继续我之前在链接中的查询：Java 10 上的 Swing 问题 https://stackoverflow.com/questions/51555030/swing-issue-on-java-10?noredirect=1#comment90144695_51555426，我发现了更多问题（仅突出显示错误），这次我发现问题主要出现在迁移到 java 10 后的 Collection API 中。

下面是错误。想知道从 Java 8 迁移时 Java 10 是否有任何重大变化（从 Collection/Generics 的角度来看）。

    [javac] C:\WorkSpace\JAVA10\oswm\rel20.10_Patches\WorkManager\src\com\osm\ui\tree\WMTreeNode.java:159: error: breadthFirstEnumeration() in WMTreeNode cannot override breadthFirstEnumeration() in DefaultMutableTreeNode
    [javac]     public Enumeration<?extends WMTreeNode> breadthFirstEnumeration() {
    [javac]                                             ^
    [javac]   return type Enumeration<? extends WMTreeNode> is not compatible with Enumeration<TreeNode>
    [javac] C:\WorkSpace\JAVA10\oswm\rel20.10_Patches\WorkManager\src\com\osm\ui\tree\WMTreeNode.java:158: error: method does not override or implement a method from a supertype
    [javac]     @Override
    [javac]     ^
    [javac] C:\WorkSpace\JAVA10\oswm\rel20.10_Patches\WorkManager\src\com\osm\ui\tree\WMTreeNode.java:164: error: depthFirstEnumeration() in WMTreeNode cannot override depthFirstEnumeration() in DefaultMutableTreeNode
    [javac]     public Enumeration<?extends WMTreeNode> depthFirstEnumeration() {
    [javac]                                             ^
    [javac]   return type Enumeration<? extends WMTreeNode> is not compatible with Enumeration<TreeNode>
    [javac] C:\WorkSpace\JAVA10\oswm\rel20.10_Patches\WorkManager\src\com\osm\ui\tree\WMTreeNode.java:163: error: method does not override or implement a method from a supertype
    [javac]     @Override
    [javac]     ^

    [javac]                           ^
    [javac] C:\WorkSpace\JAVA10\oswm\rel20.10_Patches\WorkManager\src\com\osm\ui\tree\WMTreeNode.java:298: error: no suitable method found for sort(Vector<TreeNode>)
    [javac]             Collections.sort(children);


[javac] C:\WorkSpace\JAVA10\oswm\rel20.10_Patches\WorkManager\src\com\osm\datamgmt\ui\load\LoadNode.java:90: error: breadthFirstEnumeration() in LoadNode cannot override breadthFirstEnumeration() in DefaultMutableTreeNode
    [javac]     public Enumeration<?extends LoadNode> breadthFirstEnumeration() {
    [javac]                                           ^
    [javac]   return type Enumeration<? extends LoadNode> is not compatible with Enumeration<TreeNode>
    [javac] C:\WorkSpace\JAVA10\oswm\rel20.10_Patches\WorkManager\src\com\osm\datamgmt\ui\load\LoadNode.java:95: error: depthFirstEnumeration() in LoadNode cannot override depthFirstEnumeration() in DefaultMutableTreeNode
    [javac]     public Enumeration<?extends LoadNode> depthFirstEnumeration() {


 [javac]   where T is a type-variable:
    [javac]     T extends Object declared in class Class
    [javac] C:\WorkSpace\JAVA10\oswm\rel20.10_Patches\WorkManager\src\com\osm\automation\plot\CopyCountJobParameterDescriptor.java:113: error: incompatible types: Integer cannot be converted to CAP#1
    [javac]             final boolean outOfRange = (model.getMinimum().compareTo(i) > 0) || (model.getMaximum().compareTo(i) < 0);
    [javac]                                                                      ^
    [javac]   where CAP#1 is a fresh type-variable:
    [javac]     CAP#1 extends Object from capture of ?
    [javac] C:\WorkSpace\JAVA10\oswm\rel20.10_Patches\WorkManager\src\com\osm\automation\plot\CopyCountJobParameterDescriptor.java:113: error: incompatible types: Integer cannot be converted to CAP#1
    [javac]             final boolean outOfRange = (model.getMinimum().compareTo(i) > 0) || (model.getMaximum().compareTo(i) < 0);


    [javac] C:\WorkSpace\JAVA10\oswm\rel20.10_Patches\WorkManager\src\com\osm\datamgmt\ui\load\SnapshotLoadNode.java:48: error: breadthFirstEnumeration() in SnapshotLoadNode cannot override breadthFirstEnumeration() in DefaultMutableTreeNode
    [javac]     public Enumeration<?extends SnapshotLoadNode> breadthFirstEnumeration() {
    [javac]                                                   ^
    [javac]   return type Enumeration<? extends SnapshotLoadNode> is not compatible with Enumeration<TreeNode>
    [javac] C:\WorkSpace\JAVA10\oswm\rel20.10_Patches\WorkManager\src\com\osm\datamgmt\ui\load\SnapshotLoadNode.java:53: error: depthFirstEnumeration() in SnapshotLoadNode cannot override depthFirstEnumeration() in DefaultMutableTreeNode
    [javac]     public Enumeration<?extends SnapshotLoadNode> depthFirstEnumeration() {


