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Calling methods without reference variable [duplicate]

This question already has answers here:
Calling a method inside another method in same class
(5 answers)
Why am I able to access a method without object in java [duplicate]
(2 answers)
Closed 3 years ago.
Could someone explain to me one thing about call methods, viz. When I was learning at Neatbeans, calling a method was always done using a reference variable, where I had to create a real object before, for example:
public class Question {
public static void main(String[] args) {
Test test = new Test();
test.method();
}
}
class Test {
void method() {
System.out.println("Test");
}
}
In this case, I had to create an object, assign its reference to the test variable, and then call the method.
However, in Android Studio, to call a method, I do not have to create a reference variable or an object, I only directly call the method ... for example:
public class SecondActivity extends AppCompatActivity {
EditText editText;
Button button2;
String name;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_second);
editText = findViewById(R.id.editText);
button2 = findViewById(R.id.button);
button2.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View view) {
prepareResult();
}
});
}
public void prepareResult() {
Intent i = new Intent();
name = editText.getText().toString();
i.putExtra("name", name);
setResult(RESULT_OK, i);
finish();
}
}
In this case, I do not create an object, and I do not assign its reference to 'X' variables, I immediately call the prepareResult method. Why is this happening?

In Java, when you call another method in the same class, you do not need to reference by it's class object.
Where when you call the method from same class, you can access method directly.

It's all a matter of scope. In your first example, you were trying to use the method method() from the class Test from within another class, Question.
In the second example, you can call prepareResult() directly because the method from where that call is issued, onCreate(), belongs to the same class SecondActivity.
This is possible because, essentially, they are in the same scope. All methods and variables in a particular class are visible amongst each other. Visibility outside of the class depends on the access modifiers, public, private, protected or the default package-private
You can find more details in this Oracle Java tutorial:
https://docs.oracle.com/javase/tutorial/java/javaOO/accesscontrol.html

In Java, whenever you call a function which is outside the activity, you are calling. You have to use a reference variable to call the other class constructor to build an object. It can be done in your case.
Test test = new Test();
test.method();
OR
new Test().method();
Whenever you call a function which is inside the same class(within where you defined the function), you can call directly using its name directly because you do not have to call the constructor of the class which is already created. Like you have done in your other code.
You can see the class do not change here.
public class SecondActivity extends AppCompatActivity {
EditText editText;
Button button2;
String name;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_second);
editText = findViewById(R.id.editText);
button2 = findViewById(R.id.button);
button2.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View view) {
prepareResult();
}
});
}
public void prepareResult() {
Intent i = new Intent();
name = editText.getText().toString();
i.putExtra("name", name);
setResult(RESULT_OK, i);
finish();
}
}

How to use setOnClickListener to the buttons in android using java [duplicate]

I have trouble understanding this code. I get that findViewById will get the button widget and then it'll cast it. Then, it's going to use the button to call the setOnClickListener method. However, I don't know what is that argument being passed into the setOnClickListener and I have never seen code like that before. How is it that it creates a new object but is able to create a method of its own within another method's argument? Would be great if someone could explain that. Also, what type of object is the setOnClickListener method taking in?
btn = (Button)findViewById(R.id.firstButton);
btn.setOnClickListener(new View.OnClickListener()
{
#Override
public void onClick(View v)
{
tv.setText(months[rand.nextInt(12)]);
tv.setTextColor(Color.rgb(rand.nextInt(255)+1, rand.nextInt(255)+1, rand.nextInt(255)+1));
}
});

It works like this. View.OnClickListenere is defined -
public interface OnClickListener {
void onClick(View v);
}
As far as we know you cannot instantiate an object OnClickListener, as it doesn't have a method implemented. So there are two ways you can go by - you can implement this interface which will override onClick method like this:
public class MyListener implements View.OnClickListener {
#Override
public void onClick (View v) {
// your code here;
}
}
But it's tedious to do it each time as you want to set a click listener. So in order to avoid this you can provide the implementation for the method on spot, just like in an example you gave.
setOnClickListener takes View.OnClickListener as its parameter.

