Allow options for != ternary check

Provide more configuration options for `SpringTernaryCheck` and change
the default to only ban `==` when comparing to `null`.

Closes spring-iogh-46

spring-javaformat/spring-javaformat-checkstyle/src/main/java/io/spring/javaformat/checkstyle/check/SpringTernaryCheck.java

@@ -16,6 +16,8 @@
 
 package io.spring.javaformat.checkstyle.check;
 
+import java.util.Locale;
+
 import com.puppycrawl.tools.checkstyle.api.AbstractCheck;
 import com.puppycrawl.tools.checkstyle.api.DetailAST;
 import com.puppycrawl.tools.checkstyle.api.TokenTypes;
@@ -29,6 +31,8 @@
  */
 public class SpringTernaryCheck extends AbstractCheck {
 
+	private EqualsTest equalsTest = EqualsTest.NEVER_FOR_NULLS;
+
 	@Override
 	public int[] getDefaultTokens() {
 		return getAcceptableTokens();
@@ -61,7 +65,7 @@ private void visitQuestion(DetailAST ast) {
 				log(ast.getLineNo(), ast.getColumnNo(), "ternary.missingParen");
 			}
 		}
-		if (hasType(ast.getFirstChild(), TokenTypes.EQUAL)) {
+		if (hasType(ast.getFirstChild(), TokenTypes.EQUAL) && !isEqualsTestAllowed(ast)) {
 			log(ast.getLineNo(), ast.getColumnNo(), "ternary.equalOperator");
 		}
 	}
@@ -73,8 +77,53 @@ private boolean isAllowedGrandParent(DetailAST grandParent) {
 				|| hasType(grandParent, TokenTypes.LITERAL_WHILE);
 	}
 
+	private boolean isEqualsTestAllowed(DetailAST ast) {
+		switch (this.equalsTest) {
+		case ANY:
+			return true;
+		case NEVER:
+			return false;
+		case NEVER_FOR_NULLS:
+			DetailAST equal = ast.findFirstToken(TokenTypes.EQUAL);
+			return equal.findFirstToken(TokenTypes.LITERAL_NULL) == null;
+		}
+		throw new IllegalStateException("Unsupported equals test " + this.equalsTest);
+	}
+
 	private boolean hasType(DetailAST ast, int type) {
 		return (ast != null && ast.getType() == type);
 	}
 
+	public void setEqualsTest(String equalsTest) {
+		try {
+			this.equalsTest = Enum.valueOf(EqualsTest.class,
+					equalsTest.trim().toUpperCase(Locale.ENGLISH));
+		}
+		catch (final IllegalArgumentException ex) {
+			throw new IllegalArgumentException("unable to parse " + equalsTest, ex);
+		}
+	}
+
+	/**
+	 * Type of equals operators allowed in the test condition.
+	 */
+	public enum EqualsTest {
+
+		/**
+		 * Equals checks can be used for any test.
+		 */
+		ANY,
+
+		/**
+		 * Equals tests can never be used.
+		 */
+		NEVER,
+
+		/**
+		 * Equals tests can never be used for {@code null} checks.
+		 */
+		NEVER_FOR_NULLS
+
+	}
+
 }

spring-javaformat/spring-javaformat-checkstyle/src/test/resources/check/TernaryEqualsEqualsAny.txt

@@ -0,0 +1 @@
++0 errors

spring-javaformat/spring-javaformat-checkstyle/src/test/resources/check/TernaryEqualsEqualsNever.txt

@@ -0,0 +1,2 @@
++TernaryEqualsEqualsNever.java:27:33: Ternary operation should use != when testing. [SpringTernary]
++1 error

spring-javaformat/spring-javaformat-checkstyle/src/test/resources/config/TernaryEqualsEqualsAny.xml

@@ -0,0 +1,9 @@
+<?xml version="1.0"?>
+<!DOCTYPE module PUBLIC "-//Puppy Crawl//DTD Check Configuration 1.2//EN" "http://www.puppycrawl.com/dtds/configuration_1_2.dtd">
+<module name="com.puppycrawl.tools.checkstyle.Checker">
+	<module name="com.puppycrawl.tools.checkstyle.TreeWalker">
+		<module name="io.spring.javaformat.checkstyle.check.SpringTernaryCheck">
+			<property name="equalsTest" value="any"/>
+		</module>
+ 	</module>
+</module>

spring-javaformat/spring-javaformat-checkstyle/src/test/resources/config/TernaryEqualsEqualsNever.xml

@@ -0,0 +1,9 @@
+<?xml version="1.0"?>
+<!DOCTYPE module PUBLIC "-//Puppy Crawl//DTD Check Configuration 1.2//EN" "http://www.puppycrawl.com/dtds/configuration_1_2.dtd">
+<module name="com.puppycrawl.tools.checkstyle.Checker">
+	<module name="com.puppycrawl.tools.checkstyle.TreeWalker">
+		<module name="io.spring.javaformat.checkstyle.check.SpringTernaryCheck">
+			<property name="equalsTest" value="never"/>
+		</module>
+ 	</module>
+</module>

spring-javaformat/spring-javaformat-checkstyle/src/test/resources/source/TernaryEqualsEquals.java

@@ -15,7 +15,7 @@
  */
 
 /**
- * This is a valid example of a ternary expression.
+ * This is an invalid example of a ternary expression.
  *
  * @author Phillip Webb
  */
@@ -24,7 +24,12 @@ public class TernaryEqualsEquals {
 	public void test() {
 		boolean a = true;
 		boolean b = false;
-		int c = (a == b ? 1 : 2);
+		int c = (a != b ? 1 : 2);
+	}
+
+	public void test2() {
+		Boolean a = true;
+		int c = (a == null ? 1 : 2);
 	}
 
 }

spring-javaformat/spring-javaformat-checkstyle/src/test/resources/source/TernaryEqualsEqualsAny.java

@@ -0,0 +1,35 @@
+/*
+ * Copyright 2017-2018 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/**
+ * This is an invalid example of a ternary expression.
+ *
+ * @author Phillip Webb
+ */
+public class TernaryEqualsEqualsAlways {
+
+	public void test() {
+		boolean a = true;
+		boolean b = false;
+		int c = (a != b ? 1 : 2);
+	}
+
+	public void test2() {
+		Boolean a = true;
+		int c = (a == null ? 1 : 2);
+	}
+
+}

spring-javaformat/spring-javaformat-checkstyle/src/test/resources/source/TernaryEqualsEqualsNever.java

@@ -0,0 +1,30 @@
+/*
+ * Copyright 2017-2018 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/**
+ * This is a valid example of a ternary expression.
+ *
+ * @author Phillip Webb
+ */
+public class TernaryEqualsEqualsNever {
+
+	public void test() {
+		boolean a = true;
+		boolean b = false;
+		int c = (a == b ? 1 : 2);
+	}
+
+}