1 ///////////////////////////////////////////////////////////////////////////////////////////////
2 // checkstyle: Checks Java source code and other text files for adherence to a set of rules.
3 // Copyright (C) 2001-2026 the original author or authors.
4 //
5 // This library is free software; you can redistribute it and/or
6 // modify it under the terms of the GNU Lesser General Public
7 // License as published by the Free Software Foundation; either
8 // version 2.1 of the License, or (at your option) any later version.
9 //
10 // This library is distributed in the hope that it will be useful,
11 // but WITHOUT ANY WARRANTY; without even the implied warranty of
12 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
13 // Lesser General Public License for more details.
14 //
15 // You should have received a copy of the GNU Lesser General Public
16 // License along with this library; if not, write to the Free Software
17 // Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
18 ///////////////////////////////////////////////////////////////////////////////////////////////
19
20 package com.puppycrawl.tools.checkstyle.checks.design;
21
22 import com.puppycrawl.tools.checkstyle.StatelessCheck;
23 import com.puppycrawl.tools.checkstyle.api.AbstractCheck;
24 import com.puppycrawl.tools.checkstyle.api.DetailAST;
25 import com.puppycrawl.tools.checkstyle.api.TokenTypes;
26
27 /**
28 * <div>
29 * Implements Joshua Bloch, Effective Java, Item 17 -
30 * Use Interfaces only to define types.
31 * </div>
32 *
33 * <p>
34 * According to Bloch, an interface should describe a <em>type</em>. It is therefore
35 * inappropriate to define an interface that does not contain any methods
36 * but only constants. The Standard interface
37 * <a href="https://docs.oracle.com/javase/8/docs/api/javax/swing/SwingConstants.html">
38 * javax.swing.SwingConstants</a> is an example of an interface that would be flagged by this check.
39 * </p>
40 *
41 * <p>
42 * The check can be configured to also disallow marker interfaces like {@code java.io.Serializable},
43 * that do not contain methods or constants at all.
44 * </p>
45 *
46 * @since 3.1
47 */
48 @StatelessCheck
49 public final class InterfaceIsTypeCheck
50 extends AbstractCheck {
51
52 /**
53 * A key is pointing to the warning message text in "messages.properties"
54 * file.
55 */
56 public static final String MSG_KEY = "interface.type";
57
58 /** Control whether marker interfaces like Serializable are allowed. */
59 private boolean allowMarkerInterfaces = true;
60
61 /**
62 * Creates a new {@code InterfaceIsTypeCheck} instance.
63 */
64 public InterfaceIsTypeCheck() {
65 // no code by default
66 }
67
68 @Override
69 public int[] getDefaultTokens() {
70 return getRequiredTokens();
71 }
72
73 @Override
74 public int[] getRequiredTokens() {
75 return new int[] {TokenTypes.INTERFACE_DEF};
76 }
77
78 @Override
79 public int[] getAcceptableTokens() {
80 return getRequiredTokens();
81 }
82
83 @Override
84 public void visitToken(DetailAST ast) {
85 final DetailAST objBlock =
86 ast.findFirstToken(TokenTypes.OBJBLOCK);
87 final DetailAST methodDef =
88 objBlock.findFirstToken(TokenTypes.METHOD_DEF);
89 final DetailAST variableDef =
90 objBlock.findFirstToken(TokenTypes.VARIABLE_DEF);
91 final boolean methodRequired =
92 !allowMarkerInterfaces || variableDef != null;
93
94 if (methodDef == null && methodRequired) {
95 log(ast, MSG_KEY);
96 }
97 }
98
99 /**
100 * Setter to control whether marker interfaces like Serializable are allowed.
101 *
102 * @param flag whether to allow marker interfaces or not
103 * @since 3.1
104 */
105 public void setAllowMarkerInterfaces(boolean flag) {
106 allowMarkerInterfaces = flag;
107 }
108
109 }