Exception: java.lang.NoClassDefFoundError: com/google/gwt/dev/DevMode

Someone please help

I  am new to GWT. I am getting below exception when I try to debug.

java.lang.NoClassDefFoundError: com/google/gwt/dev/DevMode

Caused by: java.lang.ClassNotFoundException: com.google.gwt.dev.DevMode

at java.net.URLClassLoader$1.run(URLClassLoader.java:200)

at java.security.AccessController.doPrivileged(Native Method)

at java.net.URLClassLoader.findClass(URLClassLoader.java:188)

at java.lang.ClassLoader.loadClass(ClassLoader.java:306)

at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:276)

at java.lang.ClassLoader.loadClass(ClassLoader.java:251)

at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:319)

Exception in thread "main" 

Any idea on this?

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