How to turn null into false in Java?
In my code I have a org.apache.tapestry5.json.JSONObject named j . I want to read the property use-name from that Object:
Boolean isUseName = (Boolean) j.opt("use-name");
The value can either be true, false or null (if the entry is not present in the JSONObject). Now I'd like to use isUseName for a conditional statement:
if(!isUseName) {System.out.println("No Name to be used.")}
This gives me a NullPointerException if "use-name" was not in the JSONObject. One way is to simply check first if isUseName is null, eg
if(isUseName != null && !isUseName) {System.out.println("No Name to be used.")}
I was wondering, if there is a more elegant way. Is it eg possible to (automatically) set isUseName to false, if j.opt() returns null? One thing that came to my mind is using a ternary expression, but this has a redundant j.opt("use-name") :
Boolean isUseName = (j.opt("use-name") != null)
? (Boolean) j.opt("use-name")
: false;
你可以用Boolean.TRUE进行比较：
boolean useName = Boolean.TRUE.equals (j.opt ("use-name"));
Logical expressions short-circuit, so if you check null first, then you can use it after the check:
if(isUseName != null && !isUseName) {
// It's not null, and false
System.out.println("No Name to be used.");
}
or
if(isUseName == null || !isUseName) {
// It's null or false
System.out.println("No Name to be used.");
}
and similarly
if(isUseName != null && isUseName) {
// It's not null, and true
}
and
if(isUseName == null || isUseName) {
// It's either null or true
}
如何检查密钥是否存在
boolean useName = j.has("use-name") ? j.getBoolean("use-name") : false;
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