helpful-npe.md

Helpful NPE Messages

Assume we have the following classes.

class Person {

    private String name;
    private int age;

    private Address address;

    public Person() {
    }

    public Person(String name, int age) {
        this.age = age;
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Address getAddress() {
        return address;
    }

    public void setAddress(Address address) {
        this.address = address;
    }

}

class Address {

    private final City city;

    public Address(City city) {
        this.city = city;
    }

    public City getCity() {
        return city;
    }

}

class City {

    private final String name;

    public City(String name) {
        this.name = name;
    }

    public String getName() {
        return name;
    }
}

And create a Person instance like this.

Person person = new Person();

When you try to access the address of the person object.

person.getAddress().getCity().getName()

Compile the application. And run it, you will see the famous NullPointerException.

Exception in thread "main" java.lang.NullPointerException
        at com.example.demo.NpeExample.main(NpeExample.java:12)

But the above message does not indicate where causes the exception. Either person.getAddress() or person.getAddress().getCity() could raise this exception.

In Java 14, add an extra parameter -XX:+ShowCodeDetailsInExceptionMessages to java when running the application, you will get detailed messages about the java.lang.NullPointerException.

#java -XX:+ShowCodeDetailsInExceptionMessages --enable-preview com.example.demo.NpeExample

Exception in thread "main" java.lang.NullPointerException: Cannot invoke "com.example.demo.Address.getCity()" because the return value of "com.example.demo.Person.getAddress()" is null
        at com.example.demo.NpeExample.main(NpeExample.java:12)

It is very helpful when you are debugging your application.

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