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What it looks like

java.lang.IllegalArgumentException: No enum constant Color.PURPLE

Extends RuntimeException in java.lang.

What it means

IllegalArgumentException is the standard way for a method to reject an argument whose value is wrong, as opposed to null (NullPointerException) or an object in the wrong state (IllegalStateException). The JDK throws it from many places, such as new ArrayList(-1), Enum.valueOf with an unknown name and Thread.sleep(-1), and so should your own validation code.

Common causes

• Enum.valueOf("RED") when the enum has no such constant: "No enum constant Color.RED".
• A negative size, timeout or percentage passed to a constructor or method.
• A malformed value such as an invalid URI, UUID, or time zone id.
• A Spring assertion (Assert.notNull, Assert.hasText) that failed during configuration.

How to fix it

1. Read the message: well-written code states which argument was wrong and why.
2. Validate input at the edge and return a proper error to the caller instead of letting the exception escape.
3. For enums, write a lookup that returns an Optional, or catch the exception where the text enters.
4. Throw IllegalArgumentException in your own methods with a message that includes the bad value.

Broken and fixed

Color c = Color.valueOf(userInput); // "No enum constant Color.PURPLE"

Optional<Color> c = Arrays.stream(Color.values())
        .filter(x -> x.name().equalsIgnoreCase(userInput))
        .findFirst();

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In Spring and enterprise code

Spring turns an IllegalArgumentException from a controller into a 500 unless you map it. Handle it in a @ControllerAdvice and return a 400 with a clear message.

Debug it faster

Paste the whole stack trace into the stack trace beautifier to hide framework frames, jump to the root cause and find the first line of your own code. It runs in your browser, so the trace never leaves your machine.

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