如何获取已经在另一个 class 中设置的实例值？

Question

I'm a new to Java. I have 3 classes: 1-DataHolder 2-SetToDateHolder 3-GetFromDataHolder; In the "SetToDateHolder" class, I set the value to dataholder object, and I trying to access it in the "GetFromDataHolder" class, but it got null value, I know it happens because of new instance created. Can you help me to realize that I can get the value of instance which I have set in another class.

These are the classes:

public class DataHolder {

    private String myName;

    public String getMyName() {
        return myName;
    }

    public void setMyName(String myName) {
        this.myName = myName;
    }
}

public class SetToDataHolder {

    public static void main(String[] args) {
        DataHolder dataHolder = new DataHolder();
        dataHolder.setMyName("Wesley");
    }

}

public class GetFromDataHolder{
    
    public static void main(String[] args) {
        DataHolder dataHolder = new DataHolder();
        System.out.println(dataHolder.getMyName());
    }

}

Answer 1

I don't really understand why you need 2 additional classes as you already have the getter / setter in your DataHolder class, but here's how you should be able to proceed your logic:

public class DataHolder {

    private String myName;

    public String getMyName() {
        return myName;
    }

    public void setMyName(String myName) {
        this.myName = myName;
    }
}

public class SetToDataHolder {
    
    private DataHolder dataHolder;

    public SetToDataHolder(DataHolder dataHolder){
        this.dataHolder = dataHolder;
    }

    public void setMyName(String myName) {
        this.dataHolder.setMyName(myName);
    }
}

public class GetFromDataHolder{
    
    private DataHolder dataHolder;

    public GetFromDataHolder(DataHolder dataHolder){
        this.dataHolder = dataHolder;
    }

    public String getMyName() {
        this.dataHolder.getMyName();
    }
}

public class MainClass{
    
    public static void main(String[] args){
        DataHolder dataHolder = new DataHolder();

        SetToDataHolder setToDataHolder = new SetToDataHolder(dataHolder);
        setToDataHolder.setMyName("Wesley");

        GetFromDataHolder getFromDataHolder = new GetFromDataHolder(dataHolder);
        System.out.println(getFromDataHolder.getMyName());
    }
}

The idea is that your SetToDataHolder and GetFromDataHolder, have access to the same instance of DataHolder.