How to check null on StringBuilder?

问题

I want to check for null or empty specifically in my code. Does empty and null are same for StringBuilder in Java?

For example:

StringBuilder state = new StringBuilder();
StringBuilder err= new StringBuilder(); 
success = executeCommand(cmd, state, err); 

/* here executeCommand() returns empty or null in state, I cant  make changes in  <br/> executeCommand() so can I check it in my code somehow for state, if its null or empty? */<br/>

if (state == null) { //do blabla1 }
if (state.tostring().equals("")) { //do blabla2 }

Does above code make sense or how should I change it?

回答1:

No, null and empty are different for StringBuilder.

StringBuilder nullBuilder = null;
if(nullBuilder == null) {
    System.out.println("Builder is null");
}

StringBuilder emptyBuilder = new StringBuilder("");
if(emptyBuilder == null || emptyBuilder.toString().equals("")) {
    System.out.println("Builder is empty");
}

回答2:

In Java, null is a reference literal. If a variable is null then is not referring to anything.

So, if you have StringBuilder s = null, that means that s is of type StringBuilder but it is not referring to a StringBuilder instance.

If you have a non-null reference then you are free to call methods on the referred object. In the StringBuilder class, one such method is length(). In fact if you were to call length() using a null reference then the Java runtime will throw a NullPointerException.

Hence, this code is quite common:

If (s == null || s.length() == 0/*empty if the length is zero*/){
    // do something

It relies on the fact that evaluation of || is from left to right and stops once it reaches the first true condition.

回答3:

Null mean, there are no object in the heap for that reference variable. This is common to all java object, not specific to StringBuilder and Empty means, "".

In your code, you have created a StringBuilder object, so checking null is redundant. And, You can check empty by using isEmpty() method in from java String api

if(state.tostring().isEmpty()) {
//
}

And checking null is correct. Find the corrected version here

if (state == null) {
// ...bla 1
} else if (state.tostring().isEmpty()) {
//... bla 2
}

Your second if condition will throw NullPointerException, if the state is null. So if should be nested with if else

回答4:

No. empty means, that there are no characters in the StringBuilder. null means that there is no StringBuilder object at all.

A variable is only null if it has a reference type (for example String, StringBuilder, Set, as a thumbrule: all capitalized types) and it is not initialised yet or has been set explicitly to null.

回答5:

The below code may help you,

StringBuffer sb = new StringBuffer();
String str = sb.toString();
if(!"".equals(str)) {
    System.out.println("String : " + str);
} else {
    System.out.println("Empty Builder");
}

回答6:

You can try like this

StringBuilder state = new StringBuilder();
if(StringUtils.isNotBlank(state .toString())){   
//this will check for null, " ", ""
}

来源：https://stackoverflow.com/questions/21553562/how-to-check-null-on-stringbuilder

标签

java

stringbuilder