This is the best way to implement Onclicklistener for many buttons in a row
implement View.onclicklistener.
public class MainActivity extends AppCompatActivity implements View.OnClickListener {
This is a button in the MainActivity
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
bt_submit = (Button) findViewById(R.id.submit);
bt_submit.setOnClickListener(this);
}
This is an override method
#Override
public void onClick(View view) {
switch (view.getId()){
case R.id.submit:
//action
break;
case R.id.secondbutton:
//action
break;
}
}

That what manual says about setOnClickListener method is:
public void setOnClickListener (View.OnClickListener l)
Added in API level 1 Register a callback to be invoked when this view
is clicked. If this view is not clickable, it becomes clickable.
Parameters
l View.OnClickListener: The callback that will run
And normally you have to use it like this
public class ExampleActivity extends Activity implements OnClickListener {
protected void onCreate(Bundle savedValues) {
...
Button button = (Button)findViewById(R.id.corky);
button.setOnClickListener(this);
}
// Implement the OnClickListener callback
public void onClick(View v) {
// do something when the button is clicked
}
...
}
Take a look at this lesson as well Building a Simple Calculator using Android Studio.

its an implementation of anonymouse class object creation to give ease of writing less code and to save time

It works by same principle of anonymous inner class where we can instantiate an interface without actually defining a class :
Ref: https://www.geeksforgeeks.org/anonymous-inner-class-java/

How to define a specific layout onclick function globally for multiple activities

I need to define a layout for multiple activities in android and from the UI part, it is successful. But to code those elements to perform on each click listeners, I need to define it in all the java pages I use.
Can we globally define this in a java page and include it in the required pages?
menuButton = findViewById(R.id.menuButton);
menuButton.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View view) {
Intent i = new Intent(getApplicationContext(), MenuActivity.class);
startActivity(i);
finish();
overridePendingTransition(0, 0);
}
});

Yes and No.
You can create a BaseActivity which has the common logic that has to be executed for each button click.
But you need to implement the listener for the button on specific activity, since life cycle of each activity is independent of other activity.
To make the code readable better (avoiding implementing listener/setOnclickListener), you can use ButterKinfe, and create a method for OnClick() annotation, and call the method in BaseActivity.

What you essentially want to do is call the findViewById(), which can only be called if you have a reference to a Context variable. You should use your Activity Context, hence you pass this to the static function, which can then access all methods accessible via Context .
public class ExampleActivity2 extends AppCompatActivity {
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
MenuUtils.setListener(this);
}
}
Define the static class like this:
public static class MenuUtils{
public static void setListener(Context context){
menuButton = context.findViewById(R.id.menuButton);
menuButton.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View view) {
// do stuff
}
});
}
}
What you should be careful about is that any Activity you pass to this function should have a menuButton in it's layout, otherwise you run the risk of getting a NullPointerException, which is when findViewById() cannot find menuButton.

Call method from another object-java

I'm doing some Android development and I have an object, which doing a specific task. When that task is done I need to inform my main method (Main Activity), which is constantly running, that the process has been finished and pass some information about it.
This may sound a bit unclear, so I'll give you an example:
Take a look at the setOnClickListener() method in Android:
Button button = (Button) findViewById(R.id.button1);
button.setOnClickListener(new OnClickListener() {
//This method is called on click
#Override
public void onClick(View v) {
//The View is passed in an anonymous inner class
}
});
It waits for a button to be clicked and calls the onClick(View v) method. I am seeking to achieve the same code structure. How to do this?

