如何方便Netbeans Designer使用hashmap加载使用枚举反向查找的JPanel？ [英] How to facilitate Netbeans Designer to load JPanel-s that use an enum reverse-lookup using hashmap?
问题描述
这是一个多萝西·迪克斯，当我发现事情的根源时撰写这个问题不过，我决定发布它，因为它让我们办公室里的每个人都陷入僵局，我可以在google或stackoverflow搜索中找不到任何帮助。这是一个很好的例子，一旦你提出正确的问题，你可能会看到光。
虽然标题可能听起来很复杂，一个似乎没有答案的真正简单的问题。这个中心是一个枚举：
状态
public enum Status
{
INVALID（INVALID ，
ISSUED（发行），
取消（已取消）;
private final String displayName;
private final static Map< String，Status> displayMap = new HashMap（4）;
私人状态（字符串显示）{
this.displayName = display;
mapDisplayName（this.displayName，this）;
}
public String getDisplayName（）{
return displayName;
}
public static状态parseString（String statusStr）{
return displayMap.get（statusStr）;
}
private static void mapDisplayName（final String displayName，final Status state）{
displayMap.put（displayName，state）;
}
}
当然的想法是使用 displayMap 作为反向查找。与$ getDisplayName（）方法无关。
此枚举的 getDisplayName（ ）调用在子面板中用于初始化与组合框一起使用的静态数组，例如：
public class JPanelStatus extends javax.swing.JPanel {
private final String [] STATUS_LABELS = {
Status.ISSUED .getDisplayName（），
Status.CANCELLED .getDisplayName（）
};
public JPanelStatus（）{
initComponents（）;
：
jComboBoxStatus.setModel（new DefaultComboBoxModel<>（STATUS_LABELS））;
：
}
：
}
其中在主要的JPanel中引用。当我在Netbeans Designer中查看这个 JPanelStatus 子面板时，工作正常。但是，当我加载主窗体时，它会失败，并且异常（例如，预览设计]功能
显示初始化失败：
java.lang.NoClassDefFoundError：无法初始化类au.com.project.State
在au.com.project.client.JPanelStatus。< init>（JPanelStatus.java:35）
at sun.reflect.NativeConstructorAccessorImpl.newInstance0（Native Method）
at sun.reflect.NativeConstructorAccessorImpl.newInstance （NativeConstructorAccessorImpl.java:62）
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance（DelegatingConstructorAccessorImpl.java:45）
在java.lang.reflect.Constructor.newInstance（Constructor.java:423）
... ...
Netbeans IDE日志添加了以下额外信息： p
信息：无法初始化类au.com.project.State/ pre>
通过一个消除过程 - Co发布不相关的代码 - 一旦我在State枚举中注释了HashMap
put（）调用，我发现表单将加载。
这很有趣。我说。它看起来像一个副作用，从
put（）。而离开它是 - 一个小的 Spock ，它很快给了我一样的错误从没有JPanel和没有Netbeans的命令行。
错误是由我造成的，尝试从Enum构造函数中使用HashMap。它不会写成。
所以我改变了标题来命中真正的问题 - 实际上，如何使用HashMap对枚举进行反向查找？
解决方案问题来自于HashMap如何在
Status枚举中声明如何初始化枚举。
Java枚举中的第一件事必须是值列表，如下所示：INVALID，ISSUED和CANCELED。每个人都需要知道的是，在对象创建（Class或Enum）期间，首先运行的是一个 Java 。 Init是愚蠢的，通过声明性代码线性运行，先到先得。
枚举的前3个x语句调用构造函数 - 这意味着语句：
private final static Map< String，Status> displayMap = new HashMap（4）;
尚未执行，
displayMap是 null 。另外，一个static {}块在相同的1-2-3 -...序列中执行，也不起作用。
遗憾的是，Netbeans / Designer堆栈跟踪或IDE日志都没有报告NullPointerException - 单元测试。一旦你有一个NPE，它集中精神。
解决方案：displayMap displayMap 在初始构造函数调用时未初始化不能
static final，因为您不能在构造函数中初始化静态成员。必须在第一次通话时使用以下示例中的一些变体来初始化：
状态/ strong>public enum Status
{
INVALID（INVALID），
发行（已发行），
取消（已取消）;
private static Map< String，Status> displayMap;
私人状态（字符串显示）{
this.displayName = display;
mapDisplayName（this.displayName，this）;
}
：
private static void mapDisplayName（final String displayName，final Status state）{
if（null == displayMap）{
displayMap =新的HashMap（7）;
}
displayMap.put（displayName，state）;
}
}
然后它运行顺利。
警告：
不要分配
null在displayMap声明 - 这是有效的：
- 如果（null == displayMap）在第一次调用Enum构造函数时，{...} 块成功分配了HashMap。
- 在处理所有枚举值声明之后。
- Init将为声明的变量调用任何初始化。
- 如果
displayMap = null;被声明它将替换使用新的空HashMap填充HashMap，带有3个x值。
可能相关问题：
- 使用每个键/常数有多个值的Java枚举反向查询 ... I感觉到HashMap会更容易地提供多个字符串的反向查找。
This has become a Dorothy Dix as I found the root of the matter while I was composing this question. Nevertheless, I decided to post it because it had everyone in our office stumped and I could find nothing helpful in the google or stackoverflow searches. This is a good example of once you ask the right question, you may see the light.
