can someone explains how this line works in JAVA?
boolean result = value == null
It evaluates value == null, and assigns true or false to result.
Due to the evaluation order, you don't need parentheses result = (value == null).
boolean result = value == null
means you declare a boolean variable result and assigning it to the return value of value == null
value == null --> ture or false
boolean result = true|false
Java will compare the value with null first, after matching both the values then resulting value will be assign as true or false to the result variable. it will check also for data type(if needed, like you cant compare boolean values with null).
Related
Are there any dangers in checking for null and dereferencing on the same line?
If myObj is in fact null, how would this code behave?
Are there differences in how different languages handle situations like these (ie C# vs Java)
For example, something like below
if(myObj != null && myObj.someProp == "Test")
{
//...
}
&& is short-circuiting so if myObj is indeed null then the second condition will never be evaluated.
This behaviour is the same for both C# and Java.
In your example the if condition expect a boolean value that is provided by the boolean operation myObj != null && myObj.someProp == "Test".
When using the && operator the left operand is checked first. If its value equals true then the right operand is checked as it's not possible to know its state in advance. But if its value equals false then no need to check the right operand as no matter the right condition state will be, the whole operation will result to false.
This is why it's safe.
But when using the & operator both operands are always checked. Your example would look as follows with the & operator:
if(myObj != null & myObj.someProp == "Test")
{
//...
}
Doing so, when myObj variable is equals to null then the code above will fail. In this case your code won't be safe.
I hope this helps;
if a HashMap is empty and I check for .containsKey()
I get a null answer.
My Problem is that If I want to check for null I get an error message
if(containsKey == null || !containsKey){
I receive the error message
Operator '==' cannot be applied to 'boolean', 'null'
Can someone tell me why this is happening. I thought that this should work
Check that the map isn't null (not that HashMap.containsKey(T) returned null, because it didn't - it can't. It returns a boolean primitive, which can only be true or false).
if (map != null && map.containsKey(someKey)) {
// ...
}
You can use the HashMap .isEmpty() method to check if your hashmap is empty or not.
containsKey can't be null as it is the method being called. Try checking if the map itself is null.
Booleans are primitives, and primitives will never be null.
Only Object classes can be null.
Following this argument, you can do this for object class Integer:
Integer myObject = 1;
if (myObject != null){
...
}
But you cannot do this for int, which is a primitive like booleans:
int myPrimitve = 1;
if (myPrimitve == null){
...
}
Your IDE will show the error Operator == cannot be applied to int, null
I have a json in which userId property is coming as string "null" -
"userId":"null"
I have a method which checks whether my string is null or not -
public static boolean isEmpty(String value) {
return value == null || value.isEmpty();
}
But every time my above method returns back me as false for above userId? It should true since userId is null. Is there any other api in Guava or Apache Commons which can do this?
The value null is not equal to the String "null". null means that a given object has not been assigned a value, where the String "null" is a valid object. It contains the characters n u l l, which is a valid value for a String. You need to also check if the value is the literal string "null" in order to do what you want.
Correct Check
return value == null || value.isEmpty() || value.equals("null") ;
If you want to still maintain "null" as a valid username, then change whatever is sending the json to the following format, which should be interpreted as a literal null rather than a String with content "null"
"userId":null
"null" is not the same as null.
"null" is a string 4 characters in length of the word "null".
null (no quotes) is just that--nothing.
{"userId":"null"} equals String userId = "null" in java.
{} would equal String userId = null when unmarshalled.
I am .net programmer and completely new in java. I am facing problem in handling null string in java. I am assigning value from string array to string variable completeddate.
I tried all this but that didn't work.
String COMPLETEDATE;
COMPLETEDATE = country[23];
if(country[23] == null && country[23].length() == 0)
{
// ...
}
if (COMPLETEDATE.equals("null"))
{
// ...
}
if(COMPLETEDATE== null)
{
// ...
}
if(COMPLETEDATE == null || COMPLETEDATE.equals("null"))
{
// ...
}
For starters...the safest way to compare a String against a potentially null value is to put the guaranteed not-null String first, and call .equals on that:
if("constantString".equals(COMPLETEDDATE)) {
// logic
}
But in general, your approach isn't correct.
The first one, as I commented, will always generate a NullPointerException is it's evaluated past country[23] == null. If it's null, it doesn't have a .length property. You probably meant to call country[23] != null instead.
The second approach only compares it against the literal string "null", which may or may not be true given the scope of your program. Also, if COMPLETEDDATE itself is null, it will fail - in that case, you would rectify it as I described above.
Your third approach is correct in the sense that it's the only thing checking against null. Typically though, you would want to do some logic if the object you wanted wasn't null.
Your fourth approach is correct by accident; if COMPLETEDDATE is actually null, the OR will short-circuit. It could also be true if COMPLETEDDATE was equal to the literal "null".
To check null string you can use Optional in Java 8 as below:
import Optional
import java.util.Optional;
import it as above
String str= null;
Optional<String> str2 = Optional.ofNullable(str);
then use isPresent() , it will return false if str2 contains NULL otherwise true
if(str2.isPresent())
{
//If No NULL
}
else
{
//If NULL
}
reference: https://docs.oracle.com/javase/8/docs/api/java/util/Optional.html
It is not entirely clear what you are asking, but to check if a String variable is null, use the following statement.
if(myString==null)
This checks whether the object reference is null.
The following statement, which you have written is incorrect for two reasons.
if (COMPLETEDATE.equals("null"))
{
// ...
}
1. null is a keyword in Java, "null" is just a string of text.
2. .equals() checks to see if two objects are equal according to the given method's definition of equality. Null checks should always be made using the == comparison operator, as it checks reference equality.
If a variable is null, you cannot dereference it.
That means you can not invoke methods on it.
So... The following if statement will throw a NullPointerException every time the first clause is true:
if (a == null && a.length() == 0)
In other words: if a is null, you CANNOT invoke the length method on a.
I am trying for null check like below
if (isTrue == null)
compile error says : "The operator == is undefined for the argument type(s) boolean"
Please help, how to do null check.
Thanks
You can't do null check on primitive types. boolean is a primitive type.
If you absolutely need to represent a null value with a boolean variable, you need to use the wrapper class java.lang.Boolean.
So, your example would be:
Boolean isTrue;
isTrue = null; // valid
isTrue = true; // valid
isTrue = false; // valid
if (isTrue == null) {
// valid!
}
Here's the WIKIPEDIA entry for primitive wrapper classes.
The right way is
boolean isTrue;
if(!isTrue)
or
if(isTrue)
You can not check if the boolean is null or not.boolean must be true or false.
A boolean is a primative type and cannot be null.
A boolean cannot be null in java.
A Boolean, however, can be null.