Have you meet this situation: compare Integer with its primitive type int? The situation seems like the following example.
public class NullPointerExceptionCaseStudy {
public static void main(String[] args) {
Integer startNum = new Integer(10);
Countdown counter = new Countdown(startNum);
//what if startNum is assigned to null?
//***the statement 0==counter.getCount() will throw an NullPointerExcepion***
if (0 == counter.getCount()) {
System.out.printnln("Time's up");
}
}
class Countdown{
private Integer count;
public Countdown(Integer count) {
this.count = count;
}
public Integer getCount() {
return count;
}
}
}
Why the conditional statement throws an Exception when startNum is assigned to null?
According to the Java Specification (chapter 5. Conversions),
At run time, unboxing conversion proceeds as follows:
If r is a reference of type Boolean, then unboxing conversion converts r into r.booleanValue()
If r is a reference of type Byte, then unboxing conversion converts r into r.byteValue()
If r is a reference of type Character, then unboxing conversion converts r into r.charValue()
If r is a reference of type Short, then unboxing conversion converts r into r.shortValue()
If r is a reference of type Integer, then unboxing conversion converts r into r.intValue()
If r is a reference of type Long, then unboxing conversion converts r into r.longValue()
If r is a reference of type Float, unboxing conversion converts r into r.floatValue()
If r is a reference of type Double, then unboxing conversion converts r into r.doubleValue()
If r is null, unboxing conversion throws a NullPointerException
The conditional statement “if (0 == counter.getCount())” splits several steps:
1. Integer temp = counter.getCount()
2. int intTemp = temp.intValue() // If temp is null, NullPointerException is thrown
3. compare 0 with intTemp
Solution - Revise the conditional statement as below:
//Objects is introduced since 1.7 in java.util package
/*
public static boolean equals(Object a, Object b) {
return (a == b) || (a != null && a.equals(b));
}
*/
if (Objects.equals(counter.getCount(), 0) {
//your implementation
}
The same with multiplicative operators(*, /, %) and additive operators(+,-), the operands must be a type that is convertible to a primitive numeric type.