两个看来相似的问题 - 未名空间MITBBS历史存档

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两个看来相似的问题

两个看来相似的问题# Programming - 葵花宝典

G*A2007-10-31 07:10

1 楼

问题1:
assume that WhiteOnBlackLabel inherits a paint method from JLabel and also
implements its own version of paint.
JLabel label = new WhiteOnBlackLabel("A label");
label.paint(g); //for some Graphics object g
Which of those two implementations of paint will be executed in
label.paint(g); of this example?
paint() defined in label or paint() defined in WhiteOnBlacklabel?
问题2:
All of our Fruits inherit an equals method from class Object. Suppose that
Fruit declares its own equals method:
Object>

d*d2007-10-31 07:10

2 楼

同学，new出来的是个pointer吧。
你把题目改严谨一点先。不然没法回答的。

【在 G****A 的大作中提到】

: 问题1:
: assume that WhiteOnBlackLabel inherits a paint method from JLabel and also
: implements its own version of paint.
: JLabel label = new WhiteOnBlackLabel("A label");
: label.paint(g); //for some Graphics object g
: Which of those two implementations of paint will be executed in
: label.paint(g); of this example?
: paint() defined in label or paint() defined in WhiteOnBlacklabel?
: 问题2:
: All of our Fruits inherit an equals method from class Object. Suppose that

t*t2007-10-31 07:10

3 楼

不是C++吧?

【在 d*******d 的大作中提到】

: 同学，new出来的是个pointer吧。
: 你把题目改严谨一点先。不然没法回答的。

d*d2007-10-31 07:10

4 楼

oh,看上去会不会是java?

【在 t****t 的大作中提到】

: 不是C++吧?

R*Z2007-10-31 07:10

5 楼

赫赫, 都看见JLabel了, 当然是Java

【在 d*******d 的大作中提到】

: oh,看上去会不会是java?

R*Z2007-10-31 07:10

6 楼

i believe #1 should be WhiteOnBlacklabel's paint(), and #2 should call Fruit
's equalTo()
Using C++ terminology, all Java functions/methods are virtual (unless it's
declared final I guess). So if you have a class D inherited from base class
B, even if you refer to this object as B, it is acutally a D object and
will call D's methods.

【在 G****A 的大作中提到】

g*g2007-10-31 07:10

7 楼

It's called abstract in java terms. What you say holds though.

Fruit
class

【在 R***Z 的大作中提到】

: i believe #1 should be WhiteOnBlacklabel's paint(), and #2 should call Fruit
: 's equalTo()
: Using C++ terminology, all Java functions/methods are virtual (unless it's
: declared final I guess). So if you have a class D inherited from base class
: B, even if you refer to this object as B, it is acutally a D object and
: will call D's methods.

R*Z2007-10-31 07:10

8 楼

i thought Java's abstract methods are like pure virtual functions in C++,
and Java's abstract classes are like classes that contain pure virtual
functions in C++.

【在 g*****g 的大作中提到】

: It's called abstract in java terms. What you say holds though.
:
: Fruit
: class

j*g2007-10-31 07:10

9 楼

I think the two questions maybe not the same.
For question one, if the subclass' paint is using Graphics object as its
parameter, it's an overwriting. The runtime execution will call subclass'
paint method.
For the second question, two methods have different signatures. That's
overloading. Which method is called depends on excact parameter matching. In
this case, the Fruit one should be called.

G*A2007-10-31 07:10

10 楼

第二个问题正确答案是Object.

Fruit
class

【在 R***Z 的大作中提到】

G*A2007-10-31 07:10

11 楼

You got the point! For the second one, these is NO overrrding because two
menthods have different signatures.

In

【在 j****g 的大作中提到】

: I think the two questions maybe not the same.
: For question one, if the subclass' paint is using Graphics object as its
: parameter, it's an overwriting. The runtime execution will call subclass'
: paint method.
: For the second question, two methods have different signatures. That's
: overloading. Which method is called depends on excact parameter matching. In
: this case, the Fruit one should be called.

c*x2007-10-31 07:10

12 楼

the purpose of two questions are the same, they test what
polymorphysm,
1. label.paint(g); // WhiteOnBlackLabel.paint() is called.
2. fo.equals(f) // will actually call public boolean equals(Fruit fruit)