Develop Reference

new object issue

I'm new to Android Studio.
I made a TextView and a Button in Android Studio.
when I click button it supposed to trigger this method:
public void click (View view)
{
TextView tex = (TextView) findViewById(R.id.text_view);
tex.setText("Hello");
}
The code works in this way .
but when I made the method like this:
public void click (View view)
{
TextView tex = new TextView(this);
findViewById(R.id.text_view);
tex.setText("Hello");
}
the code doesn't do what it supposed to do. I mean nothing happen to the TextView text.
Can anyone explain to me what's the difference? and why this happens or what's wrong about the second case?

TextView tex = new TextView(this);
That code creates new textview instance but this textview has no connection with your view.
But TextView tex = (TextView) findViewById(R.id.text_view); finds the first descendant view with the given ID and assigns it to your local variable so you have connection.
More info https://developer.android.com/reference/android/view/View#findViewById(int)

public void click (View view)
{
TextView tex = new TextView(this);
findViewById(R.id.text_view);
tex.setText("Hello");
}
In this section, tex is a textView object but it does not have any field on the view.
TextView tex= findViewById(R.id.textView)
this line assigns a view to tex object.
The reason is quite obvious.

it seems to me that you are fairly new to programming and doesn't understand java very well, and doesn't have any programming experience of some sort. You see, Semi colons are used to terminate a line of code. So, thats one of the things you missed. Second, this:
TextView tex = (TextView) findViewById(R.id.text_view);
tex.setText("Hello");
gets a reference from a view on the first line and sets the text on the second line. While this:
TextView tex = new TextView(this);
findViewById(R.id.text_view);
tex.setText("Hello");
Creates a new view on the first line. Gets a reference of the view on the second whilst not using its reference. And sets the text on the third line to the new view that was created on the first line.
Suggestion
OOP is not a really hard subject, but, if you really want to code, better know a little more about simple programming before diving in to OOP.

https://developer.android.com/reference/android/view/View.html#findViewById(int)
findviewbyid returns a View.
TextView tex = new TextView(this);
findViewById(R.id.text_view);
when you do this, you created a new TextView but you never actualy did anything with the View returned by findViewById since you ignored it.

There are two good ways to this as listed below
setOnClickListner
Using android view binding library (Maybe in combination with ViewModel and difficult, research nice and slow ! :p)
using android:onClick="methodName()" attribute, (I personally don't prefer this, AFAIK doesn't work in fragment)
Here is the Easiest one!
Put this code into onCreate after setContentView is called
Button btn = findViewById(R.id.your_btn_id);
btn.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
//handle triggner here i.e set hello to your textView
}
});
Happy coding!

You need add tex into your layout to see it:
((ViewGroup) findViewById(R.id.some_container_id)).addView(tex)
When you call
findViewById(R.id.text_view);
and ignore returning value, this function have no effect

Button to hide a TextView

I've been searching for a solution for this for a while but cannot seem to get one working. There are one or two on here about this subject but I can't seem to get them going. I'm also a novice in Android and while I've been on and off playing with it for a few years, I still understand next to nothing about what I'm writing.
Basically I've got a TextView and a button. Ideally I'd like to put some text in the TextView, press a button it's gone, press the button again and it's back.
I've narrowed it down to needing to understand what findViewById(R.id.button2) does but honestly I'm a bit lost.
I've added my button code but apologies that this is such a noob question
public void onClick(Button v){
TextView t1 = (TextView)findViewById(R.id.editText);
v.setVisibility(View.GONE);
Button button = (Button) findViewById(R.id.button2);
button.setOnClickListener(new View.OnClickListener() {
public void onClick(View v) {
TextView t1 = (TextView)findViewById(R.id.TextView);
v.setVisibility(View.GONE);
}
});
}

Your code has a couple of issues. I'm not going to give you the code because that won't really help you learn. Instead I'll explain things and let you try to figure it out or come back with more explicit questions.
You know that xml file you set using setContentView? Some of the tags in it had a property android:id="xxxx". That xxxx is the id of that view, its used so you can find that view in your code. The findViewById function walks through all the views on screen and finds a view with that id and returns it. That gives you a reference to the view so you can change it. For example, you can set its visibility, set its background color, or set an OnClickListener.
So to have a button toggle the visibility of another view, you need to be able to do the following things:
1)Find the view who's visibility you want to change
2)Figure out what its visibility currently is
3)Figure out what you want it to be (the opposite of what it currently is
4)Set that visibility
You need to write a function that does all that. Then you need to do this
1)Find the button you want to use to change the visibility
2)Tell it to call your function when its pressed.
Figure out how to do each of those steps individually, and you should be able to put it together. Good luck.

findViewById(R.id.button2) finds the view with the id button2.
You can check inside onClick whether t1 is visible or not (t1.setVisibility(View.GONE); not v.setVisibility(View.GONE);), and toggle between View.GONE and View.VISIBLE.
Remember that your findViewById() should have a real id. They are normally set on the activity_name.xml.
You are using a onClick inside a onClick. Personally I recommend setting the listener manually with setOnClickListener.
There's a lot of work for you, start with these tutorials. Keep trying and try to understand what you are doing.

