Determining whether a Java object is null can have two meanings: (Recommended learning: java course)
First level: Directly use object == null to judge. When the object is null, it returns true, and when it is not null, it returns false.
Second level: When object != null is true, further determine whether all attributes of the object are null.
Simple judgment:
//判斷1 ： 這里會(huì)返回 true
User user = null;
if(user == null){
return true;
}else{
return false;
}
//判斷2 ： 這里會(huì)返回 false
User user = new User();
if(user == null){
return true;
}else{
return false;
}
原因： User user = new User(); 這時(shí)候已經(jīng)創(chuàng)建了一個(gè)對(duì)象，所以u(píng)ser不會(huì)為nullThe above example is relatively simple and clear at a glance, and there does not seem to be any problem.
" But in fact, when the object you need to judge is an object passed from the front end, simply using object == null to judge is not enough, because the object passed by the front end will look like the judge. The object does not is null, but all properties of the object are null. [Except boolean types, and serialized values, and there may be other special values] "
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