001/////////////////////////////////////////////////////////////////////////////////////////////// 002// checkstyle: Checks Java source code and other text files for adherence to a set of rules. 003// Copyright (C) 2001-2025 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 * <div> 029 * Implements Joshua Bloch, Effective Java, Item 17 - 030 * Use Interfaces only to define types. 031 * </div> 032 * 033 * <p> 034 * According to Bloch, an interface should describe a <em>type</em>. It is therefore 035 * inappropriate to define an interface that does not contain any methods 036 * but only constants. The Standard interface 037 * <a href="https://docs.oracle.com/javase/8/docs/api/javax/swing/SwingConstants.html"> 038 * javax.swing.SwingConstants</a> is an example of an interface that would be flagged by this check. 039 * </p> 040 * 041 * <p> 042 * The check can be configured to also disallow marker interfaces like {@code java.io.Serializable}, 043 * that do not contain methods or constants at all. 044 * </p> 045 * <ul> 046 * <li> 047 * Property {@code allowMarkerInterfaces} - Control whether marker interfaces 048 * like Serializable are allowed. 049 * Type is {@code boolean}. 050 * Default value is {@code true}. 051 * </li> 052 * </ul> 053 * 054 * <p> 055 * Parent is {@code com.puppycrawl.tools.checkstyle.TreeWalker} 056 * </p> 057 * 058 * <p> 059 * Violation Message Keys: 060 * </p> 061 * <ul> 062 * <li> 063 * {@code interface.type} 064 * </li> 065 * </ul> 066 * 067 * @since 3.1 068 */ 069@StatelessCheck 070public final class InterfaceIsTypeCheck 071 extends AbstractCheck { 072 073 /** 074 * A key is pointing to the warning message text in "messages.properties" 075 * file. 076 */ 077 public static final String MSG_KEY = "interface.type"; 078 079 /** Control whether marker interfaces like Serializable are allowed. */ 080 private boolean allowMarkerInterfaces = true; 081 082 @Override 083 public int[] getDefaultTokens() { 084 return getRequiredTokens(); 085 } 086 087 @Override 088 public int[] getRequiredTokens() { 089 return new int[] {TokenTypes.INTERFACE_DEF}; 090 } 091 092 @Override 093 public int[] getAcceptableTokens() { 094 return getRequiredTokens(); 095 } 096 097 @Override 098 public void visitToken(DetailAST ast) { 099 final DetailAST objBlock = 100 ast.findFirstToken(TokenTypes.OBJBLOCK); 101 final DetailAST methodDef = 102 objBlock.findFirstToken(TokenTypes.METHOD_DEF); 103 final DetailAST variableDef = 104 objBlock.findFirstToken(TokenTypes.VARIABLE_DEF); 105 final boolean methodRequired = 106 !allowMarkerInterfaces || variableDef != null; 107 108 if (methodDef == null && methodRequired) { 109 log(ast, MSG_KEY); 110 } 111 } 112 113 /** 114 * Setter to control whether marker interfaces like Serializable are allowed. 115 * 116 * @param flag whether to allow marker interfaces or not 117 * @since 3.1 118 */ 119 public void setAllowMarkerInterfaces(boolean flag) { 120 allowMarkerInterfaces = flag; 121 } 122 123}