Crashing Minecraft? How to Fix an IndexOutOfBoundsException

If Minecraft Java Edition closes with an IndexOutOfBoundsException, start with the crash report and stack trace. The exception means Java code tried to use an array, string, or collection position outside its valid range. In Minecraft, the failing code may belong to the game, a mod, a mod loader, or a server plugin. The message alone does not prove that your world is corrupted or that Minecraft needs more RAM.

Work through the checks below in order: capture the report, compare a clean profile, verify the Java runtime for your game version, then isolate mods or world-specific data. Back up a valuable world before testing it.

The images in this guide are generic illustrations. Their windows, paths, timestamps, version fields, and sample log lines are not screenshots from a tested installation or exact launcher interfaces.

What the Java exception means

Java indexes usually start at zero. A list with five entries has positions 0 through 4; requesting position 5 is out of range. Oracle describes IndexOutOfBoundsException as a runtime exception thrown when an index into something such as an array, string, or vector is outside its range. ArrayIndexOutOfBoundsException and StringIndexOutOfBoundsException are more specific subclasses.

A crash report may show a line such as java.lang.IndexOutOfBoundsException: Index 5 out of bounds for length 5. The exact wording varies by Java version and the code that throws the exception. This is an indexing error, not the same as OutOfMemoryError. Adding memory with -Xmx is not a general fix for an invalid index.

Six steps to diagnose the crash

1. Find the full crash report and identify the stack trace

For the official Minecraft Launcher, Java crash reports are normally in the crash-reports folder under the game directory. The latest game log is in logs/latest.log. Common default game-directory locations are:

• Windows: %appdata%\.minecraft
• macOS: ~/Library/Application Support/minecraft
• Linux: ~/.minecraft

A third-party launcher may store each instance in its own directory, so use that launcher's instance-folder option or documentation. For a server crash, inspect the server console and server log as well as the client report.

Search the report for IndexOutOfBoundsException, then read the frames immediately below it and the nearby “suspected” or mod sections, if present. Note the exact game version, loader, mod list, and what you were doing when the crash occurred. A mod name in a frame is a useful lead, but it is not automatic proof that the mod alone is at fault; incompatible interactions can involve more than one component.

2. Reproduce the problem in a clean profile

Create or select a separate, unmodified profile for the same Minecraft version, with no mods or custom loader. Try the same action in a temporary new world. Do not use your only copy of an important save as the first test.

If the clean profile works, the problem is more likely tied to a mod, loader, resource pack, custom configuration, or data used by the original profile. That narrows the search; it does not identify a specific mod by itself. If the clean profile also crashes, keep the report and check whether the crash happens in a new world, during startup, or only after a particular action.

3. Check the Java runtime against the exact release

Do not install a random Java version just because an older guide recommends it. The required runtime depends on the Minecraft release and the launcher configuration. The official launcher normally manages the runtime for its game installation. If you selected a custom Java executable, use the requirement for your exact release or return to the launcher's managed runtime setting.

For a current example, Mojang's Minecraft Java Edition 26.1 release notes state that the game requires Java 25 and includes Microsoft's OpenJDK 25 distribution. That does not mean Java 25 should be forced onto older game versions or every third-party launcher. Check the release notes and the mod loader's compatibility requirements together.

4. Isolate mods without deleting your setup

Make a copy of the launcher instance or at least the mods folder before changing it. Confirm that every mod targets the installed Minecraft version and loader, and that its required dependencies are present. Then temporarily move a batch of mods out of the test instance and reproduce the crash. If the crash stops, restore the batch in smaller groups until the trigger is narrowed down.

Keep the original files rather than deleting them. Check the identified mod's project page or issue tracker for a release matching your version, known conflicts, or an update note. If no single mod explains the crash, test likely combinations and include the full report when asking the mod author for help.

5. Protect the world and test a copy

If the crash happens only while opening one world, close Minecraft and make a separate backup of that world before trying further changes. You can copy the relevant save folder from saves to a safe location, or use the backup process recommended by Minecraft Support. Test a duplicate of the affected save and a fresh temporary world separately.

If a fresh world works but the copy of the affected world crashes, the trigger may involve saved world data, an entity or block added by a mod, or a version-specific game issue. That result does not prove the save is corrupted. Avoid deleting region files, editing level.dat, or running a repair utility unless the report points to a specific data problem and you have a verified backup.

6. Check the server side if this happens in multiplayer

If the crash occurs only when joining one server, ask the owner to check the server log at the matching time. The server may be running an incompatible plugin, mod, or game build even when your local client is current. A server operator should back up the server, verify that plugins match the server software and Minecraft version, and test changes on a copy or staging server where possible.

Players usually cannot repair a server-side plugin exception from their own computer. Share the relevant crash excerpt, your game version, and loader details with the operator. Do not post account tokens, private IPs, or unrelated personal information when sharing logs.

Fast checks: what to change and what not to assume

Common questions

Will increasing Minecraft's RAM allocation fix IndexOutOfBoundsException?

Usually, no. The exception describes an invalid index. Change memory allocation only when the log points to a separate memory problem, such as OutOfMemoryError, and follow the current Minecraft Support instructions for your setup.

Does this error mean my Minecraft world is corrupted?

No. A crash while loading one save makes that save a useful diagnostic lead, but the cause could be modded data, a compatibility issue, or a game bug. Preserve a backup and compare a copy with a new world before considering repairs.

Should I update Java or Minecraft first?

First record the exact game version, launcher, loader, and Java executable in use. Then match them to the release and mod requirements. With the official launcher, avoid overriding its managed Java runtime unless you have a specific compatibility reason.

Official references

• Oracle Java SE 25 API: IndexOutOfBoundsException
• Mojang: Minecraft Java Edition 26.1 release notes
• Fabric Documentation: Reading crash reports
• Fabric Documentation: Finding and sharing logs
• Minecraft Support: Backing up a Java Edition world
• Minecraft Support: Checking memory allocation for game crashes