Object Equals

Main.java

public class Main {

  public static void main(String[] args) {
    VulnerableClass a = new VulnerableClass(1);
    VulnerableClass b = new VulnerableClass(2);
    a.equals(b);
    HarmlessClass ha = new HarmlessClass(1);
    HarmlessClass hb = new HarmlessClass(2);
    ha.equals(hb);
    Object oa = new VulnerableClass(1);
    Object ob = new VulnerableClass(2);
    oa.equals(ob); 
                   // Object.equals() -> VulnerableClass.equals()
                   // Object.equals() -> HarmlessClass.equals()
    Object oha = new HarmlessClass(1);
    Object ohb = new HarmlessClass(2);
    oha.equals(ohb);                
  }
}

class VulnerableClass {
  int i;

  public VulnerableClass(int i) {
    this.i = i;
  }

  private void vulnerableMethod() {
    
  }
  
  @Override
  public boolean equals(Object obj) {
    // I am vulnerable
    vulnerableMethod();
    return i == ((VulnerableClass) obj).i;
  }
}

class HarmlessClass {
  @Override
  public boolean equals(Object obj) {
    return super.equals(obj); // HarmlessClass.equals() -> Object.equals() 
  }
}