Java 映射集合元素并保留对源集合的引用
我正在寻找一种方法来创建一个集合、列表、集合或映射,其中包含原始集合的转换元素,并反映该集合中的每个修改 例如,如果我有一个来自第三方API的Java 映射集合元素并保留对源集合的引用,java,collections,Java,Collections,我正在寻找一种方法来创建一个集合、列表、集合或映射,其中包含原始集合的转换元素,并反映该集合中的每个修改 例如,如果我有一个来自第三方API的列表,而另一个API需要一个列表。我知道我可以这样变换列表: List<Integer> intList = thirdPartyBean.getIntListProperty(); List<String> stringList = intList.stream().map(Integer::toString) .coll
列表
,而另一个API需要一个列表
。我知道我可以这样变换列表:
List<Integer> intList = thirdPartyBean.getIntListProperty();
List<String> stringList = intList.stream().map(Integer::toString)
.collect(Collectors.toList());
secondBean.setStringListProperty(stringList);
List<String> stringList = new MappedList<>(intList, Integer::toString, Integer::valueOf);
因此,我正在搜索类似于List.sublist(from,to)
的内容,其中结果由原始列表“支持”
我正在考虑实现我自己的列表包装器,其使用方式如下:
List<Integer> intList = thirdPartyBean.getIntListProperty();
List<String> stringList = intList.stream().map(Integer::toString)
.collect(Collectors.toList());
secondBean.setStringListProperty(stringList);
List<String> stringList = new MappedList<>(intList, Integer::toString, Integer::valueOf);
List-stringList=new-MappedList(intList,Integer::toString,Integer::valueOf);
第二个lambda用于反转转换,以支持像stringList.add(String)
这样的调用
但是在我自己实现它之前,我想知道我是否尝试重新发明轮子-也许这个问题已经有了一个通用的解决方案?考虑到您的示例,
列表,您需要在第一个列表的顶部创建一个包装器。
现在,如果希望List
反映对List
所做的所有运行时更改,您有两个解决方案
不要创建初始的列表
,使用一个方法或包装器,该方法或包装器将始终从列表
返回转换后的值,这样您就永远不会有一个静态的列表
在列表
周围创建一个包装器,该包装器应具有对列表
的引用,并重写add()、addAll()、remove()和removeAll()方法。在覆盖的方法中,更改列表的状态
这正是政府所要解决的问题
您可以围绕List
和List
创建两个包装器,并让第一个包装器观察另一个包装器的状态 我会将列表包装在另一个带有变压器的列表中
public class MappedList<S, T> extends AbstractList<T> {
private final List<S> source;
private final Function<S, T> fromTransformer;
private final Function<T, S> toTransformer;
public MappedList(List<S> source, Function<S, T> fromTransformer, Function<T, S> toTransformer) {
this.source = source;
this.fromTransformer = fromTransformer;
this.toTransformer = toTransformer;
}
public T get(int index) {
return fromTransformer.apply(source.get(index));
}
public T set(int index, T element) {
return fromTransformer.apply(source.set(index, toTransformer.apply(element)));
}
public int size() {
return source.size();
}
public void add(int index, T element) {
source.add(index, toTransformer.apply(element));
}
public T remove(int index) {
return fromTransformer.apply(source.remove(index));
}
}
private void test() {
List<Integer> intList = new ArrayList<>(Arrays.asList(1, 2, 3));
List<String> stringList = new MappedList<>(intList, String::valueOf, Integer::valueOf);
intList.add(4);
stringList.remove(0);
System.out.println(intList); // Prints [2, 3, 4]
System.out.println(stringList); // Prints [2, 3, 4]
}
公共类MappedList扩展了AbstractList{
私人最终名单来源;
变压器的专用最终功能;
变压器专用最终功能;
公共映射列表(列表源、函数from transformer、函数to transformer){
this.source=源;
this.fromTransformer=fromTransformer;
此。toTransformer=toTransformer;
}
公共T获取(整数索引){
从transformer.apply(source.get(index))返回;
}
公共T集(整数索引,T元素){
从transformer.apply(source.set(索引,toTransformer.apply(元素))返回);
}
公共整数大小(){
返回source.size();
}
公共空添加(整数索引,T元素){
源.add(索引,toTransformer.apply(元素));
}
公共T删除(整型索引){
从transformer.apply(source.remove(index))返回;
}
}
专用无效测试(){
List intList=newarraylist(Arrays.asList(1,2,3));
List stringList=new MappedList(intList,String::valueOf,Integer::valueOf);
增加(4);
stringList.remove(0);
System.out.println(intList);//Prints[2,3,4]
System.out.println(stringList);//Prints[2,3,4]
