JAVA8新特性stream流收集为Map，value为null导致空指针的问题

2023-09-24 20:01

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jdk8

新特性stream深受喜爱，平时使用比较多，其中有：

Map collect2 =
list.stream().collect(Collectors.toMap(Book::getName, Book::getIdNO,(pre, after) -> pre));

现象如下：

package MainTest.stream;import java.util.*;import java.util.function.Function;import java.util.stream.Collectors;public class StreamListToMap {    public static void main(String[] args) {        List list = new ArrayList<>();        list.add(new Book("master amber", 1987, "ASHJBYGSFDFG15"));        list.add(new Book("slaver amber", 1987, "IJFGNMKETRE58"));        list.add(new Book("master amber", 1999, "MYHGORPJFJ64"));        list.add(new Book("ssdas", 12414, null));        list.add(new Book("fkajh", 32423, null));        // 问题1：jdk8这种情况下，value为空会报错，jdk9解决了这个问题(key为null没有问题)        System.out.println("==============");        Map collect2 = list.stream().collect(Collectors.toMap(Book::getName, Book::getIdNo,(pre, after) -> pre));        for (String key : collect2.keySet()) {            System.out.println("key:"+key + "=======value:"+collect2.get(key));        }    }    public static class Book {        private String name;        private int year;        private String idNo;        Book(String name, int year, String idNo) {            this.name = name;            this.year = year;            this.idNo = idNo;        }        public String getName() {            return name;        }        public void setName(String name) {            this.name = name;        }        public int getYear() {            return year;        }        public void setYear(int year) {            this.year = year;        }        public String getIdNo() {            return idNo;        }        public void setIdNo(String idNo) {            this.idNo = idNo;        }    }}

问题：JDK8环境下，如果Book::getIdNo 为null的话会报空指针,即JDK8环境下的，该接口不支持value为null（key为null没这个问题）

R collect(Collector collector);

该接口不支持valueMapper为null，JDK9解决了这个问题

解决方案：

升级JDK9

按需求可以过滤掉null的value

使用另一个collect接口

 R collect(Supplier supplier,BiConsumer accumulator,              BiConsumer combiner);
Supplier supplier ：收集器返回的容器

BiConsumer accumulator ：消费者，R参数是返回结果类型，? super T 一般值list内元素

BiConsumer combiner：联合器，主要是并行流下使用（parallel），非并行流不使用改参数，但不能为null

package MainTest.stream;import java.util.*;import java.util.function.Function;import java.util.stream.Collectors;public class StreamListToMap {    public static void main(String[] args) {        List list = new ArrayList<>();        list.add(new Book("master amber", 1987, "ASHJBYGSFDFG15"));        list.add(new Book("slaver amber", 1987, "IJFGNMKETRE58"));        list.add(new Book("master amber", 1999, "MYHGORPJFJ64"));        list.add(new Book("ssdas", 12414, null));        list.add(new Book("fkajh", 32423, null));                // 问题1解决方案1        System.out.println("==============");        HashMap collect3 = list.stream().parallel().collect(HashMap::new, (n, v) -> n.put(v.getName(), v.getIdNo()), HashMap::putAll);        for (String key : collect3.keySet()) {            System.out.println("key:"+key + "=======value:"+collect3.get(key));        }        // 问题1解决方案2 过滤掉null的，看需求        System.out.println("==============");        HashMap collect4 = list.stream().filter(l-> Objects.nonNull(l) && Objects.nonNull(l.idNo)).collect(HashMap::new, (n, v) -> n.put(v.getName(), v.getIdNo()), HashMap::putAll);        for (String key : collect4.keySet()) {            System.out.println("key:"+key + "=======value:"+collect4.get(key));        }    }    public static class Book {        private String name;        private int year;        private String idNo;        Book(String name, int year, String idNo) {            this.name = name;            this.year = year;            this.idNo = idNo;        }        public String getName() {            return name;        }        public void setName(String name) {            this.name = name;        }        public int getYear() {            return year;        }        public void setYear(int year) {            this.year = year;        }        public String getIdNo() {            return idNo;        }        public void setIdNo(String idNo) {            this.idNo = idNo;        }    }}