BigClass.java

public class BigClass {
	private Collaborator collaborator1;
	private Collaborator collaborator2;
	private Collaborator collaborator3;
	private Collaborator collaborator4;
	private Collaborator collaborator5;
	....
		
	public void doSomething(){
		...
	}
}

public class BigClassTest {
	@Test
	public void doSomething_thrownByCollaborator1() {
		...
	}
	@Test
	public void doSomething_thrownByCollaborator2() {
		...
	}
	...
}