Develop Reference

Centering the contents of a view inside a LinearLayout

Here's the layout (below). I'm trying to reposition the location of the checkbox; move it to the right of the view. android:gravity and android:layout_gravity seem to have no effect. Any explanation? This LinearLayout is a child of a Relative Layout.
<LinearLayout
android:id="#+id/deviceLinear"
android:layout_width="fill_parent"
android:layout_height="match_parent"
android:layout_below="#id/view1" >
<ImageView
android:contentDescription="#string/devices_icon"
android:id="#+id/devices_img"
android:layout_width="0dip"
android:layout_height="match_parent"
android:layout_weight="0.15"
android:layout_gravity="center"
android:src="#drawable/hardware_phone" />
<TextView
android:id="#+id/devices"
android:layout_width="0dip"
android:layout_height="wrap_content"
android:layout_gravity="center"
android:layout_weight="0.43"
android:text="#string/devices"
android:textSize="20sp" />
<CheckBox
android:id="#+id/check_devices"
android:button="#drawable/custom_checkbox"
android:layout_width="0dip"
android:layout_height="fill_parent"
android:layout_weight="0.42"
android:onClick="onCheckboxClicked" />
</LinearLayout>

I assume that you want the CheckBox to be on the right side of the LinearLayout. Since you have given it a layout_weight it will always take up some fixed percentage of the width regardless of its size as you can see by that blue bounding box.
Try wrapping the checkbox in another LinearLayout that has android:orientation='horizontal'. Give the 0.42 weight to the wrapping LinearLayout and then set the LinearLayout's android:gravity to be right. That should keep your spacing while moving the checkbox to the far right.
Something like this:
<LinearLayout
android:orientation="horizontal"
android:layout_width="0dp"
android:layout_weight="0.42"
android:layout_height="wrap_content"
android:gravity="right" />
<CheckBox
android:layout_width="wrap_content"
android:layout_height="wrap_content" />
</LinearLayout>
You might also want to consider a RelativeLayout which enables you to position Views based on where other views are.

You are giving layout_weight="0.42" to your checkbox, that means that it will be measured with a width based on the LinearLayout's width. By this approach you will never manage to put it on the right.
The best way to achieve your goal is to use a RelativeLayout. Here is my list item layout with a checkbox on the right. Sorry about the merge tag but it is merged inside a RelativeLayout with
android:layout_width="match_parent"
android:layout_height="wrap_content"
attributes
<?xml version="1.0" encoding="utf-8"?>
<merge xmlns:android="http://schemas.android.com/apk/res/android">
<ImageView android:id="#+id/imgChannelIcon"
android:layout_alignParentLeft="true"
style="?muListItemImage60x60" />
<CheckBox android:id="#+id/chkIsChecked"
android:layout_width="wrap_content"
android:layout_height="0dp"
android:layout_alignTop="#+id/imgChannelIcon"
android:layout_alignParentRight="true"
android:layout_alignBottom="#+id/imgChannelIcon"
android:layout_marginLeft="#dimen/list_item_checkbox_margin_left"
android:layout_marginRight="#dimen/list_item_checkbox_margin_right"
android:layout_marginTop="#dimen/list_item_checkbox_margin_top"
android:layout_marginBottom="#dimen/list_item_checkbox_margin_bottom"
android:focusable="false"
android:focusableInTouchMode="false" />
<TextView android:id="#+id/lblChannelTitle"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:layout_toRightOf="#+id/imgChannelIcon"
android:layout_alignTop="#+id/imgChannelIcon"
android:layout_toLeftOf="#+id/chkIsChecked"
android:layout_alignWithParentIfMissing="true"
android:includeFontPadding="false"
android:textStyle="bold"
android:text="#string/channel_item_dummy_title"
style="?muListItemTextBig" />
<TextView android:id="#+id/lblChannelSubtitle"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:layout_toRightOf="#+id/imgChannelIcon"
android:layout_below="#+id/lblChannelTitle"
android:layout_toLeftOf="#+id/chkIsChecked"
android:layout_alignWithParentIfMissing="true"
android:includeFontPadding="false"
android:textStyle="normal"
android:text="#string/channel_item_dummy_subtitle"
style="?muListItemTextNormal" />
<TextView android:id="#+id/lblChannelCategory"
android:layout_width="0dp"
android:layout_height="0dp"
android:layout_toRightOf="#+id/imgChannelIcon"
android:layout_below="#+id/lblChannelSubtitle"
android:layout_alignBottom="#+id/imgChannelIcon"
android:layout_toLeftOf="#+id/chkIsChecked"
android:layout_alignWithParentIfMissing="true"
android:includeFontPadding="false"
android:paddingLeft="3dp"
android:gravity="center_vertical|left"
android:textStyle="italic"
android:text="#string/channel_item_dummy_category"
style="?muListItemTextNormal_Inverse" />
<TextView android:id="#+id/lblChannelUpdate"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:layout_below="#+id/imgChannelIcon"
android:layout_alignParentLeft="true"
android:layout_alignParentRight="true"
android:includeFontPadding="false"
android:paddingTop="3dp"
android:paddingBottom="3dp"
android:textStyle="italic"
android:text="#string/channel_item_dummy_update"
style="?muListItemTextTiny" />
</merge>
As you can see I first place an image with layout_alignParentLeft="true" then the checkbox with layout_alignParentRight="true" and finally I arrange the other components based on those two. From your image I can see that you can use your devices_img as the left component (but you will have to give it a fixed height and maybe some margins in order to became the Layout's height) and your check box as your right.
Keep in mind that in a RelativeLayout with wrap_content as height you cannot use the AlignParentBottom attribute.
Hope this helps...