}
请注意,如果source
可能包含null
,则来自Transformer的需要null
检查输入值
现在您没有将原始列表转换为另一个列表并与原始列表失去联系,您正在向原始列表添加转换。另一个选项是使用JavaFXObservableList
类,该类可以用一个可观察层包装现有列表,您可以在该层上定义要传播的操作
下面是一个从字符串列表传播到整数列表的示例:
List<String> strList = new ArrayList<>();
List<Integer> intList = new ArrayList<>();
ObservableList<String> strings = FXCollections.observableList(strList);
strings.addListener((ListChangeListener<String>) change -> {
if(change.next()) {
if (change.wasAdded()) {
change.getAddedSubList().stream().map(Integer::valueOf).forEach(intList::add);
} else if (change.wasRemoved()) {
change.getRemoved().stream().map(Integer::valueOf).forEach(intList::remove);
}
}
});
strList = strings;
strList.add("1");
strList.add("2");
strList.add("2");
System.out.println(intList);
strList.remove("1");
System.out.println(intList);
我不知道您使用的是哪个版本的JDK,但是如果您对使用JavaFX库没问题,您可以使用observateList
。您不需要修改现有列表,因为ObservableList
是java.util.list
的包装器。这有一个例子
import java.util.concurrent.atomic.AtomicBoolean;
import java.util.function.Function;
import javafx.collections.FXCollections;
import javafx.collections.ObservableList;
import javafx.collections.ListChangeListener.Change;
public class ObservableBiList{
//prevent stackoverflow
private static final AtomicBoolean wasChanged = new AtomicBoolean( false);
public static <T, R> void change( Change< ? extends T> c, ObservableList< R> list, Function< T, R> convert) {
if( wasChanged.get()){
wasChanged.set( false);
return;
}
wasChanged.set( true);
while( c.next()){
if( c.wasAdded() && !c.wasReplaced()){
for( T str : c.getRemoved())
list.add( convert.apply( str));
}else if( c.wasReplaced()){
for( int i=c.getFrom();i<c.getTo();i++)
list.set( i,convert.apply( c.getList().get( i)));
}else if( c.wasRemoved()){
for( T str : c.getRemoved())
list.remove( convert.apply( str));
}
}
System.out.printf( "Added: %s, Replaced: %s, Removed: %s, Updated: %s, Permutated: %s%n",
c.wasAdded(), c.wasReplaced(), c.wasRemoved(), c.wasUpdated(), c.wasPermutated());
}
public static void main( String[] args){
ObservableList< Integer> intList = FXCollections.observableArrayList();
intList.addAll( 1, 2, 3, 4, 5, 6, 7);
ObservableList< String> stringList = FXCollections.observableArrayList();
stringList.addAll( "1", "2", "3", "4", "5", "6", "7");
intList.addListener( ( Change< ? extends Integer> c) -> change( c, stringList, num->Integer.toString( num)));
stringList.addListener( ( Change< ? extends String> c) -> change( c, intList, str->Integer.valueOf( str)));
intList.set( 1, 22);
stringList.set( 3, "33");
System.out.println( intList);
System.out.println( stringList);
}
}
import java.util.concurrent.AtomicBoolean;
导入java.util.function.function;
导入javafx.collections.FXCollections;
导入javafx.collections.ObservableList;
导入javafx.collections.ListChangeListener.Change;
公共类可观察列表{
//防止堆栈溢出
私有静态最终AtomicBoolean wasChanged=新的AtomicBoolean(false);
公共静态无效更改(更改<?扩展T>c,可观察列表列表,函数转换){
如果(wasChanged.get()){
wasChanged.set(false);
返回;
}
wasChanged.set(true);
while(c.next()){
如果(c.wasAdded()&&!c.wasareplacement()){
对于(T str:c.getRemoved())
list.add(convert.apply(str));
}否则,如果(c.已替换()){
for(int i=c.getFrom();i intList=FXCollections.observearraylist();
addAll(1,2,3,4,5,6,7);
ObservableListstringList=FXCollections.observableArrayList();
添加所有(“1”、“2”、“3”、“4”、“5”、“6”、“7”);
addListener((Change<?extends Integer>c)->Change(c,stringList,num->Integer.toString(num));
