In the core Java API, which class has only one public field named TYPE?

Introduction / Context:
Java includes wrapper and utility classes for handling primitive types and special values. The class java.lang.Void is one such special class that represents the void type in reflection and other APIs. It is sometimes an interview question to ask which class has only one public field called TYPE, to check knowledge of less commonly used core classes and their structure.

Given Data / Assumptions:

• We are considering classes in the java.lang package such as Runtime, Process, Void and System.
• TYPE is a conventional name for fields that hold a Class object associated with a primitive or special type.
• Wrapper classes like Integer and Boolean also have TYPE fields, but they usually have additional instance fields.
• We are specifically asked about a class whose only public field is TYPE.

Concept / Approach:
The class java.lang.Void is a final class that cannot be instantiated. It exists mainly to provide a Class object representing the void type through its public static final field TYPE. This TYPE field refers to the Class instance for the Java keyword void. The class does not have other public fields or constructors, and its design emphasises that there are no instances of void. In contrast, classes like Runtime, Process and System have multiple members and serve other purposes in the API.

Step-by-Step Solution:
Step 1: Recall that java.lang.Void is declared as public final class Void and has a private constructor to prevent instantiation. Step 2: Recall that it defines a single public static final field named TYPE, which holds the Class object for the primitive void type. Step 3: Compare this with classes like Runtime and System, which have several methods and no such single field layout. Step 4: Recognise that Process represents operating system processes and does not exist solely to hold a TYPE field. Step 5: Conclude that Void is the class whose only public field is TYPE, matching option C.

Verification / Alternative check:
Looking at simplified declarations, java.lang.Void includes public static final Class<Void> TYPE, and a private constructor. It does not provide other public fields. In contrast, java.lang.Boolean, Integer and other wrappers have public static final TYPE fields but also have instance fields that hold the wrapped value and other members. The classes Runtime, Process and System have multiple methods and static fields for environmental properties, not just a single TYPE field. This confirms that Void is unique in having TYPE as its primary public field.

Why Other Options Are Wrong:
Option A, Runtime, is a class used to interact with the Java runtime environment and has various methods and does not have only a TYPE field. Option B, Process, represents system level processes and has methods for handling streams and exit values. Option D, System, has many public static fields such as in, out and err, as well as numerous methods. Option E, Thread, has fields and methods for controlling threads. None of these classes are designed as a simple holder for TYPE. Only Void fits the description of having one public field named TYPE.

Common Pitfalls:
Because java.lang.Void is rarely used directly in everyday code, many developers are not familiar with it. They may confuse it with wrapper classes or assume that all primitive related classes behave the same. Another pitfall is thinking that System or Runtime might have such a field because they are closely tied to the environment. In interviews, correctly identifying Void and explaining that it represents the void type through its TYPE field demonstrates attention to detail in the Java core library.

Final Answer:
In the Java API, the class java.lang.Void has only one public field named TYPE.