== Why is this an issue?

Comparing a variable to multiple cases is a frequent operation. This can be done using a sequence of if-else statements. However, for many cases like enums or simple value comparisons, a `switch` statement is the better alternative. With Java 21, the `switch` statement has been significantly improved to support pattern matching and record pattern.

Using a `switch` statement instead of an if-else chain provides benefits like clearer code, certainty of covering all cases, and may even improve performance.

This rule raises an issue when an if-else chain should be replaced by a `switch` statement.

== How to fix it

Replace the chain of if-else with a switch expression.

=== Code examples

==== Noncompliant code example

[source,java,text,diff-id=1,diff-type=noncompliant]
----
sealed interface Expression {}
record Plus(Expression left, Expression right) implements Expression {}
record Minus(Expression left, Expression right) implements Expression {}
record Div(Expression left, Expression right) implements Expression {}

int eval(Expression expr){
  if(expr instanceof Plus plus){ // Noncompliant; should be replaced by a switch expression
    return eval(plus.left) + eval(plus.right);
  }else if(expr instanceof Div div){
    return eval(div.left) / eval(div.right);
  }else if(expr instanceof Minus minus){
    return eval(minus.left) - eval(minus.right);
  } else {
    throw new IllegalArgumentException("Unknown expression");
  }
}
----

[source,java,text,diff-id=2,diff-type=noncompliant]
----
enum Color{RED,GREEN,YELLOW}

String name(Color c){
  if(c == Color.RED){ // Noncompliant; should be replaced by a switch expression
    return "red";
  }else if(c == Color.GREEN){
    return "green";
  }else if(c == Color.YELLOW){
    return "yellow";
  }else{
    throw new IllegalArgumentException("Unknown color");
  }
}
----

[source,java,text,diff-id=3,diff-type=noncompliant]
----
int points(int result){
  if(result == 2){ // Noncompliant; should be replaced by a switch expression
    return 10;
  } else if(result == 3 || result==4 ){
    return 20;
  } else if (result == 5) {
    return 50;
  }else{
    return 0;
  }
}
----

[source,java,text,diff-id=4,diff-type=noncompliant]
----
class Circle{}
class Rectangle{}
class Square{}

String name(Object shape){
  if (shape instanceof Circle) { // Noncompliant; should be replaced by a switch expression
    return "circle";
  } else if (shape instanceof Rectangle) {
     return "rectangle";
  } else if (shape instanceof Square) {
    return "square";
  } else {
    throw new IllegalArgumentException();
  }
}
----

==== Compliant solution

[source,java,text,diff-id=1,diff-type=compliant]
----
sealed interface Expression {}
record Plus(Expression left, Expression right) implements Expression {}
record Minus(Expression left, Expression right) implements Expression {}
record Div(Expression left, Expression right) implements Expression {}

int eval(Expression expr){
  return switch(expr){
    case Div(var left, var right) -> eval(left) / eval(right);
    case Plus(var left, var right) -> eval(left) + eval(right);
    case Minus(var left, var right) -> eval(left) - eval(right);
  };
}
----

[source,java,text,diff-id=2,diff-type=compliant]
----
enum Color{RED,GREEN,YELLOW}
String name(Color c){
  return switch(c){
    case RED -> "red";
    case GREEN -> "green";
    case YELLOW -> "yellow";
  };
}
----

[source,java,text,diff-id=3,diff-type=compliant]
----
int points(int result){
  return switch(result){
    case 2 -> 10;
    case 3,4 -> 20;
    case 5 -> 50;
    default -> 0;
  };
}
----

[source,java,text,diff-id=4,diff-type=compliant]
----
class Circle{}
class Rectangle{}
class Square{}

String name(Object shape){
  return switch(shape){
    case Circle c -> "circle";
    case Rectangle r -> "rectangle";
    case Square s -> "square";
    default -> throw new IllegalArgumentException();
  };
}
----


== Resources
* https://openjdk.org/jeps/440[Record Patterns]
* https://openjdk.org/jeps/441[Pattern Matching for switch]
* https://docs.oracle.com/javase/tutorial/java/nutsandbolts/switch.html[The switch Statement]

