使用在另一个 class 中调用的方法设置 edittext 文本

Question

i have the edittext initialized in the activity like:

EditText date;
DateDialog date_class;

protected void onCreate(Bundle savedInstanceState) {
        date = findViewById(R.id.date_input);
        date.setOnClickListener(this);
        date_class = new DateDialog();
........
}

and at onclicklistner:

public void onClick(View v) {
.....
date_class.SetDateString();
.....
}

the SetDateString() method in the other class (DateDialog) is:

public void SetDateString() {
String name = "bla bla bla";
Activity act= new Activity();//defining the activity object
act.setdate(name);//sends the string to the activity's method where it will set a text for the edittext
}

so it sends a string with act.setdate(name) method which is defined in the activity like:

public void setdate(String name){
        date.setText(name);// sets a text for the edittext
    }

i have simplified the code for you to explain the point, I'm using other class to do some job on this activity but it unfortunately returns this error:

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.EditText.setText(java.lang.CharSequence)' on a null object reference

Answer 1

This approach is the opposite of standard data flow and even if you got it to work it could lead to thread problems down the line. You should never be trying to manipulate the UI from any class other than itself.

If i am inferring correctly, you want to click on a text box, open a dialog, do something in a dialog, and then update your initial activity? This is a situation where you want to use an Interface.

Define an interface at the bottom of your activity, outside of the enclosing class for now.

interface DialogCallback{
    String doSomething(String name);
}

In your DateDialog class add a variable

private DialogCallback dialogCallback;  

In your main activity add this to your class initialization:

implements DialogCallback  

Do the ALT+Enter and implement the method doSomething(String name). When you initialize the DateDialog class, you need to set the callback variable, either by doing it in the init call or as a separate setter in the DateDialog class.

void setCallback(DialogCallback callback){
    this.dialogCallback = callback;
}

Then set it after you init the class:

date_class = new DateDialog();
date_class.setCallback(this);

Finally, change your function from

public void SetDateString() {
    String name = "bla bla bla";
    Activity act= new Activity();//defining the activity object
    act.setdate(name);//sends the string to the activity's method where it will  set a text for the edittext
}

to

public void setDateString(){
    String name = "foo";
    dialogCallback.doStuff(name);
}

Now it will pass that string back your main class to the "DoStuff(String name)" function that was implemented and you use that to update the UI.
Additionally, your variables should be like "dateClass" and not "date_class" and functions should not start with uppercase letters.
As far as to why the code as is does not work, my guess would be Activity = new Activity(). It makes no sense, you are not creating an instance of any specific class, even if you were, it would be "new" meaning any previously defined ui elements would not be accessible.