public static void main(String[] args) { List<DemoClass> demos = null; for (DemoClass demo : demos) { System.out.println("123"); } }

list.size()>0就行

      public static void main(String[] args) {
		DemoClass dc=null;
		List<DemoClass> demos = new ArrayList();
		demos.add(dc);
		System.out.println(demos.size());
		for (DemoClass demo : demos) {   
	         System.out.println("123"); 
	    } 
	}