javaNull / UndefinedMedium

Unexpected null values during base class initialization

Time: 0:00

BugFind the bug and fix the code

What it should do

Initialize a base class object and set a child-specific field. Example: initialize() -> void

What happens instead

Exception in thread "main" java.lang.NullPointerException

class Base {
  protected String childField;
  public Base() {
    initialize();
  }
  public void initialize() {
    System.out.println(childField);
  }
}

class Child extends Base {
  public Child() {
    super();
    childField = "Initialized";
  }
}

public class Main {
  public static void main(String[] args) {
    new Child();
  }
}

Test a custom input to verify an edge case (e.g. empty lists, negative numbers, duplicates).

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This bug belongs to the null and undefined errors pattern

Cannot read properties of undefined, NoneType has no attribute, NaN appearing from nowhere.

Learn the null and undefined errors pattern Step through your own code

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class Base { protected String childField; public Base() { initialize(); } public void initialize() { System.out.println(childField); } } class Child extends Base { public Child() { super(); childField = "Initialized"; } } public class Main { public static void main(String[] args) { new Child(); } }

Unexpected null values during base class initialization

What it should do

What happens instead

More null and undefined errors to practise

Seen this error in your own code?

Unexpected null values during base class initialization

What it should do

What happens instead

More null and undefined errors to practise

Seen this error in your own code?