Align textview in a block

I wat to create an app like this, I use textview to preview number
<TextView
android:id="#+id/generatenumber1"
android:layout_width="fill_parent"
android:layout_height="wrap_content" />
<TextView
android:id="#+id/generatenumber2"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
How to align and color them like this, http://s24.postimg.org/sr53ezhtd/sdsd.png ?
I tried android:layout_gravity="center_vertical|right but not work.

Use android:gravity="center", and add padding to the left and right to create the whitespace between items.
Layout_gravity is used by layouts to define how they put the views inside it. Gravity is used for text/data in a view.

Try this,
<TextView
android:id="#+id/txtName"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:background="#FF0000"
android:padding="10dip"
android:text="7"
android:textColor="#fff" />

RelativeLayout weight

In a layout resource XML, I have 3 RelativeLayout(s) which are inside a main RelativeLayout. The view will be shown vertically. These 3 RelativeLayout() are set next to each other, and I want them to fill the whole screen, doesnt matter what will be the screen size. My, layout view:
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="#drawable/backg"
>
<ImageView
android:id="#+id/imageView1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentLeft="true"
android:layout_alignParentTop="true"
android:layout_marginTop="#dimen/top_mr_image"
android:src="#drawable/temp" />
<RelativeLayout
android:id="#+id/r1"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:layout_alignParentBottom="true"
android:layout_alignParentLeft="true"
android:layout_below="#+id/imageView1"
android:layout_marginLeft="10dp"
android:background="#drawable/r1bg"
android:layout_weight="1"
>
<TextView
android:id="#+id/textView1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentLeft="true"
android:layout_marginLeft="#dimen/txt_mr_right"
android:layout_marginRight="#dimen/txt_mr_right"
android:layout_marginTop="39dp"
android:text="S"
android:textAppearance="?android:attr/textAppearanceLarge" />
<TextView
android:id="#+id/textView2"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentLeft="true"
android:layout_below="#id/textView1"
android:layout_marginLeft="#dimen/txt_mr_right"
android:layout_marginRight="#dimen/txt_mr_right"
android:text="T"
android:textAppearance="?android:attr/textAppearanceLarge" />
</RelativeLayout>
<RelativeLayout
android:id="#+id/r2"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:layout_alignParentBottom="true"
android:layout_alignTop="#+id/r1"
android:layout_toRightOf="#+id/r1"
android:layout_weight="1" >
</RelativeLayout>
<RelativeLayout
android:id="#+id/r3"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:layout_alignParentBottom="true"
android:layout_alignTop="#+id/r2"
android:layout_toRightOf="#+id/r2"
android:layout_weight="1" >
</RelativeLayout>
I set weight=1 and layout_width=0dp for each relativeLayout and this technique works with buttons, I thought the same will be with relativeLayout, seems my thoughts were wrong. Any idea?
UPD1: I have added an image of what I would like to have

