英文:
Method reference to an instance method of an arbitrary object of a particular type
问题
String[] arr = {"First", "Second", "Third", "Fourth"};
Arrays.sort(arr, String::compareToIgnoreCase); // 可以编译
Arrays.sort(arr, "a"::compareToIgnoreCase); // 无法编译
1) 为什么 "a"::compareToIgnoreCase 无法编译？如果我们可以说 String::compareToIgnoreCase 有一个隐式的 String 参数（this），为什么我们不能说 "a"::compareToIgnoreCase 有一个隐式的 "a" 参数？（"a" 与 "First" 进行比较，"a" 与 "Second" 进行比较，依此类推）
英文:
String[] arr = {"First", "Second", "Third", "Fourth"};
Arrays.sort(arr, String::compareToIgnoreCase); //can compile
Arrays.sort(arr, "a"::compareToIgnoreCase); //can't compile
- why the "a"::compareToIgnoreCase cannot compile? if we can said String::compareToIgnoreCase has an implicit String argument (this), why we cannot said "a"::compareToIgnoreCase has an implicit "a" as argument? ("a" compare to "First", "a" compare to "Second".....)
答案1
得分: 2
"a"::compareToIgnoreCase是对一个单一参数的方法的方法引用，它将给定的String与String"a"进行比较。隐式参数始终等于"a"。
一个Comparator的compare方法需要两个给定的String实例。
也许如果你将方法引用写成lambda表达式，会更清晰：
Arrays.sort(arr, (a, b) -> a.compareToIgnoreCase(b)); // 可以编译
Arrays.sort(arr, (x) -> "a".compareToIgnoreCase(x)); // 不能编译，因为期望有两个参数的方法
英文:
"a"::compareToIgnoreCase is a method reference to a method of a single argument, which compares a given String to the String "a". The implicit argument is always equal to "a".
A Comparator's Compare method requires two given String instances.
Maybe if you write the method references as lambda expressions it would be clearer:
Arrays.sort(arr, (a,b) -> a.compareToIgnoreCase(b)); //can compile
Arrays.sort(arr, (x) -> "a".compareToIgnoreCase(x)); // can't compile, since a method with
// two arguments is expected
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