为什么在 Java 中使用 lambda 会破坏明确赋值？

Question

尝试使用 java-1.21.0-openjdk-amd64 编译以下代码会导致这些错误

.../TransactionProfile.java:[60,22] variable emvData might already have been assigned
.../TransactionProfile.java:[61,21] variable emvData might already have been assigned
.../TransactionProfile.java:[63,3] variable emvData might not have been initialized

public abstract class TransactionProfile {

  protected DeviceContext context;
  protected PeripheralResponse response;
  protected final PeripheralMessage emvData;
  protected final transient PeripheralTags peripheralTags = PeripheralTags.getInstance();

  TransactionProfile(DeviceContext context, PeripheralResponse response) {
    this.context = context;
    this.response = response;

    if (response == null) {
      emvData = new PeripheralMessage(new LinkedHashMap<>());
    } else {
      response.get(peripheralTags.emvDataTag)
          .ifPresentOrElse(
              emv -> emvData = new PeripheralMessage(emv.getUniqueMapOfChildren()),
              () -> emvData = new PeripheralMessage(new LinkedHashMap<>()));
    }
  }

当构造函数不含 lambda 时，它可以正常编译

  TransactionProfile(DeviceContext context, PeripheralResponse response) {
    this.context = context;
    this.response = response;

    Tlv emv = null;
    if (response != null) {
      emv = response.get(peripheralTags.emvDataTag).orElse(null);
    }
    if (emv == null) {
      emvData = new PeripheralMessage(new LinkedHashMap<>());
    } else {
      emvData = new PeripheralMessage(emv.getUniqueMapOfChildren());
    }
  }

emvData 如何在第二种形式而不是第一种形式中“明确分配”？据我所知，别无选择。 https://docs.oracle.com/javase/specs/jls/se15/html/jls-16.html

Answer 1

直观的答案是Java编译器不理解ifPresentOrElse方法的

语义

。它不知道这两个 lambda 中的一个或另一个将被执行。

技术答案是，JLS第 16 章中的明确赋值规则并没有说变量 >isprove< definitely assigned in your example. The fact that you could probably 会发生赋值（假设 ifPresentOrElse

 符合 javadoc），这不是重点。