According to ZK JDK Support, the minimal compatible version is 1.6. But in the source, we still use 1.5.

ZK requires at least Java SE 6 and works with higher versions that are binary-compatible with the official Oracle JDK 1.6. If you run into any ZK issues relating to Java upgrade please report to us.

In fact, compiling on Java 8 will get a warning message. JDK-8011043

Warning:java: source value 1.5 is obsolete and will be removed in a future release

In Java 9, the support of 1.5 was dropped in JDK-8011044.

In order to use newer Java to compile, the source/target level should be changed to at least 1.6.
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https://blog.zkoss.org/2018/09/20/2018-jdk-and-browser-compatibility-survey-summary/
Since we announced we'll change to use Java 8, and 93% of customers are ready (using Java >= 8).
We can add some Java 8 support like java.time in ZK 9.