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SONARJAVA-5602 implement UseIsEmptyToTestEmptinessOfStringBuilder (#5188)
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‎its/autoscan/src/test/resources/autoscan/diffs/diff_S3033.json‎

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{
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"ruleKey": "S3033",
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"hasTruePositives": true,
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"falseNegatives": 0,
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"falsePositives": 0
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}

‎java-checks-test-sources/default/src/main/files/non-compiling/java/lang/StringBuilder.java‎

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package java.lang;
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public class StringBuilder {
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@Override
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public String toString() {
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return "";
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}
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public boolean isEmpty() {
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return true;
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}
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public void samples(){
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boolean b = toString().isEmpty(); // FN, we do not raise inside StringBuilder or StringBuffer
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}
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}

‎java-checks-test-sources/default/src/main/java/checks/UseIsEmptyToTestEmptinessOfStringBuilderCheckSample.java‎

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package checks;
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public class UseIsEmptyToTestEmptinessOfStringBuilderCheckSample {
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void noncompliantStringBuilder() {
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StringBuilder sb = new StringBuilder();
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if ("".equals(sb.toString())) { // Noncompliant {{Replace "equals()" with "isEmpty()".}}
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// ^^^^^^^^^^^^^^^^^^^^^^^^
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System.out.println("Empty StringBuilder");
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}
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if (sb.toString().isEmpty()) { // Noncompliant {{Replace "toString().isEmpty()" with "isEmpty()".}} [[quickfixes=qf1]]
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// fix@qf1 {{Replace with "isEmpty()"}}
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// edit@qf1 [[sc=11;ec=32]] {{.isEmpty()}}
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System.out.println("Empty StringBuilder");
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}
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if (sb.toString().length() == 0) { // Noncompliant {{Replace "toString().length()" with "isEmpty()".}}
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// ^^^^^^^^^^^^^^^^^^^^^^
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System.out.println("Empty StringBuilder");
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}
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if (sb.toString().length() > 0) { // FN
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}
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if (sb.toString().length() < 1) { // FN
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}
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if (!"".equals(sb.toString())) { // Noncompliant
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System.out.println("Non-empty StringBuilder");
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}
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boolean inExpression = "".equals(sb.toString()); // Noncompliant
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if (sb.toString().equals("")) { // Noncompliant {{Replace "equals()" with "isEmpty()".}}
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// ^^^^^^^^^^^^^^^^^^^^^^^^
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System.out.println("Empty StringBuilder using equals reversed");
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}
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if (!sb.toString().equals("")) { // Noncompliant
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System.out.println("Non-empty StringBuilder using equals reversed");
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}
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}
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void quickFix(){
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StringBuilder sb = new StringBuilder();
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var x = sb.toString().isEmpty() && false; // Noncompliant [[quickfixes=qf2]]
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// fix@qf2 {{Replace with "isEmpty()"}}
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// edit@qf2 [[sc=15;ec=36]] {{.isEmpty()}}
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}
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void consume(String s) {
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}
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void noncompliantStringBuffer() {
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StringBuffer sb = new StringBuffer();
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if ("".equals(sb.toString())) { // Noncompliant
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System.out.println("Empty StringBuffer");
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}
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if (sb.toString().isEmpty()) { // Noncompliant
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System.out.println("Empty StringBuffer");
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}
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}
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void compliantStringBuilder() {
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StringBuilder sb = new StringBuilder();
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if (sb.isEmpty()) { // Compliant
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System.out.println("Empty StringBuilder");
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}
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if (!sb.isEmpty()) { // Compliant
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System.out.println("Non-empty StringBuilder");
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}
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boolean expression = sb.isEmpty(); // Compliant
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if (sb.length() == 0) { // Compliant
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System.out.println("Empty StringBuilder");
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}
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boolean isName = "name".equals(sb.toString());
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boolean singleLetter = sb.toString().length() == 1;
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boolean greaterThan = sb.toString().length() > 1;
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String substring = sb.toString().substring(0, 1);
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String sbString = sb.toString();
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if ("".equals(sbString)) { // Compliant, we do not support variables assignment
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System.out.println("Empty string");
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}
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if (sb.isEmpty()) { // Compliant, we do not support variables assignment
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System.out.println("Empty string");
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}
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}
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void compliantStringBuffer() {
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StringBuffer sb = new StringBuffer();
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if (sb.isEmpty()) { // Compliant
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System.out.println("Empty StringBuffer");
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}
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if (!sb.isEmpty()) { // Compliant
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System.out.println("Non-empty StringBuffer");
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}
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}
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void complexExpressions() {
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if (getStringBuilder().toString().isEmpty()) { // Noncompliant
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System.out.println("Empty");
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}
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if ("".equals(getStringBuilder().toString())) { // Noncompliant
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System.out.println("Empty");
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}
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StringBuilder sb = new StringBuilder();
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if (sb.append("hello").toString().isEmpty()) { // Noncompliant
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System.out.println("Empty after append");
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}
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if ((1 == 2 ? sb : new StringBuilder()).toString().isEmpty()) { // Noncompliant
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System.out.println("Empty after append");
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}
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if (getStringBuilder().isEmpty()) { // Compliant
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System.out.println("Empty");
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}
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}
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StringBuilder getStringBuilder() {
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return new StringBuilder("test");
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}
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}

