Java value is always null? [duplicate]

Question

I have a value in Java that is always null but I don't understand why, because I am setting the value of the variable using the class constructor.

I have code like such:

Driver driverClass = new Driver("<file path redacted>");

And then the below:

String cfgFilePath;

public Driver(String cfgFile) {
    this.cfgFilePath = cfgFile;
}

private ArrayList<String> keys = getKeys(cfgFilePath);
private String a1 = keys.get(0);
private String b1 = keys.get(1);

For some reason, IntelliJ IDEA says that cfgFilePath is always null. I am initializing it with the Driver class constructor, so why is it null? When I run the program I get a null pointer exception.

See stackoverflow.com/a/14806340/2864740 (direct relevant answer in related question) — user2864740
– user2864740, Commented Aug 26, 2018 at 22:13

rieckpil · Accepted Answer · 2018-08-26 22:09:17Z

1

Move the initialization of keys, a1 and b1 to your constructor like:

public Driver(String cfgFile) {
    this.cfgFilePath = cfgFile;
    this.keys = getKeys(cfgFilePath);
    this.a1 = keys.get(0);
    this.b1 = keys.get(1);
}

private ArrayList<String> keys = new ArrayList<>();
private String a1;
private String b1;

answered Aug 26, 2018 at 22:09

rieckpil

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2 Comments

Emily Over a year ago

This worked, thanks! But I'd like to know how. I read the other post's explanation of the order of initialization but I am still confused as to why that means that those variables are always null at all times.

rieckpil Over a year ago

These attributes are null because your attribute keys does need cfgFilePath to work properly. You set cfgFilePath within your constructor but the keys attribute tries to initialize its attribute before you call the constructor and at this time the cfgFilePath is not set (the String attribute is null per default if it's not initialized). You have a predefined order of execution you want to achieve -> 1. set cfgFilePath and 2. initialize the other attributes.

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