// The good part:
// String(javaString).valueOf()
// Follow the same pattern to convert other Java types to corresponding JavaScript primitives

var javaString = new java.lang.String("test");
print('javaString:', javaString);
print('typeof javaString:', typeof javaString);                           // Object
print('javaString instanceof String:', (javaString instanceof String));   // true (really a java.lang.String object)
print('String(javaString).valueOf() === "test":', String(javaString).valueOf() === "test");   // true: Converted to JS primitive