Java类和对象

• 最简单的类class A{}//没有任何属性和行为

• 对象A obj = new A();

• 类是定义，是规范，是“死”的东西

• 对象是实例，是类的一个实现，是一个具体的东西

• 打个比方

  • 类等价于一个土豆丝菜谱

  • 对象是根据类制作出的对象，等价于一盘土豆丝

• A obj1 = new A(); Aobj2 = new A();

• 以上有2个对象，他们的类型都是A，但是这是两个不同的对象，在内存中有不同的存放地址。因此，没有两个对象是完全一样的。

• A obj = new A();

• obj可以看作是内存中一个对象（包括若干个数据）的句柄

• 在C/C++中，obj称为指针，在Java中称为Reference

• 对象赋值是Reference赋值，而基本类型是直接值拷贝

• 产生一个对象，A obj = new A（）；

  • 99%的情况是用new关键字，还有1%是用克隆和反射生成

• new出对象后，内部属性值默认是

  • short 0 int 0 long 0L

  • boolean false

  • char ‘\u000’

  • byte 0

  • float 0.0f

  • double 0.0d

    public class ArgumentPassingTest {
    
    	public static void main(String[] args) {
    		int a = 1, b = 2;
    		swap(a,b);
    		System.out.println("a is " + a + ", b is " + b);  //a=1, b=2
    		
    		MyNumber obj1 = new MyNumber();
    		MyNumber obj2 = new MyNumber();
    		obj2.num = 10;
    		swap(obj1, obj2);
    		System.out.println("obj1 is " + obj1.num + ", obj2 is " + obj2.num);  //  obj1  10,   obj2  5
    
    	}
    	public static void swap(int m, int n)
    	{
    		int s = m;
    		m = n;
    		n = s;
    	}
    	public static void swap(MyNumber obj3, MyNumber obj4)
    	{
    		int s = obj3.num;
    		obj3.num = obj4.num;
    		obj4.num = s;
    	}
    
    }
    

     

    public class Initialization {
    	
    	boolean v1;
    	byte v2;
    	char v3;
    	double v4;
    	float v5;
    	int  v6;
    	long v7;
    	short v8; 
    	
    	
    	public static void main(String[] args) {
    		Initialization obj = new Initialization();
    		System.out.println("The initial value of boolean variable is " + obj.v1);
    		System.out.println("The initial value of byte variable is " + obj.v2);
    		System.out.println("The initial value of char variable is " + obj.v3);  //\u0000 space
    		System.out.println("The initial value of double variable is " + obj.v4);
    		System.out.println("The initial value of float variable is " + obj.v5);
    		System.out.println("The initial value of int variable is " + obj.v6);
    		System.out.println("The initial value of long variable is " + obj.v7);
    		System.out.println("The initial value of short variable is " + obj.v8);
    		
    		int a;
    		System.out.println(a);  //error, a没有初始化
    
    	}
    
    }
    

     

    public class ReferenceTest {
    
    	public static void main(String[] args) {
    		
    		int num1 = 5;
    		int num2 = num1;
    		System.out.println("num1: " + num1 + ", num2: " + num2);
    		num2 = 10;
    		System.out.println("num1: " + num1 + ", num2: " + num2);
    		
    		
    		MyNumber obj1 = new MyNumber();
    		MyNumber obj2 = new MyNumber();
    		System.out.println(obj1.num);
    		System.out.println(obj2.num);
    		System.out.println("======接下来输出新值=====");
    		
    		obj2 = obj1;
    		obj2.num = 10;
    		
    		System.out.println(obj1.num);
    		System.out.println(obj2.num);
    
    	}
    
    }