RelativeLayout does not pay attention to android:layout_weight. (That's a property of LinearLayout.LayoutParams, but not of RelativeLayout.LayoutParams.)
You should be able to get the layout you want with a much simpler view hierarchy. It's not clear what you are trying to do, since the last two RelativeLayouts are empty. If you need a purely vertical organization, I'd suggest using LinearLayout instead of RelativeLayout.
EDIT Based on your edit, it looks like you want a horizontal layout of three compound views, each one clickable. I think something like the following will work:
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="horizontal"
android:layout_width="match_parent"
android:layout_height="match_parent"
>
<!-- First column -->
<LinearLayout
android:id="#+id/firstColumn"
android:orientation="vertical"
android:gravity="center_horizontal"
android:layout_width="0dp"
android:layout_height="match_parent"
android:layout_weight="1"
>
<ImageView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:src="..." />
<TextView
android:layout_width="wrap_content"
android:layout_height="match_parent"
android:text="text 1"
. . . />
</LinearLayout>
<!-- Second column -->
<LinearLayout . . . >
. . .
</LinearLayout>
</LinearLayout>
If the contents of the buttons aren't correct, you can replace the second-level LinearLayout views with RelativeLayout if that helps organize the layout better.

RelativeLayouts do not support weight. You need to use a LinearLayout as a parent container if you want to use weights.

Solution is very simple. I have been looking for weight distribution in relative layout.
It's a small trick for all these kind situations.
Use LinearLayout with android:orientation="horizontal"

You can use Horizontally oriented LinearLayout Manager in the Recycler View, and place each RelativeLayout in each item, of its Adapter.
The Link: How to build a Horizontal ListView with RecyclerView?
If your RelativeLayouts are set to a fixed width and height, that is to the size of the Screen, that you can get from DisplayMatrics, that will be OK.
The Link: Get Screen width and height
If the contents of your RelativeLayouts are different, then you can use getItemViewType() method.
Please see: How to create RecyclerView with multiple view type?
Happy Coding :-)

Center multiple items in a RelativeLayout without putting them in a container?

I have a RelativeLayout containing a pair of side-by-side buttons, which I want to be centered within the layout. I could just put the buttons in a LinearLayout and center that in the RelativeLayout, but I want to keep my xml as clean as possible.
Here's what I tried, this just puts the "apply" button in the center and the "undo" button to the left of it:
<RelativeLayout
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:layout_marginTop="15sp">
<TextView
android:id="#+id/instructions"
android:text="#string/instructions"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginBottom="15sp"
android:layout_centerHorizontal="true"
/>
<Button
android:id="#+id/apply"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_centerInParent="true"
android:text="#string/apply"
android:textSize="20sp"
android:layout_below="#id/instructions"
/>
<Button
android:id="#+id/undo"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_centerInParent="true"
android:text="#string/undo"
android:textSize="20sp"
android:layout_toLeftOf="#id/apply"
android:layout_below="#id/instructions"
/>
</RelativeLayout>

android:gravity will align the content inside the view or layout it is used on.
android:layout_gravity will align the view or layout inside of his parent.
So adding
android:gravity="center"
to your RelativeLayout should do the trick...
Like this:
<RelativeLayout
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:gravity="center"
android:layout_marginTop="15sp">
</RelativeLayout>

Here is an extension of BrainCrash's answer. It is a non nested option that groups and centers all three horizontally and vertically. In addition, it takes the top TextView and centers it horizontally across both buttons. If desired, you can then center the text within the TextView with android:gravity="center". I also removed the margins, added color, and set the RelativeLayout height to fill_parent to highlight the layout. Tested on API 11.
<RelativeLayout
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:gravity="center"
android:background="#android:color/black"
>
<TextView
android:id="#+id/instructions"
android:text="TEST"
android:textSize="20sp"
android:background="#android:color/darker_gray"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_centerHorizontal="true"
android:layout_alignLeft="#+id/undo"
android:layout_alignRight="#+id/apply"
android:gravity="center"
/>
<Button
android:id="#+id/apply"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_centerInParent="true"
android:text="APPLY"
android:textSize="20sp"
android:layout_below="#id/instructions"
/>
<Button
android:id="#+id/undo"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_centerInParent="true"
android:text="UNDO"
android:textSize="20sp"
android:layout_toLeftOf="#id/apply"
android:layout_below="#id/instructions"
/>
</RelativeLayout>

android:layout_gravity="center"
will almost give what you're looking for.