You mentioned "process". If you are truly doing something in a different process, then you need to look at interprocess communications (IPC). Otherwise, you can use an interface:
Create a class called MyListener:
public interface MyListener {
void onComplete();
}
In your class that will notify your activity:
MyListener myListener;
public void setMyListener(MyListener myListener){
this.myListener = myListener;
}
Then, when you are ready to notify your main activity, call this line:
myListener.onComplete();
Last, in your MainActivity implement MyListener:
public class MyListener extends Activity implements MyListener {
///other stuff
#Override
public void onComplete(){
// here you are notified when onComplete it called
}
}
Hope this helps. Cheers.

This is exactly Listener pattern that you use with views in android. What you want to do is declare an interface in your class that's doing the job, and pass an instance of this interface. Raw example:
TaskDoer.java:
public class TaskDoer {
public interface OnTaskDoneListener {
void onDone(Data data);
}
public void doTask(OnTaskDoneListener listener) {
// do task...
listener.onDone(data);
}
}
Activity:
public void doTaskAndGetResult() {
new TaskDoer().doTask(new TaskDoer.OnTaskDoneListener() {
public void onDone(Data data) {
// do something
}
}
}

In android, when is a context different from "this"?

I am at baby step level of programming on Android (and in Java in general). I do understand that Activity inherits from the Context class. However in every code snippet I have come across, every time a context must be mentionned, it is set to "this".
My question is : when is a context different from "this" ? Could you provide an real life example of context needing to be different from "this"?
Thank you very much.

Typically, you will want to use this when you are "inside" of an Activity. However, when you are using for example a Helper class, the reference this will not work. An example can be something like this:
public class MyActivity extends Activity
{
#Override
public void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
this.setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_NOSENSOR);
}
}
A case, where you cannot:
public class MyHelper
{
/* some code of yours */
public void lockOrientation(Activity activity)
{
activity.setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_NOSENSOR);
}
}
The above code locks the orientation to the current orientation. Notice that you need to supply the method with an Activity parameter, since you cannot use:
this.setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_NOSENSOR);
In the first example, you could use this to achieve this, because you were "inside" of an Activity.
Another type of example, how do you set onClickListener.
First example, when you use this:
public class MyActivity extends Activity implements View.OnClickListener
{
#Override
public void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
Button btn=(Button)findViewById(R.id.mybutton);
btn.setOnClickListener(this);
}
#Override
public void onClick(View v)
{
//handle the click event
}
}
In this example, you can use this because in the first line, we wrote implements View.OnClickListener, so the class inherits from the given interface. Without the implements thingie, you couldn't do it. An example of setting the onClickListener without this:
public class MyActivity extends Activity
{
#Override
public void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
Button btn=(Button)findViewById(R.id.mybutton);
btn.setOnClickListener(new View.OnClickListener()
{
#Override
public void onClick(View v)
{
//handle the click event
}
});
}
}
In the second example, we are defining an Anonymous Inner Class, which will handle the click event of the button. Notice that in this case, our Activity does NOT implements View.OnClickListener.

In Outer Class you directly use "this" reference
In Inner Class Or Abstract Class implementation Or Interface implementation use "classname.this" reference

Example:
class Example{
int number = 0;
public Example(int number){
this.number = number;
}
}
notice that number in the contructor and number in the class are not the same. Altough they have the same name. Saying number = number doesn't make sense. Be using this you can asses number in the class.

For example when you are implementing an OnClickListener the "this" is different.

this is a reference to the current object — the object whose method or constructor is being called.
Inside an Activity's method this can be used as a Context object because Activity inherits from ContextThemeWrapper, which inherits from ContextWrapper, which inherits from Context.
A Fragment on the other hand does not inherit from Context. So to get the Context inside a Fragment you would have to call getActivity() for example.
This applies to any object you are calling this from.
Consider you are inside the OnClick() method of a View.OnClickListener and you want to start an Activity:
button.setOnClickListener (new View.OnClickListener() {
public void onClick(View v) {
Intent intent = new Intent(this, NextActivity.class); // wrong because 'this' is the OnClickListener object
Intent intent = new Intent(CurrentActivity.this, NextActivity.class); // correct because 'this' is the CurrentActivity object
startActivity(intent);
}
});