‎java-checks/src/main/java/org/sonar/java/checks/UseIsEmptyToTestEmptinessOfStringBuilderCheck.java‎

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/*
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* SonarQube Java
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* Copyright (C) 2012-2025 SonarSource SA
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* mailto:info AT sonarsource DOT com
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*
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* This program is free software; you can redistribute it and/or
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* modify it under the terms of the Sonar Source-Available License Version 1, as published by SonarSource SA.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
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* See the Sonar Source-Available License for more details.
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*
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* You should have received a copy of the Sonar Source-Available License
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* along with this program; if not, see https://sonarsource.com/license/ssal/
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*/
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package org.sonar.java.checks;
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import java.util.List;
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import java.util.Optional;
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import org.sonar.check.Rule;
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import org.sonar.java.ast.parser.ArgumentListTreeImpl;
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import org.sonar.java.checks.helpers.QuickFixHelper;
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import org.sonar.java.checks.methods.AbstractMethodDetection;
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import org.sonar.java.model.LiteralUtils;
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import org.sonar.java.reporting.JavaQuickFix;
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import org.sonar.java.reporting.JavaTextEdit;
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import org.sonar.plugins.java.api.JavaVersion;
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import org.sonar.plugins.java.api.JavaVersionAwareVisitor;
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import org.sonar.plugins.java.api.semantic.MethodMatchers;
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import org.sonar.plugins.java.api.tree.BinaryExpressionTree;
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import org.sonar.plugins.java.api.tree.MemberSelectExpressionTree;
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import org.sonar.plugins.java.api.tree.MethodInvocationTree;
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import org.sonar.plugins.java.api.tree.Tree;
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import static org.sonar.java.model.LiteralUtils.isEmptyString;
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@Rule(key = "S3033")
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public class UseIsEmptyToTestEmptinessOfStringBuilderCheck extends AbstractMethodDetection implements JavaVersionAwareVisitor {
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private static final String JAVA_LANG_STRING = "java.lang.String";
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private static final MethodMatchers TO_STRING = MethodMatchers.create()
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.ofTypes("java.lang.StringBuilder", "java.lang.StringBuffer")
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.names("toString")
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.withAnyParameters()
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.build();
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private static final MethodMatchers STRING_EQUALS = MethodMatchers.create()
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.ofTypes(JAVA_LANG_STRING)
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.names("equals")
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.withAnyParameters()
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.build();
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private static final MethodMatchers STRING_IS_EMPTY = MethodMatchers.create()
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.ofTypes(JAVA_LANG_STRING)
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.names("isEmpty")
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.withAnyParameters()
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.build();
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private static final MethodMatchers STRING_LENGTH = MethodMatchers.create()
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.ofTypes(JAVA_LANG_STRING)
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.names("length")
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.withAnyParameters()
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.build();
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private static final String MESSAGE_EQUALS = "Replace \"equals()\" with \"isEmpty()\".";
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private static final String MESSAGE_IS_EMPTY = "Replace \"toString().isEmpty()\" with \"isEmpty()\".";
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private static final String MESSAGE_LENGTH = "Replace \"toString().length()\" with \"isEmpty()\".";
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@Override
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protected MethodMatchers getMethodInvocationMatchers() {
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return TO_STRING;
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}
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@Override
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public boolean isCompatibleWithJavaVersion(JavaVersion version) {
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return version.isJava15Compatible();
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}
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@Override
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protected void onMethodInvocationFound(MethodInvocationTree toStringInvocation) {
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MethodInvocationTree mit = argumentSide(toStringInvocation)
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.or(() -> methodSelectSide(toStringInvocation))
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.orElse(null);
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if (mit == null) {
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return;
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}
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if (STRING_EQUALS.matches(mit) && isEqualsWithEmptyString(mit)) {
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reportIssue(mit, MESSAGE_EQUALS);
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} else if (STRING_IS_EMPTY.matches(mit) && toStringInvocation.methodSelect() instanceof MemberSelectExpressionTree sel) {
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var operator = sel.operatorToken();
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var edit = JavaTextEdit.replaceBetweenTree(operator, mit, ".isEmpty()");
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QuickFixHelper.newIssue(context)
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.forRule(this)
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.onTree(mit)
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.withMessage(MESSAGE_IS_EMPTY)
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.withQuickFixes(() -> List.of(
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JavaQuickFix.newQuickFix("Replace with \"isEmpty()\"").addTextEdit(edit).build()
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))
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.report();
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} else if (STRING_LENGTH.matches(mit) && isComparedToZero(mit)) {
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reportIssue(mit, MESSAGE_LENGTH);
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}
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}
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// example: "".equals(sb.toString()) and toStringInvocation=sb.toString() -> "".equals(sb.toString())
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private static Optional<MethodInvocationTree> argumentSide(MethodInvocationTree toStringInvocation) {
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return Optional.ofNullable(toStringInvocation.parent())
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.filter(ArgumentListTreeImpl.class::isInstance)
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.map(ArgumentListTreeImpl.class::cast)
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.map(ArgumentListTreeImpl::parent)
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.filter(MethodInvocationTree.class::isInstance)
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.map(MethodInvocationTree.class::cast);
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}
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// example: sb.toString().equals("") and toStringInvocation=sb.toString() -> sb.toString().equals("")
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private static Optional<MethodInvocationTree> methodSelectSide(MethodInvocationTree toStringInvocation) {
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return Optional.ofNullable(toStringInvocation.parent())
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.filter(MemberSelectExpressionTree.class::isInstance)
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.map(MemberSelectExpressionTree.class::cast)
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.map(MemberSelectExpressionTree::parent)
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.filter(MethodInvocationTree.class::isInstance)
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.map(MethodInvocationTree.class::cast);
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}
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private static boolean isEqualsWithEmptyString(MethodInvocationTree equalsInvocation) {
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Tree arg = equalsInvocation.arguments().get(0);
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return isEmptyString(arg) ||
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(equalsInvocation.methodSelect() instanceof MemberSelectExpressionTree sel && isEmptyString(sel.expression()));
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}
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private static boolean isComparedToZero(MethodInvocationTree lengthInvocation) {
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Tree parent = lengthInvocation.parent();
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if (parent != null && parent.is(Tree.Kind.EQUAL_TO, Tree.Kind.NOT_EQUAL_TO)) {
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BinaryExpressionTree binary = (BinaryExpressionTree) parent;
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return LiteralUtils.isZero(binary.rightOperand()) || LiteralUtils.isZero(binary.leftOperand());
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}
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return false;
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}
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}