Here is a combination of the above answer's that solved my specific situation:
Centering two separate labels within a layout that also includes a button in the left most position of the same layout (button, label, label, from left to right, where the labels are centered relative to the layout containing all three views - that is, the button doesn't push the labels off center).
I solved this by nesting two RelativeLayout's, where the outer most layout included the
Button and an Inner-RelativeLayout.
The Inner-RelativeLayout contained the two text labels (TextView's).
Here is a snippet that provides the details of how the centering and other layout stuff was done:
see: RelativeLayout Gravity not applied? and
Gravity and layout_gravity on Android
for the difference's between gravity and layout_gravity.
Tweak the paddingLeft on the btn_button1 Button to see that the TextView's do not move.
(My apologies to havexz for the downvotes. I was too hasty in thinking that just b/c your suggestions didn't solve the exact question being ask, that they do help to solve very similar situations (the answer here solves a very specific situation, and only the combination of all these answer's solved my problem. I tried upvoting, but it won't let me unless I edit the answer's, which I don't want to do.)
<RelativeLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:id="#+id/rl_outer"
android:layout_width="fill_parent"
android:layout_height="50dip"
android:background="#FF0000FF">
<Button
xmlns:android="http://schemas.android.com/apk/res/android"
android:id="#+id/btn_button1"
android:layout_width="wrap_content"
android:layout_height="fill_parent"
android:background="#FF00FF00"
android:text="<"
android:textSize="20sp"
android:paddingLeft="40dip"/>
<RelativeLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:id="#+id/rl_inner"
android:layout_width="wrap_content"
android:layout_height="fill_parent"
android:background="#FFFF00FF"
android:layout_centerInParent="true"
android:gravity="center">
<TextView
xmlns:android="http://schemas.android.com/apk/res/android"
android:id="#+id/tv_text1"
android:layout_width="wrap_content"
android:layout_height="fill_parent"
android:background="#FF505050"
android:textSize="20sp"
android:textColor="#FFFFFFFF"
android:text="Complaint #"
android:gravity="center"/>
<TextView
xmlns:android="http://schemas.android.com/apk/res/android"
android:id="#+id/tv_text2"
android:layout_width="wrap_content"
android:layout_height="fill_parent"
android:background="#FF505050"
android:textSize="20sp"
android:textColor="#FFFFFFFF"
android:layout_toRightOf="#id/tv_text1"
android:gravity="center"/>
</RelativeLayout>
</RelativeLayout>

LinearLayout is a good option. Other than that there are options like create an invisible view and center that and then align left button to the left it and right on the right of it. BUT those are just work arounds.

How do I center text horizontally and vertically in a TextView?

How do I center the text horizontally and vertically in a TextView, so that it appears exactly in the middle of the TextView in Android?

I'm assuming you're using XML layout.
<TextView
android:layout_width="match_parent"
android:layout_height="match_parent"
android:gravity="center"
android:text="#string/**yourtextstring**"
/>
You can also use gravity center_vertical or center_horizontal according to your need.
As #stealthcopter commented, in java: .setGravity(Gravity.CENTER);.
And for Kotlin users, .gravity = Gravity.CENTER

android:gravity="center"
This will do the trick

You can also set it up dynamically using:
textView.setGravity(Gravity.CENTER_VERTICAL | Gravity.CENTER_HORIZONTAL);

android:layout_centerInParent="true"
This works when used with a RelativeLayout where the layout's height & width are set to wrap_content.

You can also use the combination:
android:gravity="left|center"
Then, if textview width is more than "fill_parent" the text will still be aligned to left (not centered as with gravity set only to "center").

