Java 8 Collection stream Collectors.toMap

问题

Why this code is not compile for me?

I try to convert List to Map using stream and toMap option

List<CountryToPaymentMethodsDisplayRules>
   paymentMethodDisplayRulesByCountryList = 
       gatway.getCountryPaymentMethodsDisplayRulesByCountry();

Map<PaymentMethod, CountryToPaymentMethodsDisplayRules>
   countryToPaymentMethodsDisplayRulesMap = paymentMethodDisplayRulesByCountryList
       .stream()
       .collect(Collectors.toMap(type -> type.getCountryToPaymentMethodsDisplayRules().getPaymentMethod(),
                                 type -> type));

public interface PaymentMethod extends Serializable {
}

public enum PaymentMethodType implements PaymentMethod, Serializable {
}

public interface CountryToPaymentMethodsDisplayRules {
    public PaymentMethod getPaymentMethod();
}

public class CountryToPaymentMethodsDisplayRulesEntity implements CountryToPaymentMethodsDisplayRules, PersistentEntity<Long>, Serializable {
    @Type(type = "com.plimus.core.payment.PaymentMethodTypeUserType")
    @Column(name = "PAYMENT_TYPE")
    private PaymentMethod paymentMethod;
}

What is wrong here?

回答1:

You just need to provide the Collections.toMap() method with a method reference and an identity. Try this:

 Map<PaymentMethod, CountryToPaymentMethodsDisplayRules>
    countryToPaymentMethodsDisplayRulesMap = paymentMethodDisplayRulesByCountryList
    .stream()
    .collect(Collectors.toMap(CountryToPaymentMethodsDisplayRules::getPaymentMethod,x->x);

回答2:

Found the problem thanks

Map<PaymentMethod, CountryToPaymentMethodsDisplayRules>
countryToPaymentMethodsDisplayRulesMap = paymentMethodDisplayRulesByCountryList
   .stream()
   .collect(Collectors.toMap(type -> type.getPaymentMethod(),
                             type -> type));

来源：https://stackoverflow.com/questions/42855807/java-8-collection-stream-collectors-tomap