If a serializable class defines a field serialVersionUID this field has to be final, static and of type long. For serialization to work, readObject() and writeObject() should be private. An externalizable class must have a public zero-argument constructor. A non-serializable class, extended by a serializable class, must have a zero-argument constructor. Find non-serializable classes that have a serializable inner-class. Sorted Collections using this comparator will also not be serializable. Replace these methods by their locale-parameterized versions. Container classes like java.util.HashMap or java.util.Hashtable will only call equals on objetcs with the same hashCode. To override java.lang.Object#equals(java.lang.Object) the parameter has to be java.lang.Object and the return type boolean. By convention, classes that implement this interface should override java.lang.Object#clone() (which is protected) with a public method. See {@link java.lang.Object#clone()} for details on overriding this. When implementing Object#clone() you should inheriting from java.lang.Cloneable Find threads that inherit the empty definition of java.lang.Thread#run(). Direct calls to java.lang.Runnable#run() instead of using java.lang.Thread#start(). The method wait() should not be called on a java.util.concurrent.locks.Condition, instead the await methods should be used. Since the Format classes are not synchronized these instances should not be non-private accessible even if the method returning the instance is synchronized itself. Method calls like java.lang.Integer#getInteger(java.lang.String) do not transform strings to the concrete numbers but determine the int/short/float/... value of the system property with the specified name.