如何使用Java流将地图映射到另一个地图？

Question

我正在用Stream学习Java，我有一张地图




Answer 1

你可以这样做：

Customer c1 = new Customer("İsmail", "Y.");
Customer c2 = new Customer("Elvis", "?.");
        
Map<String, List<Customer>> customerList = new HashMap<>();
customerList.put("U1", Arrays.asList(c1));
customerList.put("U2", Arrays.asList(c1, c2));
        
Map<String, List<CustomerObj>> customerObjList = customerList
        .entrySet().stream().collect(
                Collectors.toMap(Map.Entry::getKey,
                        entry -> entry.getValue().stream()
                                .map(CustomerObj::new)
                                // or .map(c -> new CustomerObj(c))
                                .collect(Collectors.toList())
                )
        );
    }

java prettyprint-override">private static class Customer {

    private String firstName;
    private String lastName;

    public Customer(String firstName, String lastName) {
        this.firstName = firstName;
        this.lastName = lastName;
    }

    public String getFirstName() {
        return firstName;
    }

    public String getLastName() {
        return lastName;
    }
}

private static class CustomerObj {

    private String fullName;

    public CustomerObj(Customer customer) {
        this.fullName = String.join(" ",
                customer.getFirstName(), customer.getLastName()
        );
    }

    public String getFullName() {
        return fullName;
    }
}

@双簧管的答案在Java16中工作，这个答案和你的模型相似，更容易理解。

Answer 2

请参考这个链接https://www.baeldung.com/java-merge-maps，它会帮助你。

Answer 3

您可以使用key mapperMap将map1的条目流式传输，并使用Collectors.toMap进行收集。条目::getKey。对于值映射器，您可以获取条目值列表，然后将stream从Client映射到CustomObj，最后收集到List。

下面是使用整数代替客户和String代替Customer Obj的示例：

Map<String, List<Integer>> map1 = Map.of(
        "A", List.of(1, 2), 
        "B", List.of(3, 4), 
        "C", List.of(5, 6));
    
Map<String, List<String>> map2 = map1.entrySet().stream().collect(
        Collectors.toMap(Map.Entry::getKey, 
                entry -> entry.getValue().stream()
                        .map(String::valueOf).toList()));