Apply gravity:
TextView txtView = (TextView) findViewById(R.id.txtView);
txtView.setGravity(Gravity.CENTER_HORIZONTAL);
For vertical:
txtView.setGravity(Gravity.CENTER_VERTICAL);
In XML:
<TextView
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:gravity="center"
android:text="#string/Hello_World"
/>

There are two ways of doing this.
The first in the XML code. You need to pay attention at the Gravity Attribute. You also can find this attribute in the Graphic Editor; it may be easier than the XML EDITOR.
<TextView
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:gravity="center_vertical|center_horizontal"
android:text="Your Text"
/>
For your specific scenario, the values of gravity will be:
center_vertical|center_horizontal
In the graphical editor you will find all the possible values, even see their results.

If you are using TableLayout make sure to set the gravity of the TableRows to center, too.
Otherwise it will not work. At least it didn't work with me until I set the gravity of the TableRow to center.
For example, like this:
<TableRow android:id="#+id/tableRow2" android:layout_width="wrap_content" android:layout_height="wrap_content" android:gravity="center">
<TextView android:text="#string/chf" android:id="#+id/tv_chf" android:layout_weight="2" android:layout_height="wrap_content" android:layout_width="fill_parent" android:gravity="center"></TextView>
</TableRow>

You need to set TextView Gravity (Center Horizontal & Center Vertical) like this:
android:layout_centerHorizontal="true"
and
android:layout_centerVertical="true"
And dynamically using:
textview.setGravity(Gravity.CENTER);
textView.setGravity(Gravity.CENTER_VERTICAL | Gravity.CENTER_HORIZONTAL);

In my opinion,
android:gravity="center"
is better than,
android:layout_centerInParent="true"
which is better than,
android:layout_centerHorizontal="true"
android:layout_centerVertical="true"
at least for formatting text.

For Linear Layout:
In XML use something like this
<TextView
android:id="#+id/textView1"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:gravity="center_vertical|center_horizontal"
android:text="Your Text goes here"
/>
To do this at run time use something like this in your activity
TextView textView1 =(TextView)findViewById(R.id.texView1);
textView1.setGravity(Gravity.CENTER_VERTICAL | Gravity.CENTER_HORIZONTAL);
For Relative Layout: in XML use some thing like this
<TextView
android:id="#+id/textView1"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:layout_centerInParent="true"
android:text="Your Text goes here"
/>
To do this at run time use something like this in your activity
TextView textView1 =(TextView)findViewById(R.id.texView1);
RelativeLayout.LayoutParams layoutParams = RelativeLayout.LayoutParams)textView1.getLayoutParams();
layoutParams.addRule(RelativeLayout.CENTER_IN_PARENT, RelativeLayout.TRUE);
textView1.setLayoutParams(layoutParams);

Use in the XML file.
Layout file
<TextView
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:gravity="center"
android:text="#string/stringtext"/>
or:
Use this inside the Java class
TextView textView =(TextView)findViewById(R.id.texviewid);
textView.setGravity(Gravity.CENTER_VERTICAL | Gravity.CENTER_HORIZONTAL);

Use this for relative layout
android:layout_centerInParent="true"
and for other layout
android:gravity="center"

If the TextView's height and width are wrap content then the text within the TextView always be centered. But if the TextView's width is match_parent and height is match_parent or wrap_content then you have to write the below code:
For RelativeLayout:
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent">
<TextView
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:gravity="center"
android:text="Hello World" />
</RelativeLayout>
For LinearLayout:
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent">
<TextView
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:gravity="center"
android:text="Hello World" />
</LinearLayout>

While using gravity works for TextView, there's an alternate method implemented in API level 17 -
textView.setTextAlignment(View.TEXT_ALIGNMENT_CENTER);
Don't know the difference, but it works too. However only for API level 17 or higher.

In RelativeLayout, it will be nice with it.
And another Button and anything else you can add.
The following works nicely for me.
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:background="#ff314859"
android:paddingLeft="16dp"
android:paddingRight="16dp">
<TextView
android:id="#+id/txt_logo"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:text="your text here"
android:textSize="30dp"
android:gravity="center"/>
...other button or anything else...
</RelativeLayout>

Use android:textAlignment="center"
<TextView
android:text="HOW WAS\nYOUR\nDAY?"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:textAlignment="center"
android:id="#+id/textView5"
/>

Easiest way (which is surprisingly only mentioned in comments, hence why I am posting as an answer) is:
textview.setGravity(Gravity.CENTER)

You can just set the gravity of your textview into CENTER.

