The configLookup member of the ATNConfigSet class can be lazily instantiated to improve memory consumption

[yososs]

The following code was added to Array2DHashSet.java to parse the large input source.

    @Override
   protected void finalize() throws Throwable {
       if(buckets!=null) {
           System.out.println("buckets:"+buckets.length+", n:"+n +"\t:"+this.getClass().getSimpleName());
       }
   }

By outputting the usage logs of the subclasses of the Array2DHashSet class, I found the following

• About 1/4 of the instantiated ConfigHashSet is unused (n=0).

This ratio is considered to be a large value for lazy instantiation.

• https://github.com/antlr/antlr4/blob/master/runtime/Java/src/org/antlr/v4/runtime/misc/Array2DHashSet.java

• https://github.com/antlr/antlr4/blob/master/runtime/Java/src/org/antlr/v4/runtime/atn/ATNConfigSet.java

Notes on modification

• configLookup should be changed to protected scope when lazy initialization is done.
• The current configLookup == null condition should be replaced with readonly.