public class C {

private int i=20;private static int cnt=0;//要写成static减少每个对象开辟空间，写成private，保证不能被外部的类直接改写，必须通过对象才能改public C() { ++cnt;}public C(int i) { this.i=i;++cnt;}public static int getC(){ return cnt; // return this.cnt;错误，静态方法里的属性属于类而不是对象，不能加this}

}

public class Exercise2 {

public static void main(String[] args) {    System.out.printf("当前对象的个数为%d", C.getC());    C cc1=new C();    System.out.printf("当前对象的个数为%d",  C.getC());    C cc2=new C(4);    System.out.printf("当前对象的个数为%d",  C.getC());}

}