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如下代码#xff0c;虽然定义为非空 NonNull#xff0c;但依然会返回空对象#xff0c;导致调用侧被检测为空引用。
实际上不是Collections的问题是三目运算符返回了null对象。
import java.util.Collections;NonNullprivate ListString getInfo() {IccReco…问题
如下代码虽然定义为非空 NonNull但依然会返回空对象导致调用侧被检测为空引用。
实际上不是Collections的问题是三目运算符返回了null对象。
import java.util.Collections;NonNullprivate ListString getInfo() {IccRecords iccRecords mPhone.getIccRecords();if(iccRecords ! null) {//省略逻辑String[] simSpdi iccRecords.getServiceProviderDisplayInformation();return simSpdi ! null ? Arrays.asList(simSpdi) : null; //根因是这里返回null}return Collections.EMPTY_LIST;}
源码案例
frameworks/opt/telephony/src/java/com/android/internal/telephony/cdnr/CarrierDisplayNameResolver.java NonNullprivate ListString getEfSpdi() {for (int i 0; i mEf.size(); i) {if (mEf.valueAt(i).getServiceProviderDisplayInformation() ! null) {return mEf.valueAt(i).getServiceProviderDisplayInformation();}}return Collections.EMPTY_LIST;}NonNullprivate String getEfSpn() {for (int i 0; i mEf.size(); i) {if (!TextUtils.isEmpty(mEf.valueAt(i).getServiceProviderName())) {return mEf.valueAt(i).getServiceProviderName();}}return ;}NonNullprivate ListOperatorPlmnInfo getEfOpl() {for (int i 0; i mEf.size(); i) {if (mEf.valueAt(i).getOperatorPlmnList() ! null) {return mEf.valueAt(i).getOperatorPlmnList();}}return Collections.EMPTY_LIST;}NonNullprivate ListPlmnNetworkName getEfPnn() {for (int i 0; i mEf.size(); i) {if (mEf.valueAt(i).getPlmnNetworkNameList() ! null) {return mEf.valueAt(i).getPlmnNetworkNameList();}}return Collections.EMPTY_LIST;} 代码解析
注意Collection和Collections是不同的。
libcore/ojluni/src/main/java/java/util/Collections.java 工具类libcore/ojluni/src/main/java/java/util/Collection.java 接口
Collections中定义了内部类EmptyList在静态List常量EMPTY_LISTimmutable不可变列表初始化时会new一个没有指定类型的EmptyList。
public class Collections {/*** The empty list (immutable). This list is serializable.** see #emptyList()*/SuppressWarnings(rawtypes)public static final List EMPTY_LIST new EmptyList();/*** Returns an empty list (immutable). This list is serializable.** pThis example illustrates the type-safe way to obtain an empty list:* pre* Listlt;Stringgt; s Collections.emptyList();* /pre** implNote* Implementations of this method need not create a separate {code List}* object for each call. Using this method is likely to have comparable* cost to using the like-named field. (Unlike this method, the field does* not provide type safety.)** param T type of elements, if there were any, in the list* return an empty immutable list** see #EMPTY_LIST* since 1.5*/SuppressWarnings(unchecked)public static final T ListT emptyList() {return (ListT) EMPTY_LIST;}/*** serial include*/private static class EmptyListEextends AbstractListEimplements RandomAccess, Serializable {java.io.Serialprivate static final long serialVersionUID 8842843931221139166L;public IteratorE iterator() {return emptyIterator();}public ListIteratorE listIterator() {return emptyListIterator();}// Preserves singleton propertyjava.io.Serialprivate Object readResolve() {return EMPTY_LIST;}}} 解决方案
将 Collections.EMPTY_LIST 替换成 Collections.emptyList()。
虽然它们都可以用于表示一个空的不可变列表但 Collections.emptyList() 是更优先的选择因为它提供了类型安全性和更好的代码可读性。 附Collections 源码
一些值得学习的语法Android 扩展的排序跟Java 集合原生排序的实现差异
libcore/ojluni/src/main/java/java/util/Collections.java
import dalvik.system.VMRuntime;/*** This class consists exclusively of static methods that operate on or return* collections. It contains polymorphic algorithms that operate on* collections, wrappers, which return a new collection backed by a* specified collection, and a few other odds and ends.** pThe methods of this class all throw a {code NullPointerException}* if the collections or class objects provided to them are null.** pThe documentation for the polymorphic algorithms contained in this class* generally includes a brief description of the iimplementation/i. Such* descriptions should be regarded as iimplementation notes/i, rather than* parts of the ispecification/i. Implementors should feel free to* substitute other algorithms, so long as the specification itself is adhered* to. (For example, the algorithm used by {code sort} does not have to be* a mergesort, but it does have to be istable/i.)** pThe destructive algorithms contained in this class, that is, the* algorithms that modify the collection on which they operate, are specified* to throw {code UnsupportedOperationException} if the collection does not* support the appropriate mutation primitive(s), such as the {code set}* method. These algorithms may, but are not required to, throw this* exception if an invocation would have no effect on the collection. For* example, invoking the {code sort} method on an unmodifiable list that is* already sorted may or may not throw {code UnsupportedOperationException}.** pThis class is a member of the* a href{docRoot}/java.base/java/util/package-summary.html#CollectionsFramework* Java Collections Framework/a.** author Josh Bloch* author Neal Gafter* see Collection* see Set* see List* see Map* since 1.2*/public class Collections {// Suppresses default constructor, ensuring non-instantiability.private Collections() {}// Algorithms// Android-added: List.sort() vs. Collections.sort() app compat.// Added a warning in the documentation.// Collections.sort() calls List.sort() for apps targeting API version 26// (Android Oreo) but the other way around for app targeting 25 (Nougat)./*** Sorts the specified list into ascending order, according to the* {linkplain Comparable natural ordering} of its elements.* All elements in the list must implement the {link Comparable}* interface. Furthermore, all elements in the list must be* imutually comparable/i (that is, {code e1.compareTo(e2)}* must not throw a {code ClassCastException} for any elements* {code e1} and {code e2} in the list).** pThis sort is guaranteed to be istable/i: equal elements will* not be reordered as a result of the sort.** pThe specified list must be modifiable, but need not be resizable.** implNote* This implementation defers to the {link List#sort(Comparator)}* method using the specified list and a {code null} comparator.* Do not call this method from {code List.sort()} since that can lead* to infinite recursion. Apps targeting APIs {code 25} observe* backwards compatibility behavior where this method was implemented* on top of {link List#toArray()}, {link ListIterator#next()} and* {link ListIterator#set(Object)}.** param T the class of the objects in the list* param list the list to be sorted.* throws ClassCastException if the list contains elements that are not* imutually comparable/i (for example, strings and integers).* throws UnsupportedOperationException if the specified lists* list-iterator does not support the {code set} operation.* throws IllegalArgumentException (optional) if the implementation* detects that the natural ordering of the list elements is* found to violate the {link Comparable} contract* see List#sort(Comparator)*/public static T extends Comparable? super T void sort(ListT list) {// Android-changed: List.sort() vs. Collections.sort() app compat.// Call sort(list, null) here to be consistent with that methods// (changed on Android) behavior.// list.sort(null);sort(list, null);}}
