请看下面两段代码
interface Name extends Comparable 
{
public int compareTo(Object o);
}
class SimpleName implements Name {
private String base;
public SimpleName(String base) {
this.base = base;
}
public int compareTo(Object o) {
return base.compareTo(((SimpleName)o).base);
}
}
class ExtendedName extends SimpleName {
private String ext;
public ExtendedName(String base, String ext) {
super(base); this.ext = ext;
}
public int compareTo(Object o) {
int c = super.compareTo(o);
if (c == 0 && o instanceof ExtendedName)
return ext.compareTo(((ExtendedName)o).ext);
else
return c;
}
}
class Client {
public static void main(String[] args) {
Name m = new ExtendedName("a","b");
Name n = new ExtendedName("a","c");
assert m.compareTo(n) < 0;
}
}
interface Name extends Comparable<Name> {
public int compareTo(Name o);
}
class SimpleName implements Name {
private String base;
public SimpleName(String base) {
this.base = base;
}
public int compareTo(Name o) {
return base.compareTo(((SimpleName)o).base);
}
}
// use legacy class file for ExtendedName
class Test {
public static void main(String[] args) {
Name m = new ExtendedName("a","b");
Name n = new ExtendedName("a","c");
assert m.compareTo(n) == 0; // answer is now different!
}
}
注意到不一样的地方呢吗？compareTo方法的参数不一样。上面的代码在调用m.compareTo(n)的时候会调用ExtendedName中的，而下面这种不会，因为ExtendedName中的compareTo参数是Object，相当于重载了compareTo的方法，而父类SimpleName中的compareTo方法由于参数不同仍然被保留了，所以将Name类型的参数传给compareTo的时候会优先调用SimpleName中的compareTo(Name)来进行比较。
所以在一些继承里面，建议使用Object做参数的类型，称之为Binary Compatibility。
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posted on 2007-10-28 21:09
TiGERTiAN 阅读(911)
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