Circular static init in ColumnType prevents correct singleton init

[uosis]

There is a circular dependency between singleton fields in ColumnType and singleton fields in its implementations. This makes correct initialization impossible.

Observe the following peculiar behavior (using Scala REPL for ease of illustration; eq in Scala is == in Java, i.e. reference equality):

> import $ivy.`tech.tablesaw:tablesaw-core:0.43.1`
> assert(tech.tablesaw.columns.numbers.DoubleColumnType.instance() eq tech.tablesaw.api.ColumnType.DOUBLE)
java.lang.AssertionError: assertion failed

This fails because static init of ColumnType is called from within static init of DoubleColumnType, while DoubleColumnType.INSTANCE field is still null, so ColumnType.DOUBLE receives a new instance from here, instead of the singleton value that eventually ends up in DoubleColumnType.INSTANCE field.

Let's try the other way:

> import $ivy.`tech.tablesaw:tablesaw-core:0.43.1`
> assert(tech.tablesaw.api.ColumnType.DOUBLE eq tech.tablesaw.columns.numbers.DoubleColumnType.instance())
// yay no error! but...
> tech.tablesaw.columns.numbers.DoubleColumnType.DEFAULT_PARSER.columnType()
res2: ColumnType = null

Now static init of DoubleColumnType is called from within static init of ColumnType, while ColumnType.DOUBLE is still null, so we get null here.

This breaks tablesaw-parquet here because it's doing reference equality comparisons.

> import $ivy.`tech.tablesaw:tablesaw-core:0.43.1`
> import $ivy.`net.tlabs-data:tablesaw_0.43.1-parquet:0.10.1`
> val table = tech.tablesaw.api.Table.create("t").addColumns(tech.tablesaw.api.DoubleColumn.create("c", 1, 2, 3))
> new net.tlabs.tablesaw.parquet.TablesawParquetWriter().write(table, net.tlabs.tablesaw.parquet.TablesawParquetWriteOptions.builder("test.parquet").build())
java.lang.NullPointerException: Cannot invoke "tech.tablesaw.api.DoubleColumn.getDouble(int)" because "this.doubleColumns[colIndex]" is null
  net.tlabs.tablesaw.parquet.TableProxy.getDouble(TableProxy.java:225)

I am not sure what is the correct solution here. Should tablesaw-parquet not assume singleton instances and use equals instead of ==? I am guessing most things are generally working because in most cases DoubleColumnType.instance() will run first, causing DoubleColumnType.instance() and ColumnType.DOUBLE to have different instances, but nothing getting null value.

Possibly similar to #430.

[ccleva]

Fixed on tablesaw-parquet side with v0.11.0.

[uosis]

Fixed on tablesaw-parquet side with v0.11.0.

While this is great and probably the most expedient solution (thanks @ccleva for a quick fix!), the underlying problem of broken init is still there.

I am not aware of any practical issues it is causing at this point however, so this is probably not a very high priority.