1､直接调用外部类非静态方法报错

public class OuterClass {
    public void printMessage(String message) {
        System.out.println(message);
    }
    private static class InnerClass {
        public void sayHello() {
            printMessage("Hello world!"); //error: Cannot make a static reference to the non-static method printMessage(String)
        }
    }
}

2､通过实例来调用外部类非静态方法

由于InnerClass是static,需要通过其它方式获取实例引用，常用的方式就是通过参数，例如：

public class OuterClass {
    private void printMessage(String message) {
        System.out.println(message);
    }
    private static class InnerClass {
        private void sayHello(OuterClass outer) {
            outer.printMessage("Hello world!"); // allowed
        }
    }
}

另外，如果InnerClass没有嵌套在里面OuterClass，它将无法访问该private方法,例如：

public class OuterClass {
    private void printMessage(String message) {
        System.out.println(message);
    }
}
class InnerClass {
    private void sayHello(OuterClass outer) {
        outer.printMessage("Hello world!"); // ERROR: The method printMessage(String) from the type OuterClass is not visible
    }
}