Common methods for converting objects to String in Java

During development, we often encounter the problem of converting data from collection classes List and Map to String. If we do not handle it well here, we often encounter null pointer exception java.lang.NullPointerException. Here is a summary of commonly used conversions to String. method, and the problem of how to judge null after conversion.

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Commonly used methods to convert objects to String in Java:

Method 1: String objStr = (String) obj:

　　　　Forcible type conversion, the object obj is null, and the result is also null, but obj must ensure that its essence is a value of String type, that is, a converted value.

For example, you cannot force conversion (String) 123

Method 2: String objStr = obj.toString():

　　　When calling the toString method of an object, you must ensure that this class or the parent class has overridden the toString method of the Object class. If the toString method has not been overridden,

The toString method of the Object class will be called by default and return getClass().getName() + '@' + Integer.toHexString(hashCode()),

It is not the actual string representation of obj. At the same time, it must also ensure that the object obj cannot be null, otherwise call toString The method will report a null pointer exception java.lang.NullPointerException.

Method three: String objStr = String.valueOf(obj):

The object obj is null, and the conversion result is the string "null", otherwise, return ## The value of #obj.toString().

Note that if obj is null, the converted value here is already the "null" of the string. Obj == null cannot be used to detect null, nor can apache.commons.lang3

# be used. ## 　　　StringUtils.isBank(CharSequence cs) instead, str.equals("null") should be used.

Already know that obj is of String type:

Use

Method 1

to convert to String. After converting to String, the null condition is: if (objStr != null) Use method 2 with caution

For situations where you don’t know the specific type, You can use

Method 3

, but the null condition of String after conversion is changed to: if (!objStr.equals('null'))

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