What is the use of the Holder class in a HashMap?

Question

I found that in HashMapwe have an inner class Holderwith the following description:

/**
 * holds values which can't be initialized until after VM is booted.
 */

How and when should we use this class? what is its use. Explain, please.

Answer 1

This is due to enhancements to Java 7u6 and is removed in Java 8.

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Relevant documentation:

Collection Extensions in Java SE 7

An alternative hash function improves the performance of these implementation maps when a large number of key hash collisions occur.

For Java SE 7u6, this alternative hash function is implemented as follows:

- . -1. -. -, jdk.map.althashing.threshold . 512. 512 512 -. 0, -.

...

Java SE 8

- String, 7u6, JDK 8, jdk.map.althashing.threshold. -, , .

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, ? - , . Oracle HashMap, . - jdk.map.althashing.threshold.

Oracle ? - . VM , , . , / , / .

, VM. , sun.misc.VM.isBooted().