While the title might sounds complicated, it is a really simple problem that seemed to have no answer. At the centre of this is an enum:
Status
public enum Status
{
INVALID ( "INVALID" ),
ISSUED ( "Issued" ),
CANCELLED ( "Cancelled");
private final String displayName;
private final static Map<String,Status> displayMap = new HashMap( 4 );
private Status( String display ){
this.displayName = display;
mapDisplayName( this.displayName, this );
}
public String getDisplayName(){
return displayName;
}
public static Status parseString( String statusStr ) {
return displayMap.get( statusStr );
}
private static void mapDisplayName( final String displayName, final Status state ){
displayMap.put( displayName, state );
}
}
The idea of course is to use the displayMap as a reverse-lookup. Nothing to do with the getDisplayName() method at all.
This enum's getDisplayName() call is used in a sub-panel to initialise a static array used with a combobox, like:
public class JPanelStatus extends javax.swing.JPanel {
private final String[] STATUS_LABELS = {
Status.ISSUED .getDisplayName(),
Status.CANCELLED .getDisplayName()
};
public JPanelStatus(){
initComponents();
:
jComboBoxStatus.setModel( new DefaultComboBoxModel<>( STATUS_LABELS ) );
:
}
:
}
Which is referenced in the main JPanel. When I view this JPanelStatus sub-panel in the Netbeans Designer, it works fine. As does the [Preview Design] function.
However when I load the main form, it fails and the exception show an initialisation failure:
java.lang.NoClassDefFoundError: Could not initialize class au.com.project.State
at au.com.project.client.JPanelStatus.<init>(JPanelStatus.java:35)
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
... ... ...
The Netbeans IDE log added the following extra information :-p
INFO: Could not initialize class au.com.project.State
Through a process of elimination -- Commenting-out unrelated code -- I discovered the form will load once I comment-out the HashMap put() call in the State enum.
"That is interesting.", I say. It looked like a side-effect from the put(). And in away it is -- A small Spock which quickly gave me the same error from the command line without the JPanel and without Netbeans.
The error is caused by me, trying to use a HashMap from within the Enum constructor. It won't work as written.
So I changed the title to hit at the true problem -- Which is actually, how to use a HashMap to do a reverse-lookup for an enum?
The problem comes from how the HashMap is declared within the Status enum due to HOW enums are initialised.
The first thing in a Java Enum must be the list of values, here: "INVALID", "ISSUED", and "CANCELLED". The next thing everyone needs to know is that there is a secret Java stage that runs first during Object creation (Class or Enum). Init is dumb, is runs linearly through the declarative code first-come, first-served.
The first 3 x statements of an enum call the constructor -- That means the statement:
private final static Map<String,Status> displayMap = new HashMap( 4 );
Has NOT yet been executed, and displayMap is null. Also, a static { } block is executed in that same 1-2-3-... sequence and does not work either.
Unfortunately none of the Netbeans/Designer stack-trace or IDE log reported a NullPointerException -- The unit test does. Once you have a NPE, it focuses the mind. displayMap is uninitialised when the first constructor call is made.
Solution: The displayMap cannot be static final, because you may not initialise static members in a constructor. It must be initialised on the first call, using some variation of the example shown:
Status
public enum Status
{
INVALID ( "INVALID" ),
ISSUED ( "Issued" ),
CANCELLED ( "Cancelled");
private static Map<String,Status> displayMap;
private Status( String display ){
this.displayName = display;
mapDisplayName( this.displayName, this );
}
:
private static void mapDisplayName( final String displayName, final Status state ){
if( null == displayMap ){
displayMap = new HashMap( 7 );
}
displayMap.put( displayName, state );
}
}
And then it all runs quite smoothly.
Caveat:
Do NOT assign null in the displayMap declaration -- That was counter productive:
- The
if( null == displayMap ){...}block successfully assigns the HashMap during the first call to the Enum constructor. - After all the enum values declarations are processed.
- Init will call any initialises for declared variables.
- If
displayMap = null;is declared it replaces the populated HashMap, with 3 x values, with a new empty HashMap. grrr
Possibly related question:
- Reverse lookup for Java Enums with more than one value per key/constant? ... I felt that a HashMap would deliver the reverse-lookup of multiple strings a lot more easily.
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