java 切换枚举值：case 表达式必须是常量表达式
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switch over value of enum: case expressions must be constant expressions
提问by Ian2thedv
I have an enum with the following structure:
我有一个具有以下结构的枚举：
public enum Friends {
Peter("Peter von Reus", "Engineer"),
Ian("Ian de Villiers", "Developer"),
Sarah("Sarah Roos", "Sandwich-maker");
private String fullName;
private String occupation;
private Person(String fullName, String occupation) {
this.fullName = fullName;
this.occupation = occupation;
}
public String getFullName() {
return this.fullName;
}
public String getOccupation() {
return this.occupation;
}
}
I would now like to use switchto determine, if a variable nameis associated with a certain enum:
我现在想用以switch确定变量name是否与某个相关联enum：
//Get a value from some magical input
String name = ...
switch (name) {
case Friends.Peter.getFullName():
//Do some more magical stuff
...
break;
case Friends.Ian.getFullName():
//Do some more magical stuff
...
break;
case Friends.Sarah.getFullName():
//Do some more magical stuff
...
break;
}
This seems perfectly legal to me, but I'm getting the error case expressions must be constant expressionsin Eclipse.
I can get around this with a simple set of if statements but I would like to know the reason for this error and how things might go south if this was allowed.
这对我来说似乎完全合法，但我case expressions must be constant expressions在 Eclipse 中遇到了错误。我可以用一组简单的 if 语句来解决这个问题，但我想知道这个错误的原因以及如果允许的话事情会如何发展。
NOTE: I cannot change the structure of Friends
注意：我不能改变结构 Friends
回答by Elliott Frisch
If I understand your question, you can use valueOf(String)like
如果我理解你的问题，你可以使用valueOf(String)像
String name = "Peter";
Friends f = Friends.valueOf(name);
switch (f) {
case Peter:
System.out.println("Peter");
break;
case Ian:
System.out.println("Ian");
break;
case Sarah:
System.out.println("Sarah");
break;
default:
System.out.println("None of the above");
}
Also, this
还有，这
private Person(String fullName, String occupation) {
this.fullName = fullName;
this.occupation = occupation;
}
Should be
应该
private Friends(String fullName, String occupation) {
this.fullName = fullName;
this.occupation = occupation;
}
Because Person!= Friends.
因为Person!= Friends。
Edit
编辑
Based on your comment, you will need to write a static method to get the correct Friendsinstance,
根据您的评论，您需要编写一个静态方法来获取正确的Friends实例，
public static Friends fromName(String name) {
for (Friends f : values()) {
if (f.getFullName().equalsIgnoreCase(name)) {
return f;
}
}
return null;
}
Then you can call it with,
然后你可以调用它，
String name = "Peter von Reus";
Friends f = Friends.fromName(name);
valueOf(String)will match the name of the enum field. So "Ian", "Sarah" or "Peter".
valueOf(String)将匹配枚举字段的名称。所以“伊恩”、“莎拉”或“彼得”。
回答by Jon Skeet
This seems perfectly legal to me
这对我来说似乎完全合法
Well it's not - a method call is nevera constant expression. See JLS 15.28for what constitutes a constant expression. And a case value always has to be a constant expression.
好吧，它不是 - 方法调用从来都不是常量表达式。请参阅JLS 15.28了解什么是常量表达式。并且 case 值必须始终是一个常量表达式。
The simplest fix would be to have a Friend.fromFullNamestatic method, which perhaps looked the Friendup in a HashMap<String, Friend>. (You don't haveto have that method in Friendof course... it's just that would be the most conventional place.) Then you could switch over the enum rather than the name.
最简单的解决方法是使用Friend.fromFullName静态方法，它可能Friend在HashMap<String, Friend>. （你不具备有这种方法中Friend，当然......它只是将是最传统的地方。）然后，你可以在枚举切换而不是名称。
As a side note, your enum name should be in the singular and with ALL_CAPSmembers, so Friend.PETERetc.
作为旁注，您的枚举名称应为单数并带有ALL_CAPS成员Friend.PETER等。
