Step 1: install jdk-12.0.2_windows-x64_bin
Step 2: Set variable in path: Advanced System Setting/Environment Variable/System Variable/path
Path: C:\Program Files\Java\jdk-12.0.2\bin
Step 3: javac *.java
Step 4: java ChatServer
Step 5-1: java ChatClient user1
Step 5-2: java ChatClient user2
Step 5-3: java ChatClient user3