Java8 中使用Stream 让List 转 Map使用问题小结

List<SdsTest> sdsTests = new ArrayList<>();
    SdsTest sds1 = new SdsTest("aaa","aaa");
    SdsTest sds2 = new SdsTest("bbb",null);

    sdsTests.add(sds1);
    sdsTests.add(sds2);

    Map<String, String> map = sdsTests.stream().collect(Collectors.toMap(SdsTest::getName, SdsTest::getAge));    
    System.out.println(map.toString());

---------
运行错误：
Exception in thread "main" java.lang.NullPointerException
    at java.util.HashMap.merge(HashMap.java:1216)
    at java.util.stream.Collectors.lambda$toMap$150(Collectors.java:1320)
    .....

default V merge(K key, V value, BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
    Objects.requireNonNull(remappingFunction);
    // 在这里判断了value不可为null
    Objects.requireNonNull(value);
    V oldValue = get(key);
    V newValue = (oldValue == null) ? value : remappingFunction.apply(oldValue, value);
    ...

Map<String, String> map = sdsTests.stream().collect(Collectors.toMap(SdsTest::getName, sdsTest -> sdsTest.getAge() == null ? "0" : sdsTest.getAge()));

Map<String, String> nmap = sdsTests.stream().collect(HashMap::new,(k, v) -> k.put(v.getName(), v.getAge()), HashMap::putAll);
// TODO 下游业务从Map取值要做NPE判断

Map<String, Optional<String>> opmap = sdsTests.stream().collect(Collectors.toMap(SdsTest::getName, sdsTest -> Optional.ofNullable(sdsTest.getAge())));
System.out.println("bbb.age=" + opmap.get("bbb").orElse("0"));
------------
输出：
bbb.age=0

List<SdsTest> sdsTests = new ArrayList<>();
    SdsTest sds1 = new SdsTest("aaa","aaa");
    SdsTest sds2 = new SdsTest("aaa","ccc");

    sdsTests.add(sds1);
    sdsTests.add(sds2);

    Map<String, String> map = sdsTests.stream().collect(Collectors.toMap(SdsTest::getName, SdsTest::getAge));    
    System.out.println(map.toString());

---------
运行错误：
Exception in thread "main" java.lang.IllegalStateException: Duplicate key aaa
        at java.util.stream.Collectors.lambda$throwingMerger$92(Collectors.java:133)
        at java.util.stream.Collectors$$Lambda$6/1177096266.apply(Unknown Source)
        at java.util.HashMap.merge(HashMap.java:1245)
            .....

public static <T, K, U>
Collector<T, ?, Map<K,U>> toMap(Function<? super T, ? extends K> keyMapper,
                            Function<? super T, ? extends U> valueMapper) {
     // 注意这里的throwingMerger()
     return toMap(keyMapper, valueMapper, throwingMerger(), HashMap::new);
}

 private static <T> BinaryOperator<T> throwingMerger() {
      return (u,v) -> { throw new IllegalStateException(String.fORMat("Duplicate key %s", u)); };
 }
...

SdsTest sds1 = new SdsTest("aaa","aaa");
SdsTest sds2 = new SdsTest("bbb","bbb");
SdsTest sds3 = new SdsTest("aaa","ccc");

sdsTests.add(sds1);
sdsTests.add(sds2);
sdsTests.add(sds3);

// 写法一
Map<String, String> nmap = sdsTests.stream().collect(HashMap::new,(k, v) -> k.put(v.getName(), v.getAge()), HashMap::putAll);
System.out.println("nmap->:" + nmap.toString());

// 写法二
Map<String, String> nmap1 = sdsTests.stream().collect(Collectors.toMap(SdsTest::getName, SdsTest::getAge, (k1, k2) -> k2));
System.out.println("nmap1->:" + nmap1.toString());
...
----------------------
输出：
nmap->:{aaa=ccc, bbb=bbb}
nmap1->:{aaa=ccc, bbb=bbb}

...
Map<String, String> nmap1 = sdsTests.stream().collect(Collectors.toMap(SdsTest::getName, SdsTest::getAge, (k1, k2) -> k1 + "," + k2));
System.out.println("nmap1->:" + nmap1.toString());
...
----------------
输出：
nmap1->:{aaa=aaa,ccc, bbb=bbb}

......
Map<String, List<String>> map = sdsTests.stream().collect(Collectors.toMap(SdsTest::getName,
    s -> {
        List<String> ages = new ArrayList<>();
        ages.add(s.getAge());
        return ages;
    },
    (List<String> v1, List<String> v2) -> {
        v1.addAll(v2);
        return v1;
    }));
System.out.println("map->"+map.toString());
------------
输出：
map->{aaa=\[aaa, ccc\], bbb=\[bbb\]}