java.lang.IllegalStateException: Duplicate key 95.5 at java.util.stream.Collectors.lambda$throwingM

Java8 Collectors.toMap的坑

Exception in thread "main" java.lang.IllegalStateException: Duplicate key 95.5
	at java.util.stream.Collectors.lambda$throwingMerger$0(Collectors.java:133)
	at java.util.HashMap.merge(HashMap.java:1254)
	at java.util.stream.Collectors.lambda$toMap$58(Collectors.java:1320)
	at java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169)
	at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1382)
	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 Main.main(Main.java:34)
           

public static void main(String[] args) {
        Student student = new Student();
        student.setId("1");
        student.setSocre(BigDecimal.valueOf(95.5));
        student.setAge(23);

        Student student2 = new Student();
        student2.setId("1");
        student2.setSocre(BigDecimal.valueOf(98.5));
        student2.setAge(32);

        Student student3 = new Student();
        student3.setId("1");
        student3.setSocre(BigDecimal.valueOf(92.5));
        student3.setAge(43);

        List<Student> list = new ArrayList<>();
        list.add(student);
        list.add(student2);
        list.add(student3);

        Map<String,BigDecimal> result = list.stream().collect(Collectors.toMap(Student::getId, Student::getSocre));
        System.out.println(result);
    }
           

按照正常思維，往一個map裡put一個已經存在的key，會把原有的key對應的value值覆寫，然而通過一次線上問題，發現Java8中的Collectors.toMap反其道而行之，它預設給抛異常，抛異常…

解決辦法：

如果不想抛異常的話，自己傳進去一個方法即可，上述代碼可以改成：

（1）後面覆寫前面的：

Map<String,BigDecimal> result = list.stream().collect(Collectors.toMap(Student::getId, Student::getSocre,(oldValue, newValue) -> newValue));
//結果：{1=92.5}
           

（2）前面的保留，後面的丢棄：

Map<String,BigDecimal> result = list.stream().collect(Collectors.toMap(Student::getId, Student::getSocre,(oldValue, newValue) -> oldValue));
//結果：{1=95.5}