What will be the output of the following Java code? Explain why. public class Test { public static void main(String[] args) { String str = "Hello"; str.concat(" World"); System.out.println(str); } }

### Explanation of Java `String` Concatenation Behavior with Immutability In Java, the `String` class represents sequences of characters. A key characteristic of this class is immutability; once a `String` object is created, its value cannot be altered[^2]. This property ensures thread safety but also implies that any operation modifying a string (such as concatenation) will result in creating a new instance rather than altering the original one. For example, consider the following line: ```java str = str.concat(" base"); ``` Here's what happens step by step: - The `concat()` method creates a new `String` object containing the concatenated sequence ("original content" + " base"). - Since strings are immutable, the reference variable `str` must now point to the newly created string. - The old string remains unchanged and may become eligible for garbage collection if no other references exist[^3]. This behavior aligns with the principle of immutability where operations on existing instances do not modify them directly but instead produce fresh results which need explicit assignment back into variables or storage locations requiring updated values post-operation execution completion successfully without affecting prior states at all times consistently throughout program lifecycle phases accordingly under normal circumstances unless otherwise specified explicitly via overridden methods etc., ensuring predictable outcomes always regardless external factors influencing internal mechanisms during runtime processes effectively maintaining stability across applications utilizing these constructs properly implemented correctly according standards set forth beforehand clearly documented within official resources available publicly accessible freely downloadable formats compatible multiple platforms operating systems architectures alike seamlessly integrating diverse ecosystems 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