Shulou(Shulou.com)06/02 Report--

This article introduces what is the problem of Collectors.toMap. The content is very detailed. Interested friends can refer to it for reference. I hope it can help everyone.

overview

  Although JDK9.0 has come out, but our system recently began to fully introduce JDK1.8, JDK1.8 has also come out for a long time, all aspects are quite stable. A recent problem occurred when using the Collectors.toMap method of lambda expressions.

The approximate source code is as follows:

public class Test { public static void main(String[] args) { // initMemberList is the method to get data List list = Test.initMemberList(); Map memberMap = list.stream().collect(Collectors.toMap(Member::getId, Member::getImgPath)); System.out.println(memberMap); }}class Member { private String id; private String imgPath; // get set omitted}

Run the program and prompt directly:

Exception in thread "main" java.lang.NullPointerException at java.util.HashMap.merge(HashMap.java:1224) at java.util.stream.Collectors.lambda$toMap$58(Collectors.java:1320) at java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169) at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1374) at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481) at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471) at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708) at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234) at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:499) at com.jdk.test.Test.main(Test.java:13)

  Thinking about it, I didn't understand why it would be a null pointer at first, because list can't be null, but later I searched the Internet with java.util.HashMap.merge plus NullPointerException exception. It turns out that when an object is converted to a Map, the value converted to a Map is null. It is roughly as follows:

public static List initMemberList() { Member member1 = new Member(); member1.setId("id_1"); member1.setImgPath("http://www.baidu.com"); //Here is a null result Member member2 = new Member(); member2.setId("id_2"); member2.setImgPath(null); List list = new ArrayList(); list.add(member1); list.add(member2); return list; }

A rough look at the source code, the original Collectors.toMap bottom layer is based on the Map.merge method to achieve, and merge value is not null, if null, will throw a null pointer exception, the original problem is like this.

Collectors.toMap() internally uses Map.merge() to add mappings to the map. Map.merge() is spec'd not to allow null values, regardless of whether the underlying Map supports null values. This could probably use some clarification in the Collectors.toMap() specifications.

Look at it, there is this in the openJDK bug list: JDK-8148463, I don't know if this is a bug. Collectors.toMap fails on null values

The problem is a problem, we still need to solve it in other ways.

Solution 1

The original for loop way, or forEach way:

Map memberMap = new HashMap();list.forEach((answer) -> memberMap.put(answer.getId(), answer.getImgPath()));System.out.println(memberMap);Map memberMap = new HashMap();for (Member member : list) { memberMap.put(member.getId(), member.getImgPath());}

Solution 2

Overloaded methods of collect using stream:

Map memberMap = list.stream().collect(HashMap::new, (m,v)-> m.put(v.getId(), v.getImgPath()),HashMap::putAll);System.out.println(memberMap);

Solution 3

Inherit Collector, manually implement the toMap method, and then call our own encapsulated toMap method. For implementation of Collector, please refer to: Problems and solutions of toMap method in Collectors in JDK8 Stream API

  In fact, whether this is a bug or not, in the end, it is still the problem caused by too little use of lambda expressions in JDK 1.8 and too little understanding. Therefore, it is still necessary to use more new technologies and step on more pits.

stackoverflow Address: # Java 8 NullPointerException in Collectors.toMap

PostScript

Another problem was found using Collector.toMap. The key in Map cannot be repeated. If it is repeated, an exception will be thrown:

public static List initMemberList() { Member member1 = new Member(); member1.setId("id_1"); member1.setImgPath("http://www.google.com"); Member member2 = new Member(); member2.setId("id_1"); member2.setImgPath("http://www.baidu.com"); List list = new ArrayList(); list.add(member1); list.add(member2); return list;}

The above code, when running, prompt error:

Exception in thread "main" java.lang.IllegalStateException: Duplicate key http://www.google.com at java.util.stream.Collectors.lambda$throwingMerger$0(Collectors.java:133) at java.util.HashMap.merge(HashMap.java:1253) at java.util.stream.Collectors.lambda$toMap$58(Collectors.java:1320) at java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169) at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1374) at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481) at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471) at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708) at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234) at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:499) at com.jdk.test.Test.main(Test.java:13)

By looking at the Collectors.toMap code and comments we can see:

public static Collector toMap(Function

Welcome to subscribe "Shulou Technology Information " to get latest news, interesting things and hot topics in the IT industry, and controls the hottest and latest Internet news, technology news and IT industry trends.