Two Inner Classes

Main.java

public class Main {

  public static void main(String[] args) {
    a().evaluate();
  }

  public static Expression a() {
    final Expression e = SimpleExpression.builder();
    return new Expression() {
      @Override
      public Expression evaluate() {
        return e.evaluate();
      }
    };
  }

  public static Expression b() {
    final Expression e = SimpleExpression.builder();
    return new Expression() {
      @Override
      public Expression evaluate() {
        return e.evaluate();
      }
    };
  }
}

interface Expression {
  Expression evaluate();
}

class SimpleExpression implements Expression {
  @Override
  public Expression evaluate() {
    Vuln.vulnerableMethod();
    return null;
  }

  static SimpleExpression builder() {
    return new SimpleExpression();
  }

}

class Vuln {
  static void vulnerableMethod() {
    System.out.println("pwn");
  }
}