What does java.lang.NullPointerException mean?
"java.lang.NullPointerException" mainly occurs at runtime. This exception belongs to a subclass of RuntimeException, so it is an exempt exception. When the application tries to use null where an object is required, This exception will be thrown.
Several reasons for "java.lang.NullPointerException"
1. The string variable is not initialized
2. Interface type objects are not initialized with a specific class, such as:
Map map // 会报错 Map map = new Map(); //则不会报错了
3. When the value of an object is empty, you do not judge it to be empty.
4. Comparison between strings and text. Text can be a string or an element of Enum. An exception will occur as follows
String str = null; if（str.equals（“Test”））{ //这里的代码将不会被触发，因为会抛出java.lang.NullPointerException异常。 }
5. Prioritize using the String.valueOf() method instead of toString( )
When program code requires a string representation of an object, avoid using the object's toString method. If the reference of your object is equal to null, NullPointerException will be thrown. Use the static String.valueOf method. This method will not throw any exception and print "null"
6. The class is declared type, default class = null; In this way, the system can only give you a null pointer exception when calling the method in the class. Just instantiate it: class = new Class();
7. Return null, the method Do not define the return value as a general type, but use an array. In this way, if you want to return null, you can avoid many unnecessary NullPointerException
Recommended tutorial: "Java Tutorial"
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