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Java Basics

This section contains the basics of the Java programming language, which can help in certain situations (such as reading log errors) when managing a Minecraft server.

important

Some parts are really simplified and serve to understand the very basics even for beginners who are not interested in programming. Therefore, some parts or breakdowns may be misleading or even factually wrong. So please take note of this fact, and if you are interested in this topic in more depth, or from a different perspective than Minecraft server administration itself, take this section with a lot of caution.

Types

Basic data types

  • String

The String data type is in simple terms a text, for example: This is a String, or A

  • Boolean

Boolean can have 2 values - true or false, which can be interpreted as “on/off”, “yes/no”, or “true/false” depending on the context.

  • Integer

Simply put, Integer is a whole number, for example 56.

  • Float
  • Double

Both Float and Double denote a decimal number, for example 3.6343642 (the difference between them is not important, but simply put, Double has more decimal places).

Other data types

Class

In a very simplified (and factually incorrect) way, you can think of a class as a file responsible for executing a particular piece of code with a particular purpose. This class is located in package, which you can think of as the folder in which the file (class) is.

Object

An object is everything in Java except the primitive data types (int, char, byte, short, long, float, double, boolean).

Stacktrace

As explained in this great post on StackOverflow (which explains stacktrace well, so I definitely recommend reading it), stacktrace is, in simple terms, a list of method calls that the application made when an Exception (error) occurred. From stacktrace, you can tell where, when, and why the Exception occurred.

For example, such a stacktrace might look like this:

[19:37:03] [Server thread/ERROR]: Could not load 'plugins/UltraStaffChat-5.2.2.jar' in folder 'plugins'
org.bukkit.plugin.InvalidDescriptionException: Invalid plugin.yml
	at org.bukkit.plugin.java.JavaPluginLoader.getPluginDescription(JavaPluginLoader.java:178) ~[paper-api-1.18.2-R0.1-SNAPSHOT.jar:?]
	at org.bukkit.plugin.SimplePluginManager.loadPlugins(SimplePluginManager.java:159) ~[paper-api-1.18.2-R0.1-SNAPSHOT.jar:?]
	at org.bukkit.craftbukkit.v1_18_R2.CraftServer.loadPlugins(CraftServer.java:418) ~[paper-1.18.2.jar:git-Paper-388]
	at net.minecraft.server.dedicated.DedicatedServer.initServer(DedicatedServer.java:288) ~[paper-1.18.2.jar:git-Paper-388]
	at net.minecraft.server.MinecraftServer.runServer(MinecraftServer.java:1164) ~[paper-1.18.2.jar:git-Paper-388]
	at net.minecraft.server.MinecraftServer.lambda$spin$0(MinecraftServer.java:316) ~[paper-1.18.2.jar:git-Paper-388]
	at java.lang.Thread.run(Unknown Source) ~[?:?]
Caused by: java.io.FileNotFoundException: jar does not contain plugin.yml
	... 7 more

How to read a stacktrace can be found at this link: https://docs.papermc.io/paper/dev/reading-stacktraces

The fact that ‘java’ appears in stacktrace does not mean there is a problem with Java as some people think, it just lists the methods called from the Java library.

Some examples of reading stacktrace can be found here.

Exception

According to the official Java documentation, the definition of Exception is following: “An exception is an event, which occurs during the execution of a program, that disrupts the normal flow of the program’s instructions”.

This may sound complicated, but it simply means that an Exception means that an error has occurred.

You may encounter exceptions in stacktraces, such as this:

[00:00:00] [Server thread/ERROR]: Encountered an unexpected exception
org.spongepowered.configurate.serialize.SerializationException: [entities, spawning, monster-spawn-max-light-level] of type java.lang.Integer: java.lang.NumberFormatException: for input string: "default"
	at org.spongepowered.configurate.serialize.NumericSerializers.parseNumber(NumericSerializers.java:346) ~[configurate-core-4.1.2.jar:?]
	at org.spongepowered.configurate.serialize.NumericSerializers.lambda$static$9(NumericSerializers.java:254) ~[configurate-core-4.1.2.jar:?]
	at org.spongepowered.configurate.serialize.FunctionScalarSerializer.deserialize(FunctionScalarSerializer.java:40) ~[configurate-core-4.1.2.jar:?]
	...

Specifically, here we are dealing with SerializationException and NumberFormatException, implying that an error occurred during config serialization (loading) due to the inability to convert the default value (of the key entities.spawning.monster-spawn-max-light-level) to a number (the default value cannot be converted to a number). This error is broken down in more detail here.

Package

Packages in Java is used to organize code. In simple terms, a package can be thought of as a folder containing source code files (classes).

Packages appear in stacktraces, and can be used to identify exactly where the error occurred. For example in the following stacktrace the com.comphenix.protocol, xyz.kyngs.librelogin.paper and org.bukkit.plugin.java package appears.

[16:35:48 ERROR]: Error occurred while enabling LibreLogin v0.18.1 (Is it up to date?)
java.lang.NoClassDefFoundError: com/comphenix/protocol/ProtocolLibrary
        at xyz.kyngs.librelogin.paper.PaperLibreLogin.disable(PaperLibreLogin.java:109) ~[LibreLogin.jar:?]
        at xyz.kyngs.librelogin.paper.PaperBootstrap.onDisable(PaperBootstrap.java:87) ~[LibreLogin.jar:?]
        at org.bukkit.plugin.java.JavaPlugin.setEnabled(JavaPlugin.java:283) ~[paper-api-1.20.2-R0.1-SNAPSHOT.jar:?]
        at xyz.kyngs.librelogin.paper.PaperBootstrap.disable(PaperBootstrap.java:96) ~[LibreLogin.jar:?]

You can identify which specific program (plugin/mod/server/library) it is by the beginning of the package name. The name of the package contains the name of the author (comphenix), or the program itself (librelogin), or both (kyngs.librelogin).

The beginning of a package is usually the page where the developer can be found backwards, so for example an author page using the com.comphenix package can be found at https://comphenix.com, or a developer with a package starting with com.github.hakan-krgn will have the username hakan-krgn on GitHub (https://github.com/hakan-krgn).

However, if the package name doesn’t point to a specific/working page, or doesn’t include a specific plugin name, or even the author, this information is usually still very useful, because almost always the package name is all that is needed to find the program using Google.

Method

A method in Java is, simply put, a function that performs a particular “task” with a particular purpose.