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Get a List collection containing objects, and then select two properties. When performing List conversion to Map, the following error is reported: java.lang.IllegalStateException: Duplicate key Possible problems Originally, I wanted to solve this problem by adding formal parameters (entity1, entity2) - > entity1), but I still want to sol ...

1, Anonymous class Anonymous classes in Java are classes without names. They are just syntax sugar to avoid template code declaring and instantiating a one-time class. (1) It is an internal or local class without a name, and only one object is created for it. (2) An anonymous class is an expression, which means that the class needs to ...

         Today, I looked at the new features of Java 8 and saw that some of the sample code called these function interfaces. I was confused~ So the blogger checked the information to see what these function interfaces are useful for, so that it is easy to understand later. There is no more nonsense. Let ...

summary 1. The biggest highlight of JDK8 is the introduction of Optional class to solve NPE (NullPointerException)2. The Alibaba development manual also recommends the use of the Optional class to prevent NPE3. To sum up, solving NPE is the function of the Optional class and a highlight of JDK8 Optional Preview 1. Declaration of the Optional ...