与Collection和Generics相关的Java 10迁移问题

Question

继续我之前在链接中的查询:Java 10上的Swing问题,我发现了一些问题(仅突出显示错误),这次我发现在转移到Java 10之后问题主要出现在Collection API中.

以下是错误.想知道Java 10从Java 8迁移时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正在扩展javax.swing.tree.DefaultMutableTreeNode.

Answer 1

问题

如果仔细比较DefaultMutableTreeNodeJava 8和10之间的代码,您将看到返回类型被广泛化:

// 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() { /*...*/ }

您的代码在Java 8上编译,因为出于兼容性原因,可能会将泛型添加到原始类型.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,因此这些是可以预期的.

它不再编译Java 10,因为原始API想要返回a Enumeration<TreeNode>,但是你的覆盖不会这样做.相反,它返回Enumeration一个子类型.现在,Enumeration因为它实际上不是一个问题,因为它是只读的(返回更具体的类型当然总是好的),但编译器不知道(一个方法可以接受更具体的类型 - 不会好的).

为了更详细地理解最后一个括号,假设它是a List<TreeNode>和a List<SnapshotLoadNode>.然后,您的API将用作a DefaultMutableTreeNode,返回一个调用者可以添加TreeNode-s 的列表.但是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.没有原始类型,缺乏类型安全性是不可接受的,因此编译器抱怨.

解决方案

如果DefaultMutableTreeNode是我的代码我将breadthFirstEnumeration改为返回Enumeration<? extends TreeNode>(我不知道,为什么不是这样).这将使您的代码编译.

由于这不是一个选项,看起来你必须这样做:

• 松散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() { /*...*/ }