What is a NullPointerException, and how do I fix it?

Use the -cp parameter to add the JAR to the classpath, so that the JVM can load its internal classes and resources, such as java-cplibrary.jarcom.example.Main, which supports multiple JARs separated by semicolons or colons, and can also be configured through CLASSPATH environment variables or MANIFEST.MF.

UseFile.createNewFile()tocreateafileonlyifitdoesn’texist,avoidingoverwriting;2.PreferFiles.createFile()fromNIO.2formodern,safefilecreationthatfailsifthefileexists;3.UseFileWriterorPrintWriterwhencreatingandimmediatelywritingcontent,withFileWriterover

Use the implements keyword to implement the interface. The class needs to provide specific implementations of all methods in the interface. It supports multiple interfaces and is separated by commas to ensure that the methods are public. The default and static methods after Java 8 do not need to be rewrite.

JavaSPI is a built-in service discovery mechanism in JDK, and implements interface-oriented dynamic expansion through ServiceLoader. 1. Define the service interface and create a file with the full name of the interface under META-INF/services/, and write the fully qualified name of the implementation class; 2. Use ServiceLoader.load() to load the implementation class, and the JVM will automatically read the configuration and instantiate it; 3. The interface contract should be clarified during design, support priority and conditional loading, and provide default implementation; 4. Application scenarios include multi-payment channel access and plug-in verification; 5. Pay attention to performance, classpath, exception isolation, thread safety and version compatibility; 6. In Java9, provide can be used in combination with module systems.

Javagenericsprovidecompile-timetypesafetyandeliminatecastingbyallowingtypeparametersonclasses,interfaces,andmethods;wildcards(?,?extendsType,?superType)handleunknowntypeswithflexibility.1.UseunboundedwildcardwhentypeisirrelevantandonlyreadingasObject

This article explores in-depth the mechanism of sending multiple HTTP requests on the same TCP Socket, namely, HTTP persistent connection (Keep-Alive). The article clarifies the difference between HTTP/1.x and HTTP/2 protocols, emphasizes the importance of server-side support for persistent connections, and how to correctly handle Connection: close response headers. By analyzing common errors and providing best practices, we aim to help developers build efficient and robust HTTP clients.

Use the Properties class to read Java configuration files easily. 1. Put config.properties into the resource directory, load it through getClassLoader().getResourceAsStream() and call the load() method to read the database configuration. 2. If the file is in an external path, use FileInputStream to load it. 3. Use getProperty(key,defaultValue) to handle missing keys and provide default values ​​to ensure exception handling and input verification.

This tutorial details how to efficiently process nested ArrayLists containing other ArrayLists in Java and merge all its internal elements into a single array. The article will provide two core solutions through the flatMap operation of the Java 8 Stream API: first flattening into a list and then filling the array, and directly creating a new array to meet the needs of different scenarios.