Java threads 18+

package features;

public class ThreadLadder {
	static Object a1 = new Object();
	static Object a2 = new Object();
	static Object a3 = new Object();
	static Object a4 = new Object();
	static Object a5 = new Object();

	public static void main(String[] args) throws InterruptedException {
		new Thread(ThreadLadder::syncLadder, "1").start();
		Thread.sleep(500);
		new Thread(ThreadLadder::reverseSyncLadder, "2").start();
	}

	public static void syncLadder() {
		try {
			synchronized(a1) {
				System.out.println(Thread.currentThread().getName() + ": a1");
				synchronized(a2) {
					System.out.println(Thread.currentThread().getName() + ": a2");
					synchronized(a3) {
						System.out.println(Thread.currentThread().getName() + ": a3");
						synchronized(a4) {
							System.out.println(Thread.currentThread().getName() + ": a4");
							synchronized(a5) {
								System.out.println(Thread.currentThread().getName() + ": a5");
								Thread.sleep(2000);
								System.out.println(Thread.currentThread().getName() + ": a5");
							}
							System.out.println(Thread.currentThread().getName() + ": a4");
							Thread.sleep(100);
						}
						System.out.println(Thread.currentThread().getName() + ": a3");
						Thread.sleep(100);
					}
					System.out.println(Thread.currentThread().getName() + ": a2");
					Thread.sleep(100);
				}
				System.out.println(Thread.currentThread().getName() + ": a1");
				Thread.sleep(100);
			}
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
	}
	public static void reverseSyncLadder() {
		synchronized(a5) {
			System.out.println(Thread.currentThread().getName() + ": a5");
			synchronized(a4) {
				System.out.println(Thread.currentThread().getName() + ": a4");
				synchronized(a3) {
					System.out.println(Thread.currentThread().getName() + ": a3");
					synchronized(a2) {
						System.out.println(Thread.currentThread().getName() + ": a2");
						synchronized(a1) {
							System.out.println(Thread.currentThread().getName() + ": a1");
							try {
								Thread.sleep(2000);
							} catch (InterruptedException e) {
								e.printStackTrace();
							}
							System.out.println(Thread.currentThread().getName() + ": a1");
						}
					}
				}
			}
		}
	}
}