Use Collections Singleton List

Description

Replaces Arrays.asList (opens new window) with 0 or 1 parameters respectively with Collections.emptyList() (opens new window) or Collections.singletonList(..) (opens new window).

Note

Arrays.asList creates a fixed size list while Collections.singletonList creates an immutable list and therefore does not allow operations like set(int index, E element) (opens new window).

Benefits

Improves the efficiency of creating short lists.

Tags

Tags

Java 1.3, Old Language Constructs, Performance, Marker

Code Changes

List with one element

Pre

List<String> list = Arrays.asList("value");

After

List<String> list = Collections.singletonList("value");

Empty List

Pre

List<String> list = Arrays.asList();

After

List<String> list = Collections.emptyList();

Static import to Arrays.asList

Pre

import static java.util.Arrays.asList;

class MyClass {
    List<String> emptyList = asList();
    List<String> singletonList = asList("value");
}

After

import java.util.Collections;

class MyClass {
    List<String> emptyList = Collections.emptyList();
    List<String> singletonList = Collections.singletonList("value");
}

Bytecode JDK 1.8

Pre

public void original() {
    List<String> list = Arrays.asList("value");
}

 0 iconst_1
 1 anewarray #2 <java/lang/String>
 4 dup
 5 iconst_0
 6 ldc #3 <value>
 8 aastore
 9 invokestatic #4 <java/util/Arrays.asList>
12 astore_1
13 return

Post

public void transformed() {
    List<String> list = Collections.singletonList("value");
}

0 ldc #3 <value>
2 invokestatic #5 <java/util/Collections.singletonList>
5 astore_1
6 return

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Properties

Property	Value
Rule ID	UseCollectionsSingletonList
First seen in jSparrow version	3.8.0
Minimum Java version	1.3
Remediation cost	2 min