java 多行 lambda 比较器
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Multiline lambda comparator
提问by luanjot
I am starting with the lambda expressions in Java and there is something that I consider bizarre and I am sure that I am doing something wrong or it has a workaround.
我从 Java 中的 lambda 表达式开始，有一些我认为很奇怪的东西，我确信我做错了什么或者它有一个解决方法。
To define a comparator, I can do:
要定义比较器，我可以这样做：
col.setComparator((CustomCell o1, CustomCell o2) ->
((Comparable) o1.getValue()).compareTo(o2.getValue())
);
Which is great, however, if I just add two "{". I get a compilation error:
但是，如果我只添加两个“{”，那就太好了。我收到一个编译错误：
col.setComparator((CustomCell o1, CustomCell o2) -> {
((Comparable) o1.getValue()).compareTo(o2.getValue());
});
The error is not related to the "{", but to setComparator:
该错误与“{”无关，而是与setComparator：
The method setComparator(Comparator<CustomCell>) in the type
TableColumnBase<CustomParentCell,CustomCell> is not applicable for the arguments
((CustomCell o1, CustomCell o2) -> {})
I have tried using the multiline statements before for actionevents and it does work:
我之前曾尝试将多行语句用于 actionevents，它确实有效：
setOnAction(event -> {
// do something
});
Is it because it only has one argument?
是因为它只有一个参数吗？
回答by James_D
The method you are implementing with setOnActionis
您正在实施的方法setOnAction是
public void handleEvent(ActionEvent event) ;
It has a return type of void: i.e. it doesn't return anything:
它的返回类型为void: 即它不返回任何内容：
The method you are implementing with setComparatoris
您正在实施的方法setComparator是
public int compare(CustomCell cell1, CustomCell cell2) ;
which returns a value. To use the longer form, you must have an explicit return statement for methods that return a value:
它返回一个值。要使用更长的形式，您必须对返回值的方法有一个显式的 return 语句：
col.setComparator((CustomCell o1, CustomCell o2) -> {
return ((Comparable) o1.getValue()).compareTo(o2.getValue());
});