Look like you need a toogle button feature, here is a piece of code.
Important: you must pay heed to #GabeSechan and #SkyDriver2500 answers.
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.your_layout);
//your other code
Button button = (Button) findViewById(R.id.button2);
final TextView t1 = (TextView) findViewById(R.id.editText);
button.setOnClickListener(new View.OnClickListener() {
public void onClick(View v) {
t1.setVisibility(t1.getVisibility() == View.VISIBLE ? View.GONE : View.VISIBLE);
}
});
}

I'm not sure if the code will help you now. But just in case, here it is
final boolean[] isTvVisible = {false};
final TextView t1 = (TextView)findViewById(R.id.editText);
t1.setVisibility(View.GONE);
Button button = (Button) findViewById(R.id.button2);
button.setOnClickListener(new View.OnClickListener() {
public void onClick(View v) {
if (isTvVisible[0]) {
t1.setVisibility(View.GONE);
isTvVisible[0] = false;
} else {
t1.setVisibility(View.VISIBLE);
isTvVisible[0] = true;
}
}
});

How can I change Text of Textview from other funcion?

this is my first question so I hope to make it clear.
I have one textView with some numerical text and next to it one button with one click listener and what I want is that when you click on the button the numerical value (>=0) of the TextView decrements in one.
Here is part of my code:
TextView Counter = new TextView(this);
if (intSeries != 0)
Counter.setText(Integer.toString(intSeries));
else
Counter.setText("0");
Counter.setId(4);
tablaContador.addView(Counter,Tr);
Button Done = new Button(this);
Done.setText("-1");
if (intSeries != 0)
Done.setVisibility(View.VISIBLE);
else
Done.setVisibility(View.GONE);
Done.setId(6);
Done.setOnClickListener(this);
And this is the onClick funcion (part of it):
#Override
public void onClick(final View v) {
// TODO Auto-generated method stub
switch (v.getId()) {
case 6:{
TextView text = (TextView)findViewById(4);
int series = Integer.parseInt(text.getText().toString());
series--;
text.setText(series);
if (series==0){
Button boton = (Button)findViewById(6);
boton.setVisibility(View.GONE);
}
}
}
}
The error is when I try to make the setText inside the onClick function, I hope it can be fixed or maybe recieve other idea to do it.
Thank you so much.

I would avoid all this hardcoding of Ids, use resources instead.
Your call to
text.setText(series)
is passing an int. The only valid setText(int resId) overload expects a resource associated with the int value, i.e. a string resource.
Convert your series value to a string.
Something like:
text.setText(Integer.toString(series));

You should setup series as an integer. And increase/descrease it as you wish. When you want to change the button's text convert the int to String.
Instead of:
text.setText(series);
use:
text.setText(String.valueOf(series));

Variablenames in java can't start with a capital letter. That is reserved for classnames.
Counter -> counter
Done -> done

I tried this and it worked:
//Create onClickListener
OnClickListener pickChoice = new OnClickListener()
{
public void onClick(View v)
{
TextView txt = (TextView) findViewById(4);
int number = Integer.valueOf(txt.getText().toString());
txt.setText(String.valueOf(number -1));
}
};
//Create layout
LinearLayout lnLayout = new LinearLayout(this);
lnLayout.setOrientation(LinearLayout.VERTICAL);
TextView txt = new TextView(this);
txt.setId(4);
txt.setText("0");
lnLayout.addView(txt);
Button Done = new Button(this);
Done.setText("-1");
Done.setId(6);
Done.setOnClickListener(pickChoice);
lnLayout.addView(Done);
setContentView(lnLayout);
Where are you creating your button inside? an activity? the part where you pass the onClickListener to the button doesn't make sense, maybe the button is getting a wrong listener and gets you an error every time you press the button ?
The code should be easy to understand, if there is anything you need me to explain please ask :)

How to update just a part of the activity

Well, the title says everything, but there's a little problem, I can't use a fragment or a loader, because I'm developing for android 2.3, so, how can I update just a part of the activity?

You can still use a fragment even in 2.3 by using the android-support-v4 jar

You can reference the views by ID. As an example to change a EditText you can:
EditText text = (EditText) findViewById(R.id.input);
test.setText("New Text");
Just change the cast to the proper type, so for a button:
Button btn = (Button) findViewById(R.id.button);
So for your code if you have a button with the xml element android:onclick="increment" you can have a function such as:
public void increment(View view) {
TextView number = (TextView) findViewById(R.id.number);
number.setText(Integer.toString(Integer.parseInt(number.getText()) + 1));
}

editing a global variable

Hi im new to android and I have a program that has a global variable define and it works, so I can set it and get it in every activity, BUT it dosnt like to be changed in an on click listener. I made it so on the screen there is an edittext and when someone presses a button I want the edittext text to be put into the global variable. here is my code:
Button SiteButton = (Button) findViewById(R.id.SiteButton);
SiteButton.setOnClickListener(new View.OnClickListener() {
TextView textviewS = (TextView) findViewById(R.id.SiteIdT);
EditText edittextS = (EditText) findViewById(R.id.SiteIdE);
TextView textviewB = (TextView) findViewById(R.id.BusIdT);
EditText edittextB = (EditText) findViewById(R.id.BusIdE);
public void onClick(View v) {
textviewS.setText(edittextS.getText());
((Global) this.getApplication()).setgSiteId(textviewS.getText().toString());
textviewB.setText(edittextB.getText());
((Global) this.getApplication()).setgVehicleId(textviewB.getText().toString());
}
});
but the getApplication() part is showing an error. can anyone help?

You should refer to your activity this, since View.OnClickListener doesn't have such a method:
// Bad code! read below
((Global) MyActivityClassName.this.getApplication()).setgSiteId(textviewS.getText().toString());
textviewB.setText(edittextB.getText());
((Global) MyActivityClassName.this.getApplication()).setgVehicleId(textviewB.getText().toString());
By the way, how do you cast the return from getApplication() to Global? You will get a class cast exception there.