TextView gravity works as per your parent layout.
LinearLayout:
If you use LinearLayout then you will find two gravity attribute
android:gravity & android:layout_gravity
android:gravity : represent layout potion of internal text of TextView while
android:layout_gravity : represent TextView position in parent view.
If you want to set text horizontally & vertically center then use below code this
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="300dp"
android:background="#android:color/background_light"
android:layout_height="300dp">
<TextView
android:layout_width="match_parent"
android:text="Hello World!"
android:gravity="center_horizontal"
android:layout_gravity="center_vertical"
android:layout_height="wrap_content"
/>
</LinearLayout>
RelativeLayout:
Using RelativeLayout you can use below property in TextView
android:gravity="center" for text center in TextView.
android:gravity="center_horizontal" inner text if you want horizontally centered.
android:gravity="center_vertical" inner text if you want vertically centered.
android:layout_centerInParent="true" if you want TextView in center position of parent view.
android:layout_centerHorizontal="true" if you want TextView in horizontally center of parent view.
android:layout_centerVertical="true" if you want TextView in vertically center of parent view.
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="300dp"
android:background="#android:color/background_light"
android:layout_height="300dp">
<TextView
android:layout_width="match_parent"
android:text="Hello World!"
android:gravity="center"
android:layout_centerInParent="true"
android:layout_height="wrap_content"
/>
</RelativeLayout>

If you are trying to center text on a TableRow in a TableLayout, here is how I achieved this:
<TableRow android:id="#+id/rowName"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:padding="5dip" >
<TextView android:id="#+id/lblSomeLabel"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:gravity="center"
android:layout_width="0dp"
android:layout_weight="100"
android:text="Your Text Here" />
</TableRow>

If you are using Relative Layout:
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="#string/stringname"
android:layout_centerInParent="true"/>
If you are using LinearLayout
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="#string/stringname"
android:layout_gravity="center"/>

Try this way,it will work
<LinearLayout
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:orientation="vertical"
android:gravity="center">
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:textAlignment="center"/>
</LinearLayout>

Here is my answer that I had used in my app. It shows text in center of the screen.
<TextView
android:id="#+id/txtSubject"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:text="#string/subject"
android:layout_margin="10dp"
android:gravity="center"
android:textAppearance="?android:attr/textAppearanceLarge" />

The TextView's height and width are wrap content then the text within the textview always be centered, then make center in its parent layout by using:
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent">
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_centerInParent="true"
android:text="Hello.."/>
</RelativeLayout>
For LinearLayout also the code is same :
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:gravity="center">
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Hello.."/>
</LinearLayout>
and pro-grammatically parent is RelativeLayout java code this at run time use something like this in your activity
TextView textView1 =(TextView)findViewById(R.id.texView1);
RelativeLayout.LayoutParams layoutParams = RelativeLayout.LayoutParams)textView1.getLayoutParams();
layoutParams.addRule(RelativeLayout.CENTER_IN_PARENT, RelativeLayout.TRUE);
textView1.setLayoutParams(layoutParams);

Actually, we can do better by excluding fontPadding.
<TextView
android layout_height="wrap_content"
android layout_height="wrap_content"
android:includeFontPadding="false"
android:textAlignment="center"
/>

As many answers suggest above works fine.
android:gravity="center"
If you want to center it just vertically:
android:gravity="center_vertical"
or just horizontally:
android:gravity="center_horizontal"

Simply, in your XML file, set the textview gravity to center:
<TextView
android:gravity="center" />

android:gravity="center_horizontal" for align text Center horizontally.
android:gravity="center_vertical" for align text Center vertically.
android:gravity="center" for align text Center both vertically and horizontally.
<TextView
android:layout_width="match_parent"
android:gravity="center_horizontal|center_vertical"
android:layout_height="match_parent" />

You can do like this to get text centered
<TextView
android:layout_width="match_parent"
android:layout_height="match_parent"
android:gravity="center" />