When is inheritance appropriate in Java?

Frames inheritance as a behavioral is-a relationship whose subtypes must remain valid substitutes, not merely a code-reuse shortcut.

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TL;DR

Inheritance is appropriate when the subtype is genuinely a kind of the base type and can honor the base contract everywhere the base is expected. A subtype may specialize behavior, but it should not reject valid base inputs, weaken guarantees, or make ordinary base operations meaningless. In Java, a class has one direct superclass, so choosing inheritance also commits it to a hierarchy. If the relationship is only uses-a or needs-some-code-from, composition is usually clearer.

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Substitutability, base-class contracts, hierarchy stability, and the ability to recognize fragile inheritance.

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Core answer
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Overview
Inheritance is strongest when it models a stable behavioral relationship and weakest when it exists only to reuse a few methods.
Inheritance is appropriate when the subtype is genuinely a kind of the base type and can honor the base contract everywhere the base is expected. A subtype may specialize behavior, but it should not reject valid base inputs, weaken guarantees, or make ordinary base operations meaningless. In Java, a class has one direct superclass, so choosing inheritance also commits it to a hierarchy. If the relationship is only uses-a or needs-some-code-from, composition is usually clearer.
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Suppose a ReadOnlyAccount extends an Account whose contract includes . If the subtype throws for a normal base operation, it is not a good substitute even if the class names sound related. A better model may separate readable account information from withdrawal capability. Inheritance works better for a stable framework extension point or a domain hierarchy whose shared rules are real and whose differences fit the base contract.

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Using inheritance primarily to avoid duplicating a few lines.
Treating an is-a sentence as proof without testing behavior.
Allowing subtypes to throw for normal base operations.
Building deep hierarchies with mutable protected state.

When is inheritance appropriate in Java?

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