如何解决Java HashMap将值存储在不需要的键中
我正在开发一个程序,该程序可以告诉您有关包装的信息。但是,我在反向依赖方面遇到了一些问题(请参见注释:检查以前的反向依赖关系并将其保存)。主要问题是,在打印时,将值放在HashMap中之后,似乎正在以正确的方式存储它们。但是,在完成该过程后打印地图的键和值将返回其他值。代码后可以看到打印的信息。
public class MainActivity {
public static void main (String[] args) {
ArrayList<String> packages = new ArrayList<String>();
ArrayList<String> descriptions = new ArrayList<String>();
HashMap<String,ArrayList<String>> dependencies = new HashMap<>();
HashMap<String,ArrayList<String>> revDependencies = new HashMap<>();
int i = 0;
String packageStart = "Package:";
String descriptionStart = "Description:";
String dependenciesStart = "Depends:";
String packageName = "";
BufferedReader reader;
try {
reader = new BufferedReader(new FileReader(
"/var/lib/dpkg/status"));
String line = reader.readLine();
while (line != null) {
//System.out.println(line);
if (line.startsWith(packageStart)) {
packageName = line.substring(packageStart.length()).trim();
packages.add(packageName);
}
if (line.startsWith(descriptionStart)) {
descriptions.add(line.substring(descriptionStart.length()).trim());
}
if (line.startsWith(dependenciesStart)) {
String subline = line.substring(dependenciesStart.length());
String[] dependenciesWithVersion = subline.split(",");
ArrayList<String> dependenciesWOVersion = new ArrayList<String>();
ArrayList<String> currentRevDependencies = new ArrayList<String>();
String currentDependency;
for (String dependencyWithVersion : dependenciesWithVersion) {
int index = dependencyWithVersion.indexOf("(");
currentRevDependencies.clear();
//Take the version out of the package name
if (index != -1) {
currentDependency = dependencyWithVersion.substring(0,index);
} else {
currentDependency = dependencyWithVersion;
}
currentDependency.trim();
dependenciesWOVersion.add(currentDependency);
//Check for previous reverse dependencies and save them
if (revDependencies.get(currentDependency) != null){
currentRevDependencies = revDependencies.get(currentDependency);
}
currentRevDependencies.add(packageName);
revDependencies.put(currentDependency,currentRevDependencies);
System.out.print("Package: " + packageName + " Dependency: " + currentDependency + " RevDepencencies: " + revDependencies.get(currentDependency) + "\n");
}
dependencies.put(packageName,dependenciesWOVersion);
if (i == 2){
break;
}
i++;
}
line = reader.readLine();
}
} catch (Exception e) {
e.printStackTrace();
}
for (String key : revDependencies.keySet()){
System.out.print(key + " " + revDependencies.get(key) + "\n");
}
}
}
以下内容打印在屏幕上:
Package: accountsservice Dependency: dbus RevDepencencies: [accountsservice]
Package: accountsservice Dependency: libaccountsservice0 RevDepencencies: [accountsservice]
Package: accountsservice Dependency: libc6 RevDepencencies: [accountsservice]
Package: accountsservice Dependency: libglib2.0-0 RevDepencencies: [accountsservice]
Package: accountsservice Dependency: libpolkit-gobject-1-0 RevDepencencies: [accountsservice]
Package: acl Dependency: libacl1 RevDepencencies: [acl]
Package: acl Dependency: libc6 RevDepencencies: [accountsservice,acl]
Package: acpi-support Dependency: acpid RevDepencencies: [acpi-support]
acpid [acpi-support]
libaccountsservice0 [accountsservice,acl]
libpolkit-gobject-1-0 [accountsservice,acl]
libglib2.0-0 [accountsservice,acl]
libc6 [accountsservice,acl]
dbus [accountsservice,acl]
libacl1 []
似乎要存储的值不是过程完成时打印的值。
解决方法
问题
Java传递按值传递对象引用。将List
currentRevDependencies
放入revDependencies
Map
时,存储在Map
中的不是List
的副本,而是仅仅是一个对象引用。当currentRevDependencies
在外部更改时(就像您在其上调用clear
一样),地图内的列表也会更改。这意味着对于while循环的一次迭代中添加的所有键,List
引用将相同,从而导致您观察到重复的值。
如何修复
要解决此问题,请为您放入List
的每个键值对创建一个新的Map
,而不是像这样每次都清除最后一个Map
:
//[...]
String subline = line.substring(dependenciesStart.length());
String[] dependenciesWithVersion = subline.split(",");
ArrayList<String> dependenciesWOVersion = new ArrayList<String>();
String currentDependency;
for (String dependencyWithVersion : dependenciesWithVersion) {
ArrayList<String> currentRevDependencies = new ArrayList<String>(); // <- instantiate new list each iteration
//[...]
revDependencies.put(currentDependency,currentRevDependencies);
}
dependencies.put(packageName,dependenciesWOVersion);
//[...]
输出:
Package: libasan0 Dependency: gcc-4.8-base RevDepencencies: [libasan0]
Package: libasan0 Dependency: libc6 RevDepencencies: [libasan0]
Package: libasan0 Dependency: libgcc1 RevDepencencies: [libasan0]
Package: libasan0 Dependency: libstdc++6 RevDepencencies: [libasan0]
Package: libvorbisfile3 Dependency: libc6 RevDepencencies: [libvorbisfile3]
Package: libvorbisfile3 Dependency: libogg0 RevDepencencies: [libvorbisfile3]
Package: libvorbisfile3 Dependency: libvorbis0a RevDepencencies: [libvorbisfile3]
Package: libquadmath0 Dependency: gcc-4.9-base RevDepencencies: [libquadmath0]
Package: libquadmath0 Dependency: libc6 RevDepencencies: [libasan0,libquadmath0]
libc6 [libvorbisfile3]
libvorbis0a [libvorbisfile3]
gcc-4.9-base [libquadmath0]
libgcc1 [libasan0]
libc6 [libasan0,libquadmath0]
libstdc++6 [libasan0]
gcc-4.8-base [libasan0]
libogg0 [libvorbisfile3]
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