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A Java interface defines a reference type and a contract: it names operations that a class agrees to provide, while leaving the implementation to that class. You cannot instantiate an interface, but you can use it as a variable or parameter type so code can work with any implementing class.
What an interface means in Java
Think of an interface as an agreed set of operations. Callers rely on those operations; each implementing class decides how they work. Oracle’s introductory tutorial uses the example of robotic-car manufacturers and guidance-system makers agreeing on method signatures without sharing an implementation. In Java, an interface is a reference type, not an object you construct directly.
For example, Payable describes a capability: an object can report an amount in cents. Invoice supplies that behavior, and a variable declared as Payable can refer to an invoice:
interface Payable {
int amountInCents();
}
final class Invoice implements Payable {
private final int cents;
Invoice(int cents) { this.cents = cents; }
@Override public int amountInCents() { return cents; }
}
Payable item = new Invoice(1250);The declared type of item exposes the interface contract rather than the concrete class. The interface specifies the method shape and intended interaction; implementations still need to meet the documented behavioral expectations.
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How to declare and implement an interface
Declare an interface with the interface keyword. A class names the interface after implements and provides the required abstract methods unless the class itself is abstract.
- Declare the contract: write an interface and its method declarations, such as
int amountInCents();. - Implement the contract: declare the class with
implements Payableand provide a compatible public method body. - Use the abstraction: assign an implementing object to a variable, parameter, or return type declared as the interface.
A class may implement multiple interfaces, allowing it to offer several capabilities. An interface may also extend multiple interfaces. These are useful differences from class inheritance: a class has only one direct superclass, while it can implement several interfaces. Neither an interface nor an abstract class is automatically the better choice; choose based on whether the design calls for a capability contract, shared class implementation, or both.
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What kinds of methods can an interface contain?
Java SE 21 language rules distinguish abstract, default, static, and private interface methods. The Java Tutorials’ introductory interface material is based on JDK 8, so its older member overview should not be treated as a complete account of modern Java.
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|---|---|---|---|
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| Default | Yes | Instance method | Implementing classes inherit it unless they override it; an extending interface may inherit it, redeclare it as abstract, or redefine it. |
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A method without private, default, or static is implicitly abstract. In the tutorial’s declarations, constants are implicitly public static final, while abstract, default, and static methods are implicitly public.
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Why default methods have bodies
A default method provides shared instance behavior through an interface. This can help evolve an interface by adding behavior while allowing implementing classes to inherit a usable implementation. An implementing class may override it, and an extending interface can also redeclare the method as abstract or provide a different default.
What is a functional interface?
A functional interface has exactly one abstract method. That single abstract operation makes the interface a target for a lambda expression, method reference, or constructor reference. The annotation @FunctionalInterface documents that intent and asks the compiler to check the definition; it is optional when the type already meets the rule.
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Runnable task = () -> System.out.println("work");Here, the lambda supplies the behavior required by Runnable’s one abstract method. Functional interfaces can also have default and static methods; the defining count concerns abstract methods.
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Oracle’s Java Tutorials offer a readable introduction to interface declarations and implementations, but their examples are based on JDK 8. For current method categories, the Java SE 21 Language Specification is the normative reference; Oracle’s Java SE 25 API documentation describes functional-interface behavior. The distinction matters because introductory material can remain useful for the basic contract idea while omitting features introduced in later Java versions.
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