‎java-checks/src/test/java/org/sonar/java/checks/UseIsEmptyToTestEmptinessOfStringBuilderCheckTest.java‎

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/*
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* SonarQube Java
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* Copyright (C) 2012-2025 SonarSource SA
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* mailto:info AT sonarsource DOT com
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*
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* This program is free software; you can redistribute it and/or
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* modify it under the terms of the Sonar Source-Available License Version 1, as published by SonarSource SA.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
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* See the Sonar Source-Available License for more details.
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*
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* You should have received a copy of the Sonar Source-Available License
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* along with this program; if not, see https://sonarsource.com/license/ssal/
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*/
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package org.sonar.java.checks;
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import org.junit.jupiter.api.Test;
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import org.sonar.java.checks.verifier.CheckVerifier;
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import static org.sonar.java.checks.verifier.TestUtils.mainCodeSourcesPath;
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import static org.sonar.java.checks.verifier.TestUtils.nonCompilingTestSourcesPath;
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class UseIsEmptyToTestEmptinessOfStringBuilderCheckTest {
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@Test
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void test() {
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CheckVerifier.newVerifier()
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.onFile(mainCodeSourcesPath("checks/UseIsEmptyToTestEmptinessOfStringBuilderCheckSample.java"))
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.withCheck(new UseIsEmptyToTestEmptinessOfStringBuilderCheck())
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.withJavaVersion(15)
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.verifyIssues();
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}
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@Test
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void string_builder() {
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CheckVerifier.newVerifier()
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.onFile(nonCompilingTestSourcesPath("java/lang/StringBuilder.java"))
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.withCheck(new UseIsEmptyToTestEmptinessOfStringBuilderCheck())
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.withJavaVersion(15)
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.verifyNoIssues();
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}
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@Test
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void no_semantic() {
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CheckVerifier.newVerifier()
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.onFile(mainCodeSourcesPath("checks/UseIsEmptyToTestEmptinessOfStringBuilderCheckSample.java"))
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.withCheck(new UseIsEmptyToTestEmptinessOfStringBuilderCheck())
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.withoutSemantic()
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.withJavaVersion(15)
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.verifyIssues();
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}
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@Test
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void below_java_15() {
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CheckVerifier.newVerifier()
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.onFile(mainCodeSourcesPath("checks/UseIsEmptyToTestEmptinessOfStringBuilderCheckSample.java"))
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.withCheck(new UseIsEmptyToTestEmptinessOfStringBuilderCheck())
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.withJavaVersion(14)
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.verifyNoIssues();
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}
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}

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