Initial check-in with two inspections working.

src/main/java/de/platon42/intellij/plugins/cajon/inspections/AbstractAssertJInspection.kt

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+package de.platon42.intellij.plugins.cajon.inspections
+
+import com.intellij.codeInspection.AbstractBaseJavaLocalInspectionTool
+import com.intellij.psi.CommonClassNames
+import com.siyeh.ig.callMatcher.CallMatcher
+
+open class AbstractAssertJInspection : AbstractBaseJavaLocalInspectionTool() {
+
+    companion object {
+        private const val ABSTRACT_ASSERT_CLASSNAME = "org.assertj.core.api.AbstractAssert"
+        private const val IS_EQUAL_TO = "isEqualTo"
+        private const val IS_NOT_EQUAL_TO = "isNotEqualTo"
+
+        val IS_EQUAL_TO_OBJECT = CallMatcher.instanceCall(ABSTRACT_ASSERT_CLASSNAME, IS_EQUAL_TO)
+            .parameterTypes(CommonClassNames.JAVA_LANG_OBJECT)!!
+        val IS_NOT_EQUAL_TO_OBJECT = CallMatcher.instanceCall(ABSTRACT_ASSERT_CLASSNAME, IS_NOT_EQUAL_TO)
+            .parameterTypes(CommonClassNames.JAVA_LANG_OBJECT)!!
+    }
+
+    override fun getGroupDisplayName(): String {
+        return "AssertJ"
+    }
+}

src/main/java/de/platon42/intellij/plugins/cajon/inspections/AssertThatObjectIsNotNullInspection.kt

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+package de.platon42.intellij.plugins.cajon.inspections
+
+import com.intellij.codeInspection.LocalQuickFix
+import com.intellij.codeInspection.ProblemDescriptor
+import com.intellij.codeInspection.ProblemHighlightType
+import com.intellij.codeInspection.ProblemsHolder
+import com.intellij.openapi.project.Project
+import com.intellij.openapi.util.TextRange
+import com.intellij.psi.*
+import org.jetbrains.annotations.NonNls
+
+class AssertThatObjectIsNotNullInspection : AbstractAssertJInspection() {
+
+    companion object {
+        @NonNls
+        private val DISPLAY_NAME = "Comparing to non-null"
+
+        @NonNls
+        private val INSPECTION_MESSAGE = "isNotEqualTo(null) can be simplified to isNotNull()"
+
+        @NonNls
+        private val QUICKFIX_DESCRIPTION = "Replace isNotEqualTo(null) with isNotNull()"
+    }
+
+    override fun getDisplayName() = DISPLAY_NAME
+
+    override fun buildVisitor(holder: ProblemsHolder, isOnTheFly: Boolean): PsiElementVisitor {
+        return object : JavaElementVisitor() {
+            override fun visitMethodCallExpression(expression: PsiMethodCallExpression) {
+                super.visitMethodCallExpression(expression)
+                if (!IS_NOT_EQUAL_TO_OBJECT.test(expression)) {
+                    return
+                }
+
+                if (expression.argumentList.expressions[0].type == PsiType.NULL) {
+                    holder.registerProblem(
+                        expression,
+                        INSPECTION_MESSAGE,
+                        ProblemHighlightType.INFORMATION,
+                        null as TextRange?,
+                        ReplaceWithIsNotNullQuickFix()
+                    )
+                }
+            }
+        }
+    }
+
+    private class ReplaceWithIsNotNullQuickFix : LocalQuickFix {
+        override fun getFamilyName() = QUICKFIX_DESCRIPTION
+
+        override fun applyFix(project: Project, descriptor: ProblemDescriptor) {
+            val element = descriptor.startElement
+            val factory = JavaPsiFacade.getElementFactory(element.project)
+            val methodCallExpression = element as? PsiMethodCallExpression ?: return
+            val oldQualifier = methodCallExpression.methodExpression.qualifierExpression ?: return
+            val isNotNullExpression =
+                factory.createExpressionFromText("a.isNotNull()", null) as PsiMethodCallExpression
+            isNotNullExpression.methodExpression.qualifierExpression!!.replace(oldQualifier)
+            element.replace(isNotNullExpression)
+        }
+    }
+}

src/main/java/de/platon42/intellij/plugins/cajon/inspections/AssertThatObjectIsNullInspection.kt

