{"id":3826,"date":"2022-08-20T18:52:01","date_gmt":"2022-08-20T13:22:01","guid":{"rendered":"https:\/\/jassweb.com\/solved\/solved-what-is-a-nullpointerexception-and-how-do-i-fix-it\/"},"modified":"2022-08-20T18:52:01","modified_gmt":"2022-08-20T13:22:01","slug":"solved-what-is-a-nullpointerexception-and-how-do-i-fix-it","status":"publish","type":"post","link":"https:\/\/jassweb.com\/solved\/solved-what-is-a-nullpointerexception-and-how-do-i-fix-it\/","title":{"rendered":"(Solved) What is a NullPointerException, and how do I fix it?"},"content":{"rendered":"

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\n<\/p>\n

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<\/div>\n
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There are two overarching types of variables in Java:<\/p>\n

    \n
  1. \n

    Primitives<\/em>: variables that contain data. If you want to manipulate the data in a primitive variable you can manipulate that variable directly. By convention primitive types start with a lowercase letter. For example variables of type int<\/code> or char<\/code> are primitives.<\/p>\n<\/li>\n

  2. \n

    References<\/em>: variables that contain the memory address of an Object<\/code> i.e. variables that refer<\/em> to an Object<\/code>. If you want to manipulate the Object<\/code> that a reference variable refers to you must dereference<\/em> it. Dereferencing usually entails using .<\/code> to access a method or field, or using [<\/code> to index an array. By convention reference types are usually denoted with a type that starts in uppercase. For example variables of type Object<\/code> are references.<\/p>\n<\/li>\n<\/ol>\n

    Consider the following code where you declare a variable of primitive<\/em> type int<\/code> and don’t initialize it:<\/p>\n

    int x;\nint y = x + x;\n<\/code><\/pre>\n
    These two lines will crash the program because no value is specified for x<\/code> and we are trying to use x<\/code>‘s value to specify y<\/code>. All primitives have to be initialized to a usable value before they are manipulated.<\/p>\n
    Now here is where things get interesting. Reference<\/em> variables can be set to null<\/code> which means “I am referencing nothing<\/em><\/strong>“. You can get a null<\/code> value in a reference variable if you explicitly set it that way, or a reference variable is uninitialized and the compiler does not catch it (Java will automatically set the variable to null<\/code>).<\/p>\n
    If a reference variable is set to null either explicitly by you or through Java automatically, and you attempt to dereference<\/em> it you get a NullPointerException<\/code>.<\/p>\n
    The NullPointerException<\/code> (NPE) typically occurs when you declare a variable but did not create an object and assign it to the variable before trying to use the contents of the variable. So you have a reference to something that does not actually exist.<\/p>\n
    Take the following code:<\/p>\n
    Integer num;\nnum = new Integer(10);\n<\/code><\/pre>\n
    The first line declares a variable named num<\/code>, but it does not actually contain a reference value yet. Since you have not yet said what to point to, Java sets it to null<\/code>.<\/p>\n
    In the second line, the new<\/code> keyword is used to instantiate (or create) an object of type Integer<\/code>, and the reference variable num<\/code> is assigned to that Integer<\/code> object.<\/p>\n
    If you attempt to dereference num<\/code> before<\/em> creating the object you get a NullPointerException<\/code>. In the most trivial cases, the compiler will catch the problem and let you know that “num may not have been initialized<\/code>,” but sometimes you may write code that does not directly create the object.<\/p>\n
    For instance, you may have a method as follows:<\/p>\n
    public void doSomething(SomeObject obj) {\n   \/\/ Do something to obj, assumes obj is not null\n   obj.myMethod();\n}\n<\/code><\/pre>\n
    In which case, you are not creating the object obj<\/code>, but rather assuming that it was created before the doSomething()<\/code> method was called. Note, it is possible to call the method like this:<\/p>\n
    doSomething(null);\n<\/code><\/pre>\n
    In which case, obj<\/code> is null<\/code>, and the statement obj.myMethod()<\/code> will throw a NullPointerException<\/code>.<\/p>\n
    If the method is intended to do something to the passed-in object as the above method does, it is appropriate to throw the NullPointerException<\/code> because it’s a programmer error and the programmer will need that information for debugging purposes.<\/p>\n
    In addition to NullPointerException<\/code>s thrown as a result of the method’s logic, you can also check the method arguments for null<\/code> values and throw NPEs explicitly by adding something like the following near the beginning of a method:<\/p>\n
    \/\/ Throws an NPE with a custom error message if obj is null\nObjects.requireNonNull(obj, \"obj must not be null\");\n<\/code><\/pre>\n
    Note that it’s helpful to say in your error message clearly which<\/em> object cannot be null<\/code>. The advantage of validating this is that 1) you can return your own clearer error messages and 2) for the rest of the method you know that unless obj<\/code> is reassigned, it is not null and can be dereferenced safely.<\/p>\n
    Alternatively, there may be cases where the purpose of the method is not solely to operate on the passed in object, and therefore a null parameter may be acceptable. In this case, you would need to check for a null parameter<\/strong> and behave differently. You should also explain this in the documentation. For example, doSomething()<\/code> could be written as:<\/p>\n
    \/**\n  * @param obj An optional foo for ____. May be null, in which case\n  *  the result will be ____.\n  *\/\npublic void doSomething(SomeObject obj) {\n    if(obj == null) {\n       \/\/ Do something\n    } else {\n       \/\/ Do something else\n    }\n}\n<\/code><\/pre>\n
    Finally, How to pinpoint the exception & cause using Stack Trace<\/p>\n
    \n
    What methods\/tools can be used to determine the cause so that you stop
    \nthe exception from causing the program to terminate prematurely?<\/p>\n<\/blockquote>\n
    Sonar with find bugs can detect NPE.
    \nCan sonar catch null pointer exceptions caused by JVM Dynamically<\/p>\n
    Now Java 14 has added a new language feature to show the root cause of NullPointerException. This language feature has been part of SAP commercial JVM since 2006.<\/p>\n
    In Java 14, the following is a sample NullPointerException Exception message:<\/p>\n
    \n
    in thread “main” java.lang.NullPointerException: Cannot invoke “java.util.List.size()” because “list” is null<\/p>\n<\/blockquote><\/div>\n
    <\/div>\n<\/div>\n
                24<\/span> <\/p><\/div>\n<\/div>\n
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    solved What is a NullPointerException, and how do I fix it? <\/p>\n","protected":false},"excerpt":{"rendered":"
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References: variables that contain … Read more<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[320],"tags":[323,328],"class_list":["post-3826","post","type-post","status-publish","format-standard","hentry","category-solved","tag-java","tag-nullpointerexception"],"yoast_head":"\n(Solved) What is a NullPointerException, and how do I fix it? - JassWeb<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/jassweb.com\/solved\/solved-what-is-a-nullpointerexception-and-how-do-i-fix-it\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"(Solved) What is a NullPointerException, and how do I fix it? - JassWeb\" \/>\n<meta property=\"og:description\" content=\"[ad_1] There are two overarching types of variables in Java: Primitives: variables that contain data. 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