{"id":571,"date":"2021-01-07T21:51:31","date_gmt":"2021-01-07T21:51:31","guid":{"rendered":"https:\/\/bhiggs.x10hosting.com\/PracticalJavaProgramming\/?page_id=571"},"modified":"2021-01-07T22:00:52","modified_gmt":"2021-01-07T22:00:52","slug":"interfaces-callbacks","status":"publish","type":"page","link":"https:\/\/bhiggs.x10hosting.com\/PracticalJavaProgramming\/course-topics\/inheritance-polymorphism\/interfaces-callbacks\/","title":{"rendered":"Interfaces &#038; Callbacks"},"content":{"rendered":"<div id=\"ez-toc-container\" class=\"ez-toc-v2_0_83 counter-hierarchy ez-toc-counter ez-toc-grey ez-toc-container-direction\">\n<p class=\"ez-toc-title\" style=\"cursor:inherit\">Table of Contents<\/p>\n<label for=\"ez-toc-cssicon-toggle-item-6a06ee08f301c\" class=\"ez-toc-cssicon-toggle-label\"><span class=\"\"><span class=\"eztoc-hide\" style=\"display:none;\">Toggle<\/span><span class=\"ez-toc-icon-toggle-span\"><svg style=\"fill: #999;color:#999\" xmlns=\"http:\/\/www.w3.org\/2000\/svg\" class=\"list-377408\" width=\"20px\" height=\"20px\" viewBox=\"0 0 24 24\" fill=\"none\"><path d=\"M6 6H4v2h2V6zm14 0H8v2h12V6zM4 11h2v2H4v-2zm16 0H8v2h12v-2zM4 16h2v2H4v-2zm16 0H8v2h12v-2z\" fill=\"currentColor\"><\/path><\/svg><svg style=\"fill: #999;color:#999\" class=\"arrow-unsorted-368013\" xmlns=\"http:\/\/www.w3.org\/2000\/svg\" width=\"10px\" height=\"10px\" viewBox=\"0 0 24 24\" version=\"1.2\" baseProfile=\"tiny\"><path d=\"M18.2 9.3l-6.2-6.3-6.2 6.3c-.2.2-.3.4-.3.7s.1.5.3.7c.2.2.4.3.7.3h11c.3 0 .5-.1.7-.3.2-.2.3-.5.3-.7s-.1-.5-.3-.7zM5.8 14.7l6.2 6.3 6.2-6.3c.2-.2.3-.5.3-.7s-.1-.5-.3-.7c-.2-.2-.4-.3-.7-.3h-11c-.3 0-.5.1-.7.3-.2.2-.3.5-.3.7s.1.5.3.7z\"\/><\/svg><\/span><\/span><\/label><input type=\"checkbox\"  id=\"ez-toc-cssicon-toggle-item-6a06ee08f301c\"  aria-label=\"Toggle\" \/><nav><ul class='ez-toc-list ez-toc-list-level-1 ' ><li class='ez-toc-page-1 ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-1\" href=\"https:\/\/bhiggs.x10hosting.com\/PracticalJavaProgramming\/course-topics\/inheritance-polymorphism\/interfaces-callbacks\/#A_C_Callback_Example\" >A C++ Callback Example<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-2\" href=\"https:\/\/bhiggs.x10hosting.com\/PracticalJavaProgramming\/course-topics\/inheritance-polymorphism\/interfaces-callbacks\/#A_Java_Callback_Example\" >A Java Callback Example<\/a><\/li><\/ul><\/nav><\/div>\n\n<p><strong><em>Callbacks<\/em><\/strong>&nbsp;are often very useful in C and C++, and Java is no exception to this.&nbsp; However, the mechanism is different in Java.<\/p>\n\n\n\n<p>In C\/C++, it is quite common to call a function, passing an argument of type pointer to a second function, which the first function will call back at the appropriate time.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"A_C_Callback_Example\"><\/span>A C++ Callback Example<span class=\"ez-toc-section-end\"><\/span><\/h3>\n\n\n\n<p>Imagine that you wish to write a function in C++ that goes through all the values in an array, and prints out all those values that match certain criteria, where the criteria are specified by calling back a supplied function.&nbsp; The callback function is specified by passing in a pointer to that function.<\/p>\n\n\n\n<p>Here&#8217;s some C++ code that supports this, using callback functions.&nbsp;<\/p>\n\n\n<div class=\"wp-block-syntaxhighlighter-code \"><pre class=\"brush: cpp; auto-links: false; title: ; quick-code: false; notranslate\" title=\"\">\n#include &lt;iostream&gt;\nusing namespace std;\n\ntypedef void select(int value);\n\nstatic void greaterThan10(int value)\n{\n    if (value &gt; 10)\n        cout &lt;&lt; &quot; &quot; &lt;&lt; value;\n}\n\nstatic void lessThan20(int value)\n{\n    if (value &lt; 20)\n        cout &lt;&lt; &quot; &quot; &lt;&lt; value;\n}\n\nstatic void forEach(int vals&#x5B;], size_t elements, select *callMe)\n{\n    for (int i = 0; i &lt; elements; i++)\n    {\n        callMe(vals&#x5B;i]);\n    }\n}\n\nint main()\n{\n    int