public static <T, K, U>
? ? Collector<T, ?, Map<K,U>> toMap(Function<? super T, ? extends K> keyMapper,
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? Function<? super T, ? extends U> valueMapper) {
? ? ? ? return new CollectorImpl<>(HashMap::new,
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?uniqKeysMapAccumulator(keyMapper, valueMapper),
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?uniqKeysMapMerger(),
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?CH_ID);
? ? }

package stream; 
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.UUID;
import java.util.function.Function;
import java.util.stream.Collectors;
 
/**
 * @author zzl
 * @Date 2022/1/15
 * @description Java stream特性：Collectors.toMap
 */
public class CollectorsToMapTest {
    public static void main(String[] args) {
        List<TestVo> testList = new ArrayList<>();
        // 初始化list
        for (int i = 0; i < 3; i++) {
            TestVo vo = new TestVo();
            vo.setUserId(i);
            vo.setName(UUID.randomUUID().toString().replaceAll("-", ""));
            testList.add(vo);
        }
        // toMap(p1,p2),p1參數是map的key值，p2參數是map的value值,當value為對象時，可以用Function.identity()表示value值
        Map<Integer, String> map = testList.stream().collect(Collectors.toMap(TestVo::getUserId, TestVo::getName));
 
        for (Map.Entry<Integer, String> entry : map.entrySet()) {
            System.out.println("key=" + entry.getKey() + ",value=" + entry.getValue());
        }
        System.out.println("==================================================");
 
        testList = new ArrayList<>();
        testList.add(new TestVo(1, "a"));
        testList.add(new TestVo(2, "b"));
        testList.add(new TestVo(1, "c"));
        // toMap(p1,p2,p3),其中p3參數是為了解決key值沖突時,決定value取值的
        Map<Integer, String> map2 = testList.stream()
                .collect(Collectors.toMap(TestVo::getUserId, TestVo::getName, (oldValue, newValue) -> oldValue));
 
        System.out.println("(oldValue, newValue) -> oldValue的方式：key值沖突時，value取值為舊的key對應的value值");
        map2.forEach((k, v) -> System.out.println("key=" + k + ",value=" + v));
 
        System.out.println("(oldValue, newValue) -> newValue的方式：key值沖突時，value取值為新的key對應的value值");
        map2 = testList.stream()
                .collect(Collectors.toMap(TestVo::getUserId, TestVo::getName, (oldValue, newValue) -> newValue));
        map2.forEach((k, v) -> System.out.println("key=" + k + ",value=" + v));
    }
}

package stream;  
/**
 * @author zzl
 * @Date 2022/1/15
 * @description
 */
public class TestVo {
    private Integer userId;
    private String name;
 
    public Integer getUserId() {
        return userId;
    }
 
    public void setUserId(Integer userId) {
        this.userId = userId;
    }
 
    public String getName() {
        return name;
    }
 
    public void setName(String name) {
        this.name = name;
    }
 
    public TestVo(Integer userId, String name) {
        this.userId = userId;
        this.name = name;
    }
 
    public TestVo() { 
    }
}