In other words, why would you need an instance initializer? What difference or advantage do you have in writing a instance initializer over a constructor?
This seems to explain it well:
Instance initializers are a useful alternative to instance variable initializers whenever: initializer code must catch exceptions, or perform fancy calculations that can't be expressed with an instance variable initializer. You could, of course, always write such code in constructors. But in a class that had multiple constructors, you would have to repeat the code in each constructor. With an instance initializer, you can just write the code once, and it will be executed no matter what constructor is used to create the object. Instance initializers are also useful in anonymous inner classes, which can't declare any constructors at all.
Instance initializers are a useful alternative to instance variable initializers whenever:
initializer code must catch exceptions, or
perform fancy calculations that can't be expressed with an instance variable initializer. You could, of course, always write such code in constructors.
But in a class that had multiple constructors, you would have to repeat the code in each constructor. With an instance initializer, you can just write the code once, and it will be executed no matter what constructor is used to create the object. Instance initializers are also useful in anonymous inner classes, which can't declare any constructors at all.
From: JavaWorld Object initialization in Java.
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