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);

	}

}

Java类和对象

相关

标签