feb 10, 2026, 8:42 AM

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sly snow can I use generics to make it be multiple classes allowed? for example workshop<...

no

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only if u want it to always be exactly two types

sly snow Feb 10, 2026, 8:42 AM

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okay, because that is something we are supposed to add

storm goblet Feb 10, 2026, 8:43 AM

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i have some doubts that u interpreted the task correctly

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bc this is just super unusual and not the right way to approach it

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could u share the assignment?

sly snow Feb 10, 2026, 8:44 AM

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''some workshops are supposed to store a specific type of cars; others can store any type of cars''

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does that mean that either I have one specific type and do Workshop<Type> workshop = new Workshop<Type>() or for any type I do Workshop<Car> workshop = new Workshop<Car>()

storm goblet Feb 10, 2026, 8:46 AM

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sly snow ''some workshops are supposed to store a specific type of cars; others can store...

yeah. so u have that simply by generics already

storm goblet Feb 10, 2026, 8:46 AM

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sly snow does that mean that either I have one specific type and do `Workshop<Type> works...

exactly

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new Workshop<Mercedes()

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and new Workshop<Car>()

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one is mercedes only, the other allows all of them

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but strict-mixtures arent really doable

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and i dont think they ask for this

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as in "make a workshop that can only deal with mercedes and audi but not fiat"

#Get a compile time error when storing car of wrong class in workshop object