Refactoring large switch statement
You can create a KeyProcessor interface and implementation for every key (move the body of the current Processor.processStuffAboutKeyX() methods to these classes):

public interface KeyProcessor {
    void processStuff(String data);
}


Then fill a map with the available implementations:

Map&lt;Key, KeyProcessor&gt; processors = new HashMap&lt;&gt;();
processors.add(key1, new Key1Processor());
...
processors.add(key4, new Key2Processor());


And use that map in the loop:

for (Entry&lt;Key, String&gt; entry: map.entrySet()) {
    Key key = entry.getKey();
    KeyProcessor keyProcessor = processors.get(key);
    if (keyProcessor == null) {
        throw new IllegalStateException("Unknown processor for key " + key);
    }
    final String value = entry.getValue();
    keyProcessor.processStuff(value);
}


With the original 25+ cases the switch-case method is at least 75 lines long (25 case statement, 25 break statement, and at least 1-1 method call line for every case). The separate classes reduces the size and complexity which a developer see at once (+no need to scroll), the code is easier to maintain and test because you can test every case in isolation.

I think it's a lot more easier to have (and handle) a couple of small classes and corresponding *Test classes (with a few test methods for every class) than having a big class (25+ cases) and a lot of tests in one corresponding *Test class or a lot of *Test classes which test separate case branches of the same startProcessing method.

I admit that these arguments might be weak ones but without an actual implementation of the Processor class and the code which creates the input map is hard to say more.

Further readings:


Refactoring: Improving the Design of Existing Code by Martin Fowler: Replacing the Conditional Logic on Price Code with Polymorphism
Replace Conditional with Polymorphism