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+package de.platon42.intellij.plugins.cajon.inspections
+
+import com.intellij.codeInspection.LocalQuickFix
+import com.intellij.codeInspection.ProblemDescriptor
+import com.intellij.codeInspection.ProblemHighlightType
+import com.intellij.codeInspection.ProblemsHolder
+import com.intellij.openapi.project.Project
+import com.intellij.openapi.util.TextRange
+import com.intellij.psi.*
+import org.jetbrains.annotations.NonNls
+
+class AssertThatObjectIsNullInspection : AbstractAssertJInspection() {
+
+    companion object {
+        @NonNls
+        private val DISPLAY_NAME = "Comparing to null"
+
+        @NonNls
+        private val INSPECTION_MESSAGE = "isEqualTo(null) can be simplified to isNull()"
+
+        @NonNls
+        private val QUICKFIX_DESCRIPTION = "Replace isEqualTo(null) with isNull()"
+    }
+
+    override fun getDisplayName() = DISPLAY_NAME
+
+    override fun buildVisitor(holder: ProblemsHolder, isOnTheFly: Boolean): PsiElementVisitor {
+        return object : JavaElementVisitor() {
+            override fun visitMethodCallExpression(expression: PsiMethodCallExpression) {
+                super.visitMethodCallExpression(expression)
+                if (!IS_EQUAL_TO_OBJECT.test(expression)) {
+                    return
+                }
+
+                if (expression.argumentList.expressions[0].type == PsiType.NULL) {
+                    holder.registerProblem(
+                        expression,
+                        INSPECTION_MESSAGE,
+                        ProblemHighlightType.INFORMATION,
+                        null as TextRange?,
+                        ReplaceWithIsNullQuickFix()
+                    )
+                }
+            }
+        }
+    }
+
+    private class ReplaceWithIsNullQuickFix : LocalQuickFix {
+        override fun getFamilyName() = QUICKFIX_DESCRIPTION
+
+        override fun applyFix(project: Project, descriptor: ProblemDescriptor) {
+            val element = descriptor.startElement
+            val factory = JavaPsiFacade.getElementFactory(element.project)
+            val methodCallExpression = element as? PsiMethodCallExpression ?: return
+            val oldQualifier = methodCallExpression.methodExpression.qualifierExpression ?: return
+            val isNullExpression =
+                factory.createExpressionFromText("a.isNull()", null) as PsiMethodCallExpression
+            isNullExpression.methodExpression.qualifierExpression!!.replace(oldQualifier)
+            element.replace(isNullExpression)
+        }
+    }
+}

src/main/resources/META-INF/plugin.xml

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+<idea-plugin>
+    <id>de.platon42.cajon</id>
+    <name>Concise AssertJ Optimizing Nitpicker (Cajon)</name>
+    <vendor email="chrisly@platon42.de" url="http://platon42.de">Platon Computer Products</vendor>
+
+    <description><![CDATA[
+    This plugin will add inspections and quick fixes to fully make use of the AssertJ methods
+    to make the intention clear and concise.
+    ]]></description>
+
+    <!-- please see http://www.jetbrains.org/intellij/sdk/docs/basics/getting_started/build_number_ranges.html for description -->
+    <idea-version since-build="182.0"/>
+
+    <!-- please see http://www.jetbrains.org/intellij/sdk/docs/basics/getting_started/plugin_compatibility.html
+         on how to target different products -->
+    <depends>com.intellij.modules.lang</depends>
+    <depends>com.intellij.modules.platform</depends>
+    <depends>com.intellij.modules.java</depends>
+
+    <extensions defaultExtensionNs="com.intellij">
+        <localInspection groupPath="Java" shortName="AssertThatObjectIsNull" enabledByDefault="true" level="WARNING"
+                         implementationClass="de.platon42.intellij.plugins.cajon.inspections.AssertThatObjectIsNullInspection"/>
+        <localInspection groupPath="Java" shortName="AssertThatObjectIsNotNull" enabledByDefault="true" level="WARNING"
+                         implementationClass="de.platon42.intellij.plugins.cajon.inspections.AssertThatObjectIsNotNullInspection"/>
+    </extensions>
+
+    <actions>
+    </actions>
+</idea-plugin>

src/main/resources/inspectionDescriptions/AssertThatObjectIsNotNull.html

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+<html>
+<body>
+Turns assertThat(object).isNotEqualTo(null) into assertThat(object).isNotNull().
+<!-- tooltip end -->
+</body>
+</html>

src/main/resources/inspectionDescriptions/AssertThatObjectIsNull.html

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+<html>
+<body>
+Turns assertThat(object).isEqualTo(null) into assertThat(object).isNull().
+<!-- tooltip end -->
+</body>
+</html>