Replace Stream Collect By ToList

Description

Java 16 introduced Stream.toList() (opens new window) as a shorthand method for converting a Stream into an unmodifiable List.
This rule replaces invocations of collect(Collectors.toUnmodifiableList()) on streams by invocations of the new method stream.toList(). In case Collectors.toList() is used as a collector, the rule makes additional verifications whether the generated list is modified by the context or not. In the latter case, invocations of collect(Collectors.toList()) on streams are also replaced by invocations of toList().

Thus the code is simplified and readability is improved.

Requirements

• Java 16

Benefits

Removes code clutter. Improves readability.

Tags

Tags

Java 16, Old Language Constructs, Readability, Marker

Code Changes

Using Collectors.toUnmodifiableList()

Pre

List<String> List = collection
		.stream()
		.map(function)
		.filter(predicate)
		.collect(Collectors.toUnmodifiableList());

Post

List<String> List = collection
		.stream()
		.map(function)
		.filter(predicate)
		.toList();

Argument of Collections.unmodifiableList(...)

Pre

return Collections.unmodifiableList(collection
		.stream()
		.map(function)
		.filter(predicate)
		.collect(Collectors.toList()));

Post

return Collections.unmodifiableList(collection
		.stream()
		.map(function)
		.filter(predicate)
		.toList());

Using Collectors.toList()

Pre

void sendMails(List<User>users) {
	List<String> emails = users.stream()
		.map(User::getMail)
		.collect(Collectors.toList());
	for (String email : emails) {
		validate(email);
		send(email);
	}
}

Post

void sendMails(List<User>users) {
	List<String> emails = users.stream()
		.map(User::getMail)
		.toList();
	for (String email : emails) {
		validate(email);
		send(email);
	}
}

Argument of Collections.min(...)

Pre

int min = Collections.min(collection
		.stream()
		.map(function)
		.filter(predicate)
		.collect(Collectors.toList()));

Post

int min = Collections.min(collection
		.stream()
		.map(function)
		.filter(predicate)
		.toList());

Argument of List.copyOf(...)

Pre

return List.copyOf(collection
		.stream()
		.map(function)
		.filter(predicate)
		.collect(Collectors.toList()));

Post

return List.copyOf(collection
		.stream()
		.map(function)
		.filter(predicate)
		.toList());

Argument of list.addAll(...)

Pre

list.addAll(collection
		.stream()
		.map(function)
		.filter(predicate)
		.collect(Collectors.toList()));

Post

list.addAll(collection
		.stream()
		.map(function)
		.filter(predicate)
		.toList());

Enhanced For Loop with Stream.collect(...)

Pre

for (String s : collection
		.stream()
		.map(function)
		.filter(predicate)
		.collect(Collectors.toList())) {
	//...
}

Post

for (String s : collection
		.stream()
		.map(function)
		.filter(predicate)
		.toList()) {
	//...
}

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Properties

Property	Value
Rule ID	ReplaceStreamCollectByToList
First seen in jSparrow version	4.4.0
Minimum Java version	16
Remediation cost	2 min