Understanding referencing other classes in java - null pointer exception

Question

I'm struggling understanding how work with java objects from other objects. I have 3 simple classes: 1) environment object

public class Environment {

protected String envName;

public Environment(String envName){
    this.envName = envName;
}

// get and set methods
public String getenvName(){
    return envName;
}

public void setenvName(String envName){
    this.envName = envName;
}

}

2) Class that will populate this object

public class FetchConfig {

Environment environment;

public FetchConfig() {
}

public void buildConfig() {
    environment.setenvName("Steve");
}
}

3) A class with main method with will work with my Environment objects:

public class WorkWithEnvironment {

private FetchConfig config;

public static void main(String[] args) throws FileNotFoundException,
        IOException {

    WorkWithEnvironment w = new WorkWithEnvironment();
    w.setupConfig();
    w.readEnvNames();
}

private void setupConfig() throws FileNotFoundException, IOException {
    config = new FetchConfig();
    config.buildConfig();
}

private void readEnvNames() {

    System.out.println("Environment name is: "
            + config.environment.getenvName());
}

}

But when I run it, I keep getting an NPE(NullPointerException) here -> environment.setenvName("Steve");

Your FetchConfig class has a Environment field. If you don't initialize it for each instance, it remains null. — Sotirios Delimanolis
– Sotirios Delimanolis, Commented Mar 1, 2014 at 19:41

OSborn · Accepted Answer · 2014-03-01 19:42:11Z

1

You've never told FetchConfig which Environment to use. I think you meant to have environment = new Environment(); or similar in FetchConfig's default constructor.

You could also initialize the variable environment with a similar line in the buildConfig method.

answered Mar 1, 2014 at 19:42

OSborn

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Comments

indivisible · Accepted Answer · 2014-03-01 23:02:27Z

0

Your second class is trying to set values inside your Environment Class before instantiating it and so it is null when you try to assign a value to it.

public class FetchConfig {

Environment environment;

public FetchConfig() {
    environment = new Environment(null);
}

public void buildConfig() {
    environment.setenvName("Steve");
}

edited Mar 1, 2014 at 23:02

answered Mar 1, 2014 at 19:42

indivisible

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2 Comments

stk123 Over a year ago

Thx all -- actually it needed to be "Environment = new Environment(null); but I understand what I was doing incorrectly.

indivisible Over a year ago

Edited my answer. Glad we could help.

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