I realized ICU4J API reference doc uses full path for JDK class names, such as java.lang.String, instead of String (with link). This problem started ICU4J 60.1.
I found someone also experienced this - https://www.thecodingforums.com/threads/javadoc-including-java-lang-string-instead-of-just-string-for-return-values.390550/
In our build string, we define JDK doc link as below -
<property name="icu4j.api.doc.jdk.link" value="http://docs.oracle.com/javase/8/docs/api/"/>
It looks the JDK class linkage stopped working since Oracle site started redirecting http: to https. We should update the property value.
Merged to maint-64 for 64.2