Can an Interface have fields? Why or why not?
Fields in Interfaces
The answer is language-specific. In Java, interface fields are implicitly public, static and final, so they represent constants rather than per-object mutable state. Java interfaces cannot define ordinary instance fields. In C#, interfaces traditionally did not contain instance fields either; newer language features have expanded what interfaces can express, but they still are not a mechanism for storing ordinary per-instance object state. If an abstraction needs mutable instance state, an abstract class or composition is generally more appropriate.
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Java interface fields are implicitly public, static and final.
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They are constants, not per-object mutable state.
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Interfaces are primarily contracts rather than state containers.
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C# interface capabilities differ by language version, but interfaces are not intended as ordinary instance-state holders.
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Use a class or abstract class when shared instance state is part of the abstraction.
Follow-up Questions
- Why are Java interface fields static final?
- Can interfaces contain methods with implementation?
- Should constants be placed in interfaces?
- How does C# differ from Java regarding interface members?