Java中，用接口来声明变量

 接口

public interface Happy {
	void SayHappy();
}

类 

public class People implements Happy {

	@Override
	public void SayHappy() {
		// TODO Auto-generated method stub
		System.out.println("我很快乐！");
	}

}

测试类 

public class Test3 {
	public static void main(String[] args) {
		Happy happy = new People();
		happy.SayHappy();
	}
}

输出：

我很快乐！