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course/5-interfaces/exercises/4a-quiz/readme.md

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+# Interfaces Quiz
+
+Remember, interfaces are collections of method signatures. A type "implements" an interface if it has all of the methods of the given interface defined on it.
+
+```go
+type shape interface {
+  area() float64
+}
+```
+
+If a type in your code implements an `area` method, with the same signature (e.g. accepts nothing and returns a `float64`), then that object is said to *implement* the `shape` interface.
+
+```go
+type circle struct{
+	radius int
+}
+
+func (c *circle) area() float64 {
+  return 3.14 * c.radius * c.radius
+}
+```
+
+This is *different from most other languages*, where you have to *explicitly* assign an interface type to an object, like with Java:
+
+```java
+class Circle implements Shape
+```