001////////////////////////////////////////////////////////////////////////////////
002// checkstyle: Checks Java source code for adherence to a set of rules.
003// Copyright (C) 2001-2021 the original author or authors.
004//
005// This library is free software; you can redistribute it and/or
006// modify it under the terms of the GNU Lesser General Public
007// License as published by the Free Software Foundation; either
008// version 2.1 of the License, or (at your option) any later version.
009//
010// This library is distributed in the hope that it will be useful,
011// but WITHOUT ANY WARRANTY; without even the implied warranty of
012// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
013// Lesser General Public License for more details.
014//
015// You should have received a copy of the GNU Lesser General Public
016// License along with this library; if not, write to the Free Software
017// Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
018////////////////////////////////////////////////////////////////////////////////
019
020package com.puppycrawl.tools.checkstyle.checks.design;
021
022import com.puppycrawl.tools.checkstyle.StatelessCheck;
023import com.puppycrawl.tools.checkstyle.api.AbstractCheck;
024import com.puppycrawl.tools.checkstyle.api.DetailAST;
025import com.puppycrawl.tools.checkstyle.api.TokenTypes;
026
027/**
028 * <p>
029 * Implements Joshua Bloch, Effective Java, Item 17 -
030 * Use Interfaces only to define types.
031 * </p>
032 * <p>
033 * According to Bloch, an interface should describe a <em>type</em>. It is therefore
034 * inappropriate to define an interface that does not contain any methods
035 * but only constants. The Standard interface
036 * <a href="https://docs.oracle.com/javase/8/docs/api/javax/swing/SwingConstants.html">
037 * javax.swing.SwingConstants</a> is an example of an interface that would be flagged by this check.
038 * </p>
039 * <p>
040 * The check can be configured to also disallow marker interfaces like {@code java.io.Serializable},
041 * that do not contain methods or constants at all.
042 * </p>
043 * <ul>
044 * <li>
045 * Property {@code allowMarkerInterfaces} - Control whether marker interfaces
046 * like Serializable are allowed.
047 * Type is {@code boolean}.
048 * Default value is {@code true}.
049 * </li>
050 * </ul>
051 * <p>
052 * To configure the check:
053 * </p>
054 * <pre>
055 * &lt;module name="InterfaceIsType"/&gt;
056 * </pre>
057 * <p>Example:</p>
058 * <pre>
059 * public interface Test1 { // violation
060 *     int a = 3;
061 *
062 * }
063 *
064 * public interface Test2 { // OK
065 *
066 * }
067 *
068 * public interface Test3 { // OK
069 *     int a = 3;
070 *     void test();
071 * }
072 * </pre>
073 * <p>
074 * To configure the check to report violation so that it doesn't allow Marker Interfaces:
075 * </p>
076 * <pre>
077 * &lt;module name=&quot;InterfaceIsType&quot;&gt;
078 *   &lt;property name=&quot;allowMarkerInterfaces&quot; value=&quot;false&quot;/&gt;
079 * &lt;/module&gt;
080 * </pre>
081 * <p>Example:</p>
082 * <pre>
083 * public interface Test1 { // violation
084 *     int a = 3;
085 * }
086 *
087 * public interface Test2 { // violation
088 *
089 * }
090 * </pre>
091 * <p>
092 * Parent is {@code com.puppycrawl.tools.checkstyle.TreeWalker}
093 * </p>
094 * <p>
095 * Violation Message Keys:
096 * </p>
097 * <ul>
098 * <li>
099 * {@code interface.type}
100 * </li>
101 * </ul>
102 *
103 * @since 3.1
104 */
105@StatelessCheck
106public final class InterfaceIsTypeCheck
107        extends AbstractCheck {
108
109    /**
110     * A key is pointing to the warning message text in "messages.properties"
111     * file.
112     */
113    public static final String MSG_KEY = "interface.type";
114
115    /** Control whether marker interfaces like Serializable are allowed. */
116    private boolean allowMarkerInterfaces = true;
117
118    @Override
119    public int[] getDefaultTokens() {
120        return getRequiredTokens();
121    }
122
123    @Override
124    public int[] getRequiredTokens() {
125        return new int[] {TokenTypes.INTERFACE_DEF};
126    }
127
128    @Override
129    public int[] getAcceptableTokens() {
130        return getRequiredTokens();
131    }
132
133    @Override
134    public void visitToken(DetailAST ast) {
135        final DetailAST objBlock =
136                ast.findFirstToken(TokenTypes.OBJBLOCK);
137        final DetailAST methodDef =
138                objBlock.findFirstToken(TokenTypes.METHOD_DEF);
139        final DetailAST variableDef =
140                objBlock.findFirstToken(TokenTypes.VARIABLE_DEF);
141        final boolean methodRequired =
142                !allowMarkerInterfaces || variableDef != null;
143
144        if (methodDef == null && methodRequired) {
145            log(ast, MSG_KEY);
146        }
147    }
148
149    /**
150     * Setter to control whether marker interfaces like Serializable are allowed.
151     *
152     * @param flag whether to allow marker interfaces or not
153     */
154    public void setAllowMarkerInterfaces(boolean flag) {
155        allowMarkerInterfaces = flag;
156    }
157
158}