对象转型(casting); 多态; static(静态);单例模式

　　　　对象转型:

　　

public class Animals {  //父类

	private String lei;
	private double weight;  //需要get,set一下,外界才可以调用
	public Animals() {
		
		}
	public Animals(String lei, double weight) {
		super();
		this.lei = lei;
		this.weight = weight;
	}
	public void Print() {
		System.out.println("动物种类" + lei);
		System.out.println("重量" + weight);
	}
		
	/*public void Sv () {
		System.out.println("你好");
	}*/
		
	public String getLei() {
		return lei;
	}
	public void setLei(String lei) {
		this.lei = lei;
	}
	public double getWeight() {
		return weight;
	}
	public void setWeight(double weight) {
		this.weight = weight;
	}
	
}	

　　

public class Dog extends Animals {  //子类
		private String shi;
		
		public Dog() {
			
		}
		public Dog(String lei, double weight,String shi) {
			super(lei, weight);   //引用的是父类里的lei,weight构造方法
			this.shi = shi;
		}
		public void Print () {  //重写父类里的方法
			System.out.println("动物种类是" + super.getLei()); //因为是继承,super之后就可以获取
			System.out.println("重量" + super.getWeight());
			System.out.println("狗的食物"+ shi);
		}
		
		
		public String getShi() {
			return shi;
		}
		public void setShi(String shi) {
			this.shi = shi;
		}

}

　　

public class Main {

	public static void main(String[] args) {
		//父类的引用指向子类对象
		Animals an = new Dog();  //实例化Animals
		an.setLei("狗");  
		an.setWeight(15);
		
		/*an.Print();*///父类里的方法,要想设置食物,走下一步强转
		Dog dg = (Dog) an;  //子类Dog新增的属性shi和方法,不能直接访问,要强转
		dg.setShi("骨头");
		dg.Print();

        }

}
    

效果:

　　　　多态:

public class Test {
public void Printf(Animals an) {  //多态
			if(an instanceof Dog) {  //如果an这个对象是一种狗,
				//instanceof关键字是用来判断变量an是不是这个类或是这个类的子类
				Dog x =(Dog)an;  //强转成狗
				x.Print();   //输出Dog的构造方法
			} else if(an instanceof Cat ) {
				Cat c = (Cat)an;  //强转成猫
				c.Print();//输出Cat的构造方法
			}else {
				an.Print();  //animals的方法
			}
			
		}

}

//多态
		Dog dog = new Dog("狗",15,"骨头");  //先实例化一个Dog
		
		Cat cat = new Cat("猫",5,"老鼠");
			
		Test t = new Test();
		t.Printf(dog);  //dog就是an的一种
		t.Printf(cat);

　　　　静态(static):

public class Dog extends Animals {  //子类

            	//static 静态
		private String name;
		private int age;
		public static  int num;  //static
		 
		
		public Dog() {  //给num一个构造方法
			num++;
		}
		public void Print () {
			System.out.println("这是第" + num + "只狗");
		}
		public String getName() {
			return name;
		}
		public void setName(String name) {
			this.name = name;
		}
		public int getAge() {
			return age;
		}
		public void setAge(int age) {
			this.age = age;
		}

}

//static 的静态利用
		Dog d1 = new Dog();
		d1.Print(); //输出第几只狗
		System.out.println(Dog.num);  //static 直接拿类名 . num 输出,输出第一个狗
		Dog d2 = new Dog();
		System.out.println(Dog.num); //输出第二只狗
		d2.Print();

　　　　单例模式:

public class HelloWorld {
		private static HelloWorld helloworld;
		private HelloWorld () {
			
		}
		private int count;
		public static HelloWorld getinstance() {  //static的不用实例化,直接用
			if (helloworld == null ) {    //如果helloworld是空的话
				helloworld = new HelloWorld();  //给helloworld实例化
			}
			return helloworld;    //返回helloworld
		}
		public int getCount() {
			return count;
		}
		public void setCount(int count) {
			this.count = count;
		}
		
}

//单例模式
		HelloWorld hw = HelloWorld.getinstance();
		System.out.println(hw.getCount());  //输出的返回值是为0,因为没有给其赋值
		HelloWorld hw2 = HelloWorld.getinstance();
		hw2.setCount(5);
		System.out.println(	hw.getCount());*///输出的是5,因为hw与hw2指向的位置相同