OPC UA Part 6 defines 25 builtin types. Those 25 types are represented with the following java classes. Most builtin classes are immutable. byte[] is not immutable but modifications are forbidden.
IDNameJava ClassNotes
1Booleanjava.lang.Boolean
2SBytejava.lang.Byte
3Byteorg.opcfoundation.ua.builtintypes.UnsignedByte
4Int16java.lang.Short
5UInt16org.opcfoundation.ua.builtintypes.UnsignedShort
6Int32java.lang.Integer
7UInt32org.opcfoundation.ua.builtintypes.UnsignedInteger
8Int64java.lang.Long
9UInt64org.opcfoundation.ua.builtintypes.UnsignedLong
10Floatjava.lang.Float
11Doublejava.lang.Double
12Stringjava.lang.String
13DateTimeorg.opcfoundation.ua.builtintypes.DateTime
14Guidjava.util.UUID
15ByteStringbyte[]The contents must not be modified after instantiation.
16XmlElementorg.opcfoundation.ua.builtintypes.XmlElement
17NodeIdorg.opcfoundation.ua.builtintypes.NodeId
18ExpandedNodeIdorg.opcfoundation.ua.builtintypes.ExpandedNodeId
19StatusCodeorg.opcfoundation.ua.builtintypes.StatusCode
20QualifiedNameorg.opcfoundation.ua.builtintypes.QualifiedName
21LocalizedTextorg.opcfoundation.ua.builtintypes.LocalizedText
22ExtensionObjectorg.opcfoundation.ua.builtintypes.ExtensionObject
23DataValueorg.opcfoundation.ua.builtintypes.DataValue
24Variantorg.opcfoundation.ua.builtintypes.Variant
25DiagnosticInfoorg.opcfoundation.ua.builtintypes.DiagnosticInfo


Also, see class org.opcfoundation.ua.encoding.utils.BuiltinsMap for builtins id, uri and java class mappings.