public class DuoTai {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        Animal a1 = new Cat();
        a1.eat();
        
        Animal a2 = new Dog();
        a2.eat();
        
    }

}
 class Animal {
    public void eat(){
        System.out.println("吃东西");
    }
}

class Cat extends Animal {
    public void eat(){
        System.out.println("猫吃鱼");
    }
}

class Dog extends Animal {
    public void eat(){
        System.out.println("狗吃肉");
    }
}


错误类型
Exception in thread "main" java.lang.VerifyError: Bad type on operand stack
Exception Details:
  Location:
    DuoTai.main([Ljava/lang/String;)V @9: invokevirtual
  Reason:
    Type 'Cat' (current frame, stack[0]) is not assignable to 'Animal'
  Current Frame:
    bci: @9
    flags: { }
    locals: { '[Ljava/lang/String;', 'Cat' }
    stack: { 'Cat' }
  Bytecode:
    0x0000000: bb00 1059 b700 124c 2bb6 0013 bb00 1859
    0x0000010: b700 1a4d 2cb6 0013 b1                 

    at java.lang.Class.getDeclaredMethods0(Native Method)
    at java.lang.Class.privateGetDeclaredMethods(Unknown Source)
    at java.lang.Class.privateGetMethodRecursive(Unknown Source)
    at java.lang.Class.getMethod0(Unknown Source)
    at java.lang.Class.getMethod(Unknown Source)
    at sun.launcher.LauncherHelper.validateMainClass(Unknown Source)
    at sun.launcher.LauncherHelper.checkAndLoadMain(Unknown Source)

没错的，你重新试一下