addListener((Change<?extends String>c)->Change(c,intList,str->Integer.valueOf(str));
intList.set(1,22);
stringList.set(3,“33”);
System.out.println(intList);
System.out.println(stringList);
}
}
公共静态void main(字符串…参数){
List intList=observeList.createBase(新的ArrayList(Arrays.asList(1,2,3,4,5));
List stringList=ObservableList.createBase(intList,String::valueOf);
stringList.remove(0);
增加(6);
System.out.println(String.join(“,stringList));
System.out.println(intList.stream().map(String::valueOf).collect(collector.joining(“”));
}
@抑制警告({“un
public static void main(String... args) {
List<Integer> intList = ObservableList.createBase(new ArrayList<>(Arrays.asList(1, 2, 3, 4, 5)));
List<String> stringList = ObservableList.createBase(intList, String::valueOf);
stringList.remove(0);
intList.add(6);
System.out.println(String.join(" ", stringList));
System.out.println(intList.stream().map(String::valueOf).collect(Collectors.joining(" ")));
}
@SuppressWarnings({ "unchecked", "rawtypes" })
private static final class ObservableList<T, E> extends AbstractList<E> {
// original list; only this one could be used to add value
private final List<T> base;
// current snapshot; could be used to remove value;
private final List<E> snapshot;
private final Map<Function<T, ?>, List> cache;
public static <T, E> List<E> createBase(List<T> base) {
Objects.requireNonNull(base);
if (base instanceof ObservableList)
throw new IllegalArgumentException();
return new ObservableList<>(base, null, new HashMap<>());
}
public static <T, R> List<R> createBase(List<T> obsrv, Function<T, R> func) {
Objects.requireNonNull(obsrv);
Objects.requireNonNull(func);
if (!(obsrv instanceof ObservableList))
throw new IllegalArgumentException();
return new ObservableList<>(((ObservableList<T, R>)obsrv).base, func, ((ObservableList<T, R>)obsrv).cache);
}
@SuppressWarnings("AssignmentOrReturnOfFieldWithMutableType")
private ObservableList(List<T> base, Function<T, E> func, Map<Function<T, ?>, List> cache) {
this.base = base;
snapshot = func != null ? base.stream().map(func).collect(Collectors.toList()) : (List<E>)base;
this.cache = cache;
cache.put(func, snapshot);
}
@Override
public E get(int index) {
return snapshot.get(index);
}
@Override
public int size() {
return base.size();
}
@Override
public void add(int index, E element) {
if (base != snapshot)
super.add(index, element);
base.add(index, (T)element);
cache.forEach((func, list) -> {
if (func != null)
list.add(index, func.apply((T)element));
});
}
@Override
public E remove(int index) {
E old = snapshot.remove(index);
for (List<?> back : cache.values())
if (back != snapshot)
back.remove(index);
return old;
}
}
System.out.println(String.join(" ", stringList));
System.out.println(intList.stream().map(String::valueOf).collect(Collectors.joining(" ")));
}
private static final class ObservableList<E> extends AbstractList<E> {
private final List<List<?>> cache;
private final List<E> base;
public static <E> List<E> create(List<E> delegate) {
if (delegate instanceof ObservableList)
return new ObservableList<>(((ObservableList<E>)delegate).base, ((ObservableList<E>)delegate).cache);
return new ObservableList<>(delegate, new ArrayList<>());
}
public static <T, R> List<R> create(List<T> delegate, Function<T, R> func) {
List<R> base = delegate.stream().map(func).collect(Collectors.toList());
