Use StringJoiner where possible to simplify String joining

spring-core/src/main/java/org/springframework/util/ObjectUtils.java

@@ -21,6 +21,7 @@ import java.util.Arrays;
 import java.util.Collection;
 import java.util.Map;
 import java.util.Optional;
+import java.util.StringJoiner;
 
 import org.springframework.lang.Nullable;
 
@@ -709,18 +710,11 @@ public abstract class ObjectUtils {
 		if (length == 0) {
 			return EMPTY_ARRAY;
 		}
-		StringBuilder sb = new StringBuilder();
-		for (int i = 0; i < length; i++) {
-			if (i == 0) {
-				sb.append(ARRAY_START);
+		StringJoiner sj = new StringJoiner(ARRAY_ELEMENT_SEPARATOR, ARRAY_START, ARRAY_END);
+		for (Object o : array) {
+			sj.add(String.valueOf(o));
 		}
-			else {
-				sb.append(ARRAY_ELEMENT_SEPARATOR);
-			}
-			sb.append(String.valueOf(array[i]));
-		}
-		sb.append(ARRAY_END);
-		return sb.toString();
+		return sj.toString();
 	}
 
 	/**

spring-expression/src/main/java/org/springframework/expression/spel/ast/InlineList.java

@@ -19,6 +19,7 @@ package org.springframework.expression.spel.ast;
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
+import java.util.StringJoiner;
 
 import org.springframework.asm.MethodVisitor;
 import org.springframework.expression.EvaluationException;
@@ -102,17 +103,13 @@ public class InlineList extends SpelNodeImpl {
 
 	@Override
 	public String toStringAST() {
-		StringBuilder sb = new StringBuilder("{");
+		StringJoiner sj = new StringJoiner(",", "{", "}");
 		// String ast matches input string, not the 'toString()' of the resultant collection, which would use []
 		int count = getChildCount();
 		for (int c = 0; c < count; c++) {
-			if (c > 0) {
-				sb.append(",");
+			sj.add(getChild(c).toStringAST());
 		}
-			sb.append(getChild(c).toStringAST());
-		}
-		sb.append("}");
-		return sb.toString();
+		return sj.toString();
 	}
 
 	/**

spring-web/src/main/java/org/springframework/http/HttpHeaders.java

@@ -633,15 +633,11 @@ public class HttpHeaders implements MultiValueMap<String, String>, Serializable
 	 * as specified by the {@code Accept-Charset} header.
 	 */
 	public void setAcceptCharset(List<Charset> acceptableCharsets) {
-		StringBuilder builder = new StringBuilder();
-		for (Iterator<Charset> iterator = acceptableCharsets.iterator(); iterator.hasNext();) {
-			Charset charset = iterator.next();
-			builder.append(charset.name().toLowerCase(Locale.ENGLISH));
-			if (iterator.hasNext()) {
-				builder.append(", ");
+		StringJoiner joiner = new StringJoiner(", ");
+		for (Charset charset : acceptableCharsets) {
+			joiner.add(charset.name().toLowerCase(Locale.ENGLISH));
 		}
-		}
-		set(ACCEPT_CHARSET, builder.toString());
+		set(ACCEPT_CHARSET, joiner.toString());
 	}
 
 	/**

spring-webflux/src/main/java/org/springframework/web/reactive/result/condition/AbstractRequestCondition.java

@@ -17,7 +17,7 @@
 package org.springframework.web.reactive.result.condition;
 
 import java.util.Collection;
-import java.util.Iterator;
+import java.util.StringJoiner;
 
 import org.springframework.lang.Nullable;
 
@@ -50,16 +50,12 @@ public abstract class AbstractRequestCondition<T extends AbstractRequestConditio
 
 	@Override
 	public String toString() {
-		StringBuilder builder = new StringBuilder("[");
-		for (Iterator<?> iterator = getContent().iterator(); iterator.hasNext();) {
-			Object expression = iterator.next();
-			builder.append(expression.toString());
-			if (iterator.hasNext()) {
-				builder.append(getToStringInfix());
+		String infix = getToStringInfix();
+		StringJoiner joiner = new StringJoiner(infix, "[", "]");
+		for (Object expression : getContent()) {
+			joiner.add(expression.toString());
 		}
-		}
-		builder.append("]");
-		return builder.toString();
+		return joiner.toString();
 	}
 
 	/**

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/condition/AbstractRequestCondition.java

@@ -17,7 +17,7 @@
 package org.springframework.web.servlet.mvc.condition;
 
 import java.util.Collection;
-import java.util.Iterator;
+import java.util.StringJoiner;
 
 import org.springframework.lang.Nullable;
 
@@ -74,16 +74,12 @@ public abstract class AbstractRequestCondition<T extends AbstractRequestConditio
 
 	@Override
 	public String toString() {
-		StringBuilder builder = new StringBuilder("[");
-		for (Iterator<?> iterator = getContent().iterator(); iterator.hasNext();) {
-			Object expression = iterator.next();
-			builder.append(expression.toString());
-			if (iterator.hasNext()) {
-				builder.append(getToStringInfix());
+		String infix = getToStringInfix();
+		StringJoiner joiner = new StringJoiner(infix, "[", "]");
+		for (Object expression : getContent()) {
+			joiner.add(expression.toString());
 		}
-		}
-		builder.append("]");
-		return builder.toString();
+		return joiner.toString();
 	}
 
 }