说白了就是子类赋值给父类。
//在主类中people类的声明对象分别作为Chinapeople和Americanpeople类对象的上转型对象，
//Chinapeople和Americanpeople类是主类people的子类
//并让这个上转型对象调用showname方法。
People p1;
Americanpeople ap= new Americanpeople();
p1 = ap;
People p2;
Chinapeople cp = new Chinapeople();
p2 = cp;
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