可以多实现，不可以多继承---安全原因

/*
abstract class Demo
{
	abstract void show();
	abstract void func();
}
*/

interface Demo
{
	public static final int x = 4;
	public abstract void show();
	public abstract void func();
}

class DemoImpl implements Demo
{
	public  void show(){}
	public  void func(){}
}

class  InterDemo
{
	public static void main(String[] args) 
	{
		DemoImpl di = new DemoImpl();
		di.show();
		System.out.println(di.x);
		System.out.println(DemoImpl.x);
		System.out.println(Demo.x);
	}
}

/*
class A
{
	void show()
	{
		System.out.println("a");
	}
}
class B
{
	void show()
	{
		System.out.println("B");
	}
}

class C extends A,B
{
}

main()
{
	C c = new C();
	c.show();//不确定.
}
*/

interface A
{
	void show();
}
interface B
{
	void show();
}

interface E extends A,B
{
}
class D
{

}
class C extends D implements A,B
{
	public void show(){}
}

interface Smoking
{
	void smoke();
}

abstract class XueYuan
{
	void sleep(){}
	abstract void study();
	
}

class Ls extends XueYuan implements Somking
{
	void study(){}
	public void smoke(){}
}
class Wu extends XueYuan
{
	void study(){}
}