values&#x5B;] = { 4, -5, 8, 37, 56, 2, 15, 92 };\n    cout &lt;&lt; &quot;Values &gt; 10 : &quot;;\n    forEach(values, sizeof(values)\/sizeof(values&#x5B;0]), greaterThan10);\n    cout &lt;&lt; endl;\n\n    cout &lt;&lt; &quot;Values &lt; 20 : &quot;;\n    forEach(values, sizeof(values)\/sizeof(values&#x5B;0]), lessThan20);\n    cout &lt;&lt; endl;\n\n    return 0;\n}\n<\/pre><\/div>\n\n\n<p>The output of this program is:<\/p>\n\n\n\n<pre class=\"wp-block-preformatted\"><strong>Values &gt; 10 :  37 56 15 92\nValues &lt; 20 :  4 -5 8 2 15<\/strong><\/pre>\n\n\n\n<p>In other words, each of the values in the array that matches the condition enforced in each callback function.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"A_Java_Callback_Example\"><\/span>A Java Callback Example<span class=\"ez-toc-section-end\"><\/span><\/h3>\n\n\n\n<p>In Java, there are no pointers, far less pointers to functions.<\/p>\n\n\n\n<p>Instead, we pass&nbsp;<em>objects<\/em>, not (pointers to) functions, and we typically use&nbsp;<em>interfaces<\/em>.<\/p>\n\n\n\n<p>Here&#8217;s what it might look like in Java:<\/p>\n\n\n<div class=\"wp-block-syntaxhighlighter-code \"><pre class=\"brush: java; auto-links: false; highlight: [9,13,17,18,19,20,21,22,23,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47]; title: ; quick-code: false; notranslate\" title=\"\">\npackage examples;\n\npublic class CallbackExample\n{\n  public static void main(String&#x5B;] args)\n  {\n    int&#x5B;] values = { 4, -5, 8, 37, 56, 2, 15, 92 };\n    System.out.print(&quot;Values &gt; 10 : &quot;);\n    forEach(values, new GreaterThan10() );\n    System.out.println();\n\n    System.out.print(&quot;Values &lt; 20 : &quot;);\n    forEach(values, new LessThan20() );\n    System.out.println();\n  }\n\n  static void forEach(int&#x5B;] vals, Callee callMe)\n  {\n    for (int i = 0; i &lt; vals.length; i++)\n    {\n      callMe.select(vals&#x5B;i]);\n    }\n  }\n}\n\ninterface Callee\n{\n  void select(int value);\n}\n\nclass GreaterThan10 implements Callee\n{\n  public void select(int value)\n  {\n    if (value &gt; 10)\n      System.out.print(&quot; &quot; + value);\n  }\n}\n\nclass LessThan20 implements Callee\n{\n  public void select(int value)\n  {\n    if (value &lt; 20)\n      System.out.print(&quot; &quot; + value);\n  }\n}\n<\/pre><\/div>\n\n\n<p>This outputs the following:<\/p>\n\n\n\n<pre class=\"wp-block-preformatted\"><strong>Values &gt; 10 :  37 56 15 92\nValues &lt; 20 :  4 -5 8 2 15<\/strong><\/pre>\n\n\n\n<p>which is identical output to that of the C++ program.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>Callbacks&nbsp;are often very useful in C and C++, and Java is no exception to this.&nbsp; However, the mechanism is different in Java. In C\/C++, it is quite common to call a function, passing an argument of type pointer to a second function, which the first function will call back at the appropriate time. A C++ [&hellip;]<\/p>\n","protected":false},"author":2,"featured_media":0,"parent":61,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"_eb_attr":"","_uag_custom_page_level_css":"","ocean_post_layout":"left-sidebar","ocean_both_sidebars_style":"","ocean_both_sidebars_content_width":0,"ocean_both_sidebars_sidebars_width":0,"ocean_sidebar":"ocs-course-topics-sidebar","ocean_second_sidebar":"0","ocean_disable_margins":"enable","ocean_add_body_class":"","ocean_shortcode_before_top_bar":"","ocean_shortcode_after_top_bar":"","ocean_shortcode_before_header":"","ocean_shortcode_after_header":"","ocean_has_shortcode":"","ocean_shortcode_after_title":"","ocean_shortcode_before_footer_widgets":"","ocean_shortcode_after_footer_widgets":"","ocean_shortcode_before_footer_bottom":"","ocean_shortcode_after_footer_bottom":"","ocean_display_top_bar":"default","ocean_display_header":"default","ocean_header_style":"","ocean_center_header_left_menu":"0","ocean_custom_header_template":"0","ocean_custom_logo":0,"ocean_custom_retina_logo":0,"ocean_custom_logo_max_width":0,"ocean_custom_logo_tablet_max_width":0,"ocean_custom_logo_mobile_max_width":0,"ocean_custom_logo_max_height":0,"ocean_custom_logo_tablet_max_height":0,"ocean_custom_logo_mobile_max_height":0,"ocean_header_custom_menu":"0","ocean_menu_typo_font_family":"0","ocean_menu_typo_font_subset":"","ocean_menu_typo_font_size":0,"ocean_menu_typo_font_size_tablet":0,"ocean_menu_typo_font_size_mobile":0,"ocean_menu_typo_font_size_unit":"px","ocean_menu_typo_font_weight":"","ocean_menu_typo_font_weight_tablet":"","ocean_menu_typo_font_weight_mobile":"","ocean_menu_typo_transform":"","ocean_menu_typo_transform_tablet":"","ocean_menu_typo_transform_mobile":"","ocean_menu_typo_line_height":0,"ocean_menu_typo_line_height_tablet":0,"ocean_menu_typo_line_height_mobile":0,"ocean_menu_typo_line_height_unit":"","ocean_menu_typo_spacing":0,"ocean_menu_typo_spacing_tablet":0,"ocean_menu_typo_spacing_mobile":0,"ocean_menu_typo_spacing_unit":"","ocean_menu_link_color":"","ocean_menu_link_color_hover":"","ocean_menu_link_color_active":"","ocean_menu_link_background":"","ocean_menu_link_hover_background":"","ocean_menu_link_active_background":"","ocean_menu_social_links_bg":"","ocean_menu_social_hover_links_bg":"","ocean_menu_social_links_color":"","ocean_menu_social_hover_links_color":"","ocean_disable_title":"default","ocean_disable_heading":"default","ocean_post_title":"","ocean_post_subheading":"","ocean_post_title_style":"","ocean_post_title_background_color":"","ocean_post_title_background":0,"ocean_post_title_bg_image_position":"","ocean_post_title_bg_image_attachment":"","ocean_post_title_bg_image_repeat":"","ocean_post_title_bg_image_size":"","ocean_post_title_height":0,"ocean_post_title_bg_overlay":0.5,"ocean_post_title_bg_overlay_color":"","ocean_disable_breadcrumbs":"default","ocean_breadcrumbs_color":"","ocean_breadcrumbs_separator_color":"","ocean_breadcrumbs_links_color":"","ocean_breadcrumbs_links_hover_color":"","ocean_display_footer_widgets":"default","ocean_display_footer_bottom":"default","ocean_custom_footer_template":"0","footnotes":""},"class_list":["post-571","page","type-page","status-publish","hentry","entry"],"uagb_featured_image_src":{"full":false,"thumbnail":false,"medium":false,"medium_large":false,"large":false,"1536x1536":false,"2048x2048":false,"ocean-thumb-m":false,"ocean-thumb-ml":false,"ocean-thumb-l":false},"uagb_author_info":{"display_name":"Bryan Higgs","author_link":"https:\/\/bhiggs.x10hosting.com\/PracticalJavaProgramming\/author\/bryan\/"},"uagb_comment_info":0,"uagb_excerpt":"Callbacks&nbsp;are often very useful in C and C++, and Java is no exception to this.&nbsp; However, the mechanism is different in Java. In C\/C++, it is quite common to call a function, passing an argument of type pointer to a second function, which the first function will call back at the appropriate time. A C++&hellip;","_links":{"self":[{"href":"https:\/\/bhiggs.x10hosting.com\/PracticalJavaProgramming\/wp-json\/wp\/v2\/pages\/571","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/bhiggs.x10hosting.com\/PracticalJavaProgramming\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/bhiggs.x10hosting.com\/PracticalJavaProgramming\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/bhiggs.x10hosting.com\/PracticalJavaProgramming\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/bhiggs.x10hosting.com\/PracticalJavaProgramming\/wp-json\/wp\/v2\/comments?post=571"}],"version-history":[{"count":6,"href":"https:\/\/bhiggs.x10hosting.com\/PracticalJavaProgramming\/wp-json\/wp\/v2\/pages\/571\/revisions"}],"predecessor-version":[{"id":580,"href":"https:\/\/bhiggs.x10hosting.com\/PracticalJavaProgramming\/wp-json\/wp\/v2\/pages\/571\/revisions\/580"}],"up":[{"embeddable":true,"href":"https:\/\/bhiggs.x10hosting.com\/PracticalJavaProgramming\/wp-json\/wp\/v2\/pages\/61"}],"wp:attachment":[{"href":"https:\/\/bhiggs.x10hosting.com\/PracticalJavaProgramming\/wp-json\/wp\/v2\/media?parent=571"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}