Java deadlock demo

package com.example.foo;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;

public class SomeClass {
  static class Outer {
    Inner inner = new Inner();

    public synchronized String getBoth() {
      return this.toString() + " - " + this.inner.toString();
    }

    class Inner {
      public synchronized String getBoth() {
        return this.toString() + " - " + Outer.this.toString();
      }

      @Override
      public synchronized String toString() {
        Thread.yield(); // To increase risk of deadlock
        return "Inner";
      }
    }

    @Override
    public synchronized String toString() {
      Thread.yield(); // To increase risk of deadlock
      return "Outer";
    }
  }

  public static void main(String[] args) throws InterruptedException {
    final Outer o = new Outer();
    // No problem:
    System.out.println(o.getBoth()); // outer, inner
    System.out.println(o.inner.getBoth()); // inner, outer

    ExecutorService pool = Executors.newFixedThreadPool(4);
    for (int i = 0; i < 100; i++) {
      pool.execute(() -> {
        System.out.println(o.getBoth());
      });
      pool.execute(() -> {
        System.out.println(o.inner.getBoth());
      });
    }
    pool.shutdown();
    if (!pool.awaitTermination(10, TimeUnit.SECONDS))
      throw new RuntimeException("DEADLOCK!");

    System.out.println("END");
  }

}