java内部类继承的问题?报错

关于内部类的继承有些疑问想请教下大家：

1.eclipse上保存提示

No enclosing instance of type Outer is available due to some intermediate constructor invocation

public class Outer {
	public class InnerSub extends Outer{
	}
	public class InnerSub_2 extends InnerSub{
	}

	public static void main(String ... args) throws Exception{
		Outer.InnerSub sub = new Outer().new InnerSub();
		System.out.println("run...");
	}
}

2.还是上面这段代码，创建InnerSub_2的实例对象会报错，但是如果改成下面的则可以。（将InnerSub类改成static也可以使这种继承关系成立）

public class Outer {
	public class InnerSub{// extends Object{
	}
	public class InnerSub_2 extends InnerSub{
	}

	public static void main(String ... args) throws Exception{
		Outer.InnerSub sub = new Outer().new InnerSub();
		Outer.InnerSub sub2 = new Outer().new InnerSub_2();
		System.out.println("run...");
	}
}

No enclosing instance of type Outer is available due to some intermediate constructor invocation