    [javac] C:\WorkSpace\JAVA10\oswm\rel20.10_Patches\WorkManager\src\com\osm\datamgmt\ui\favorites\FavoritesTreeNode.java:30: error: breadthFirstEnumeration() in FavoritesTreeNode cannot override breadthFirstEnumeration() in DefaultMutableTreeNode
    [javac]     public Enumeration<?extends FavoritesTreeNode> breadthFirstEnumeration() {
    [javac]                                                    ^
    [javac]   return type Enumeration<? extends FavoritesTreeNode> is not compatible with Enumeration<TreeNode>
    [javac] C:\WorkSpace\JAVA10\oswm\rel20.10_Patches\WorkManager\src\com\osm\datamgmt\ui\favorites\FavoritesTreeNode.java:35: error: depthFirstEnumeration() in FavoritesTreeNode cannot override depthFirstEnumeration() in DefaultMutableTreeNode
    [javac]     public Enumeration<?extends FavoritesTreeNode> depthFirstEnumeration() {


    [javac] C:\WorkSpace\JAVA10\oswm\rel20.10_Patches\WorkManager\src\com\osm\datamgmt\ui\save\SaveNode.java:113: error: breadthFirstEnumeration() in SaveNode cannot override breadthFirstEnumeration() in DefaultMutableTreeNode
    [javac]     public Enumeration<?extends SaveNode> breadthFirstEnumeration() {
    [javac]                                           ^
    [javac]   return type Enumeration<? extends SaveNode> is not compatible with Enumeration<TreeNode>
    [javac] C:\WorkSpace\JAVA10\oswm\rel20.10_Patches\WorkManager\src\com\osm\datamgmt\ui\save\SaveNode.java:118: error: depthFirstEnumeration() in SaveNode cannot override depthFirstEnumeration() in DefaultMutableTreeNode
    [javac]     public Enumeration<?extends SaveNode> depthFirstEnumeration() {
    [javac]                                           ^


   [javac] C:\WorkSpace\JAVA10\oswm\rel20.10_Patches\WorkManager\OSMWebServices\src\java\com\osm\webservices\legacy\util\MessageAttachmentHandler.java:76: error: cannot find symbol
    [javac]                         attachment.setRawContent(new BufferedInputStream(new FileInputStream(file)),
    [javac]              

                 ^

WMTreeNode正在延伸javax.swing.tree.DefaultMutableTreeNode.

问题

如果你仔细比较一下代码DefaultMutableTreeNode在 Java 8 和 10 之间，您将看到返回类型被泛化：

// Java 8 uses `Enumeration` as a "raw type"
public Enumeration breadthFirstEnumeration() { /*...*/ }

// Java 10 uses `Enumeration` as intended: with a type parameter
public Enumeration<TreeNode> breadthFirstEnumeration() { /*...*/ }

你的代码是这样的：

public Enumeration<?extends SnapshotLoadNode> breadthFirstEnumeration() { /*...*/ }

您的代码可以在 Java 8 上编译，因为将泛型添加到raw type https://docs.oracle.com/javase/tutorial/java/generics/rawTypes.html出于兼容性原因是可能的。虽然可能存在问题（实际上与下面相同），但原始类型无论如何都被认为是不安全的，因此这些都是可以预料到的。

它不再在 Java 10 上编译，因为原始 API 想要返回一个Enumeration<TreeNode>，但你的覆盖不会这样做。相反，它返回一个Enumeration的一个子类型。现在，对于一个Enumeration这实际上不是问题，因为它是只读的（当然返回更具体的类型总是可以的），但编译器不知道这一点（方法可以接受更具体的类型 - 这是不行的）。

要更详细地理解最后一个括号，假设它是List<TreeNode> and a List<SnapshotLoadNode>反而。然后你的API，用作DefaultMutableTreeNode，将返回调用者可以添加的列表TreeNode-s 到。但你的类仍然认为它有一个子类型列表，并且会得到类转换异常 - 不好。如果没有原始类型，这种类型安全性的缺乏是不可接受的，因此编译器会抱怨。

解决方案

If DefaultMutableTreeNode我会改变我的代码吗breadthFirstEnumeration回来Enumeration<? extends TreeNode>（我不知道为什么情况并非如此）。这将使你的代码编译。

因为这不是一个选项，所以看起来你必须这样做：

• 释放更精确的类型信息并将覆盖的返回类型更改为Enumeration<TreeNode>

• 如果您需要更精确的返回类型，请创建一个新方法：

  public Enumeration<?extends SnapshotLoadNode>
      breadthFirstEnumerationAsSnapshotLoadNode() { /*...*/ }