Pattern matching
- Java course
- Intermediate
- Lesson 63/116
- 4 min
Learn type conversion with instanceof pattern matching.
Type checking with instanceof
Sometimes you need to know the actual type of a polymorphic reference. The instanceof operator does this: if (s instanceof Circle) is true when s is really a Circle. In older Java you had to cast and store a separate variable.
Since Java 16, pattern matching for instanceof exists: if (s instanceof Circle c) automatically casts s to Circle as c. It is shorter and safer.
Switch patterns (Java 21)
Since Java 21, switch expressions can test types. Writing switch (s) { case Circle c -> ...; case Square q -> ...; } runs different code for each type. This is more readable than a chain of if-instanceof.
The switch expression can return a value; case null handles null. If subclasses are not all covered, a default is required.
Practice Print a field with pattern matching
Use the abstract class Shape and Circle extends Shape from above. Add class Square extends Shape with int side, a constructor and an area method returning side * side. In main, assign Shape s = new Circle(3);, then use if (s instanceof Circle c) pattern matching to print System.out.println("Circle: " + c.r);.
Starter code
// abstract Shape and Circle as above
// Add Square class: side field and area method
// main: assign a Circle instance as Shape and print via pattern matchingSample solution
abstract class Shape {
abstract int area();
}
class Circle extends Shape {
int r;
Circle(int r) { this.r = r; }
int area() { return r * r * 3; }
}
class Square extends Shape {
int side;
Square(int side) { this.side = side; }
int area() { return side * side; }
}
public class Main {
public static void main(String[] args) {
Shape s = new Circle(3);
if (s instanceof Circle c) {
System.out.println("Circle: " + c.r);
}
}
}Expected output
Circle: 3
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Complete the pattern matching
if (s instanceof ____ c) {
System.out.println(c.r);
}if (s instanceof Circle c) {
System.out.println(c.r);
}Explanation: Pattern matching binds c as a Circle.
2 What does instanceof Circle c do?
- Casts c to Circle and defines it in scope Correct answer
- Makes c a static variable
- Removes the Circle class
- Converts Shape into Circle
Explanation: If the type check passes, pattern matching binds c as Circle.
3 Match the instanceof concepts
| Term | Meaning |
|---|---|
instanceof |
Type check operator |
pattern matching |
Binds the cast automatically |
switch (Java 21) |
Type-based branching |
default |
Required when not all types covered |