java:new

(java:new "fully.qualified.ClassName" args...)

Constructs a host (Java) object by reflection, choosing the constructor whose parameters best match the arguments, and returns an opaque java object that prints as #<java <class-name>>. Part of the JVM-only java interop package — available on the interpreter and in JVM-compiled classes, not on the WASM backend, and it needs the class to be present and reflectable at runtime. See the Java interop guide.

A java.lang.StringBuilder is constructed from the string "ab", then its length method returns 2.