請看下面兩段代碼
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!
}
}
注意到不一樣的地方呢嗎？compareTo方法的參數不一樣。上面的代碼在調用m.compareTo(n)的時候會調用ExtendedName中的，而下面這種不會，因為ExtendedName中的compareTo參數是Object，相當于重載了compareTo的方法，而父類SimpleName中的compareTo方法由于參數不同仍然被保留了，所以將Name類型的參數傳給compareTo的時候會優先調用SimpleName中的compareTo(Name)來進行比較。
所以在一些繼承里面，建議使用Object做參數的類型，稱之為Binary Compatibility。
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posted on 2007-10-28 21:09
TiGERTiAN 閱讀(913)
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