This test shows how generic type erasures work.

GenericTypeErasureTest.java

package com.impressiveinteractive.playground;

import java.util.ArrayList;
import java.util.List;
import java.util.Optional;

import org.junit.Rule;
import org.junit.Test;
import org.junit.rules.ExpectedException;

public class GenericTypeErasureTest {

	@Rule
	public ExpectedException exception = ExpectedException.none();

	@Test
	public void testCastingToListOfDifferentType() throws Exception {
		ArrayList<SockPuppet> sockPuppets = new ArrayList<>();
		sockPuppets.add(new SockPuppet());
		sockPuppets.add(new SockPuppet());
		sockPuppets.add(new SockPuppet());
		Object mysteryObject = sockPuppets;

		killAllHumans((List<Human>) mysteryObject); // Oke
	}

	private void killAllHumans(List<Human> humans) {
		Optional<Human> optional = humans.stream().findFirst(); // Still oke

		exception.expect(ClassCastException.class);
		Human human = optional.get(); // Not oke
	}

	private static class SockPuppet {}

	private static class Human {}
}