How to configure logging in a Java application with Log4j?

Spark cannot run in local mode, ClassNotFoundException: org.apache.spark.sql.SparkSession. This is the most common first step of getting stuck: even the dependencies are not correct. Only spark-core_2.12 is written in Maven, but spark-sql_2.12 is not added. SparkSession crashes as soon as it is built. The Scala version must strictly match the official Spark compiled version - Spark3.4.x uses Scala2.12 by default. If you use spark-sqljar of 2.13, the class loader cannot directly find the main class. Practical advice: Go to mvnre

This article introduces a concise and maintainable way to map the weekday string (such as "Monday") to the corresponding serial number (1-7), and use the modulo operation to realize the forward and backward offset of any number of days (such as Monday plus 4 days to get Friday), avoiding lengthy if chains and hard-coded logic.

Collections.nCopies returns an immutable view. Calling add/remove will throw UnsupportedOperationException; it needs to be wrapped with newArrayList() to modify it, and it is disabled for mutable objects.

Homebrew installs the latest stable version of openjdk (such as JDK22) by default, not the LTS version; you need to explicitly execute brewinstallopenjdk@17 or brewinstallopenjdk@21 to install the LTS version, and manually configure PATH and JAVA_HOME to be correctly recognized by the system and IDE.

What is SuppressedException: It is not "swallowed", but actively archived by the JVM. SuppressedException is not an exception loss, but the JVM quietly attaches the secondary exception to the main exception under the premise that "only one exception must be thrown" for you to verify afterwards. It is automatically triggered by the JVM in only two scenarios: one is that the resource closure in try-with-resources fails, and the other is that you manually call addSuppressed() in finally. The key difference is: the former is fully automatic and safe; the latter requires you to keep it to yourself, and it can be written as shadowing if you are not careful. try-

After a Java application is packaged as a JAR, data cannot be written directly to the resources in the JAR package (such as test.txt) because the JAR is essentially a read-only ZIP archive; the correct approach is to write variable data to an external path (such as a user directory, a temporary directory, or a configuration-specified path).

ArrayList.add() triggers expansion because grow() is called when size is equal to elementData.length. The first add allocates 10 capacity, and subsequent expansion is 1.5 times and not less than the minimum requirement, relying on delayed initialization and System.arraycopy optimization.

This article introduces typical blocking problems when using Scanner to read multiple integers in a single line. It points out that hasNextInt() will wait indefinitely when there is no subsequent input, and recommends a safe alternative with nextLine() string splitting as the core.