List<List<?>> cache = delegate instanceof ObservableList ? ((ObservableList<T>)delegate).cache : new ArrayList<>();
return new ObservableList<>(base, cache);
}
@SuppressWarnings("AssignmentOrReturnOfFieldWithMutableType")
private ObservableList(List<E> base, List<List<?>> cache) {
this.base = base;
this.cache = cache;
cache.add(base);
}
@Override
public E get(int index) {
return base.get(index);
}
@Override
public int size() {
return base.size();
}
@Override
public void add(int index, E element) {
for (List<?> back : cache)
back.add(index, element);
}
@Override
public E remove(int index) {
E old = base.remove(index);
for (List<?> back : cache)
if (back != base)
back.remove(index);
return old;
}
}
List list = new ArrayList<Object>();
list.stream().map(String::valueOf).<do_something>.collect(toList())
public static void testInteger(List<Integer> list) {
list.add(3);
list.remove(0);
}
public static void testString(List<String> list) {
list.add("4");
list.remove(0);
}
public static void main(String...args) {
List list = new ArrayList<Object>(Arrays.asList("1", "2"));
testInteger(list);
System.out.println(list.toString()); // will print: [2, 3]
testString(list);
System.out.println(list.toString()); // will print: [3, 4]
}
public static void main(String...args) {
List list = new ArrayList<Object>(Arrays.asList("1", "2"));
testInteger(list);
System.out.println(list.toString()); // will print: [2, 3]
testString(list);
System.out.println(list.toString()); // will print: [3, 4]
accessData(list); //Will crash
}
public static void accessData(List<Integer> list) {
Integer i = list.get(0); //Will work just fine
i = list.get(1); //Will result in an Class Cast Exception even tho the Method might define it as List<Integer>
}
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.Random;
import java.util.stream.Collectors;
public class Main {
static List<Integer> intListProperty;
static List<String> stringList;
public static void main(String... args) throws InterruptedException {
Main m = new Main();
m.execute();
}
private void updateAlways(Main main) {
class OneShotTask implements Runnable {
Main main;
OneShotTask(Main main) {
this.main = main;
}
public void run() {
while (main.intListProperty == main.getIntListProperty()) {}
main.intListProperty = getIntListProperty();
main.stringList = main.intListProperty.stream().map(s -> String.valueOf(s)).collect(Collectors.toList());
main.updateAlways(main);
}
}
Thread t = new Thread(new OneShotTask(main));
t.start();
}
public void execute() throws InterruptedException {
System.out.println("Starting monitoring");
stringList = new ArrayList<>();
intListProperty = new ArrayList<>();
intListProperty.add(1);
intListProperty.add(2);
intListProperty.add(3);
updateAlways(this);
while(true) {
Thread.sleep(1000);
System.out.println("\nintListProperty: " + intListProperty.toString()); // will print: [1, 2, 3, 4]
System.out.println("stringList: " + stringList.toString()); // will print: [2, 3]
}
}
// simulated
//thirdPartyBean.getIntListProperty();
private List<Integer> getIntListProperty() {
long timeInMilis = System.currentTimeMillis();
if(timeInMilis % 5000 == 0 && new Random().nextBoolean()) {
Object[] objects = intListProperty.toArray();
// change memory position
intListProperty = new ArrayList<>();
intListProperty = new ArrayList(Arrays.asList(objects));
intListProperty.add(new Random().nextInt());
}
return intListProperty;
}
}