# 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).


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

# Code Changes

# List with one element


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


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

# Empty List


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


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

# Static import to Arrays.asList


import static java.util.Arrays.asList;

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


import java.util.Collections;

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

# Bytecode JDK 1.8


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


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

Use a Java Refactoring Tool

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You can review this refactoring on your code without a license by installing jSparrow to your Eclipse IDE. Install the plug-in from Eclipse IDE: Eclipse Marketplace.

System-wide Refactoring

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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