JAVA - Execute a conditional if statement for a null value using getters and setters

Question

I have an issue with returning and replacing the value null with something with meaning from another class using getters and setters and would like to know what I have done incorrectly.

Configuration for getters and setters:

public class CarClass {

    private String make;
    private String model;
    private int yearOfMake;

    public void setMake(String make){
        if (make.isEmpty()){
            this.make = "";
            System.out.println("Error please input value");
        } else {
            System.out.println("Hello");
            this.make = make; 
        }
    }

    public String getMake(){
        return make;
    }

    public String getModel() {

        return model;
    }

    public void setModel(String model) {
        if (model.isEmpty()){
            this.model = "";
            System.out.println("Error, you have not entered a valid model");
        } else {
            this.model = model;
        }
    }

    public int getYearOfMake() {
        return yearOfMake;
    }

    public void setYearOfMake(int yearOfMake) {
        if (yearOfMake > 1900)
            this.yearOfMake = yearOfMake;
        else {
            this.yearOfMake = yearOfMake;
            System.out.println("Year of make is invalid");
        }
    }
}

Execution script for above:

public class GetterSetter {

    public static void main(String[] args) {
        CarClassTwo bmw = new CarClassTwo();
        bmw.setMake();
        System.out.println(bmw.getMake());
        CarClass benz = new CarClass();
        benz.setModel("C300");
        System.out.println(benz.getModel());
        benz.setYearOfMake(1999);
        System.out.println(benz.getYearOfMake());
        System.out.println("!!!!!!!!");
        benz.setModel("C300");
        System.out.println(benz.getModel());
        benz.setYearOfMake(1800);
        System.out.println(benz.getYearOfMake());
        System.out.println(bmw.getMake());
        System.out.println(bmw.getModel());
    }
}

Any assistance into understand what I have done incorrectly would be most appreciated.

Answer 1

When the following code is executed:

CarClassTwo bmw = new CarClassTwo();
bmw.setMake();

at that time the values of

private String make;
private String model;

are initialized to null by default.

So when bmw.setMake(); is executed, it executes if (make.isEmpty()){

which is something like null.isEmpty() --> which will throw NullPointerException.