Pattern Matching for Instanceof

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Pattern Matching for Instanceof, added in JDK 15 (JEP 394), was designed to help address the common idiom when using the instanceof operator of test, assign, convert:

Object actuallyAString = "I'm actually a string!";
	
if(actuallyAString instanceof String) {//Test if actuallyAString is a String
	String nowImAString = //Assign actuallyAString to a variable
	(String) actuallyAString; //Convert actuallyAString to a String
	
	System.out.println(nowImAString);
}

The above code, along with being verbose, creates several opportunities for introduction of bugs in the code.

Instanceof Pattern Matching

Instanceof pattern matching address this issue by extending the instanceof operator to take a predicate and a pattern variable that will be assigned when the predicate is true:

Object actuallyAString = "I'm actually a string!";
	
if(actuallyAString instanceof String nowImAString) {
	System.out.println(nowImAString);
}

Flow Scope

The pattern variable, in these examples nowImAString, is only available within the flow scope where it was declared. What this means in practice is that nowImAString is available everywhere where the compiler definitely knows its value has been assigned (i.e. everywhere where actuallyAString instanceof is true)

Normal Use Cases

Typically this will mean the pattern variable will be available in the evaluation block it was declared, and, if part of an if statement, the following code block, like in the below examples of an if statement and assigning a boolean value.

Object actuallyAString = "I'm actually a string!";
	
if(actuallyAString instanceof String nowImAString) {
	System.out.println(nowImAString);
}
	
System.out.println(nowImAString); //Compiler error, nowImAString not in scope
	
boolean isAString = (actuallyAString instanceof String nowImAString);

System.out.println(nowImAString); //Compiler error, nowImAString not in scope

AND (&&) and OR (||) Operators

However a pattern variable would leave scope to the right of an OR || operator as the pattern variable may not had been assigned:

Object actuallyAString = "I'm actually a string!";
	
if(actuallyAString instanceof String nowImAString 
	|| nowImAString.endsWith("!") //Compiler error, nowImAString not in scope after ||
) {
	System.out.println(nowImAString);
}

The pattern variable would still be in-scope thought to the right of an AND && operator:

Object actuallyAString = "I'm actually a string!";
	
if(actuallyAString instanceof String nowImAString && nowImAString.endsWith("!")) {
	System.out.println(nowImAString);
}

Unusual Use Cases

There are some unusual ways to work with a pattern variable, like in the below examples of encapsulating instanceof in a NOT ! so that the pattern variable can be referenced in an else, or throwing an exception in the body of the if so the pattern variable can be referenced outside of the evaluation and if code block.

Object actuallyAString = "I'm actually a string!";

if (!(actuallyAString instanceof String nowImAString)) {
//...
} else {
	System.out.println(nowImAString);
}

if (!(actuallyAString instanceof String nowImAString)) {
	throw new IllegalArgumentException("Must be a string!");
}

System.out.println(nowImAString);

Using the Value of a Pattern Variable Outside of Flow Scope

To use value of a pattern variable outside of flow scope, it will need to be assigned to a variable of the scope that is appropriate for your needs, like in the below example of assigning the local variable aString the value of nowImAString within the if block:

Object actuallyAString = "I'm actually a string!";
String aString = null;
	
if(actuallyAString instanceof String nowImAString) {
	aString  = nowImAString;
}
	
System.out.println(aString);