# Replace For-Loop with Stream::collect(Collectors.joining())

# Properties

Property Value
Rule ID StringBuildingLoop
First seen in jSparrow version 2.2.0
Minimum Java version 5
Remediation cost 10 min
Links S1643

# Description

This rule refactors the enhanced for loops which are only being used for concatenating the elements of collections or arrays. If the Java level is at least 8, the whole loop is transformed into a stream and Stream::collect(Collector) is used for computing the result. If the Java level is below 8 and at least 5, then a StringBuilder is used instead of the direct concatenation of strings in the loop body.

# Benefits

Strings are immutable objects, so concatenation doesn't simply add the new String to the end of the existing string. Instead, in each loop iteration, the first String is converted to an intermediate object type, the second string is appended, and then the intermediate object is converted back to a String. Performing these operations degrades the longer the String is. Using a StringBuilder instead increases performance.

# Code Changes

# At least Java 8

Pre

List<String> collectionOfStrings = generateStringList(input);
String result = "";
for(String val : collectionOfStrings) {
    result = result + val;
}

Post

List<String> collectionOfStrings = generateStringList(input);
String result = collectionOfStrings.stream().collect(Collectors.joining());

# Below Java 8

Pre

List<String> collectionOfStrings = generateStringList(input);
String result = "";
for(String val : collectionOfStrings) {
    result = result + val;
}

Post

List<String> collectionOfStrings = generateStringList(input);
StringBuilder resultSb = new StringBuilder();
for(String val : collectionOfStrings) {
    resultSb.append(val);
}
String result = resultSb.toString();

# Bytecode JDK 1.8 (Optional)

Pre

public String original(List<String> values) {
    String result = "";
    for(String val : values) {
        result = result + val;
    }
    return result;
}
 0 ldc #2
 2 astore_2
 3 aload_1
 4 invokeinterface #3 <java/util/List.iterator> count 1
 9 astore_3
10 aload_3
11 invokeinterface #4 <java/util/Iterator.hasNext> count 1
16 ifeq 53 (+37)
19 aload_3
20 invokeinterface #5 <java/util/Iterator.next> count 1
25 checkcast #6 <java/lang/String>
28 astore 4
30 new #7 <java/lang/StringBuilder>
33 dup
34 invokespecial #8 <java/lang/StringBuilder.<init>>
37 aload_2
38 invokevirtual #9 <java/lang/StringBuilder.append>
41 aload 4
43 invokevirtual #9 <java/lang/StringBuilder.append>
46 invokevirtual #10 <java/lang/StringBuilder.toString>
49 astore_2
50 goto 10 (-40)
53 aload_2
54 areturn

Post

public String transformed(List<String>values) {
    String result = values.stream().collect(Collectors.joining());
    return result;
}
 0 aload_1
 1 invokeinterface #11 <java/util/List.stream> count 1
 6 invokestatic #12 <java/util/stream/Collectors.joining>
 9 invokeinterface #13 <java/util/stream/Stream.collect> count 2
14 checkcast #6 <java/lang/String>
17 astore_2
18 aload_2
19 areturn

Automatic Application of This Rule

The automatic application of this rule is supported in the following jSparrow version:

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