Different languages have different rules for grammatical agreement with quantity. In English, for example, the quantity 1 is a special case. We write “1 book”, but for any other quantity we’d write “n books”. This distinction between singular and plural is very common, but other languages make finer distinctions. The full set supported by Android is zero, one, two, few, many, and other.
The rules for deciding which case to use for a given language and quantity can be very complex, so Android provides you with methods such as getQuantityString() to select the appropriate resource for you.
Although historically called “quantity strings” (and still called that in API), quantity strings should only be used for plurals. It would be a mistake to use quantity strings to implement something like Gmail’s “Inbox” versus “Inbox (12)” when there are unread messages, for example. It might seem convenient to use quantity strings instead of an if statement, but it’s important to note that some languages (such as Chinese) don’t make these grammatical distinctions at all, so you’ll always get the other string.
The selection of which string to use is made solely based on grammatical necessity. In English, a string for zero will be ignored even if the quantity is 0, because 0 isn’t grammatically different from 2, or any other number except 1 (“zero books”, “one book”, “two books”, and so on). Conversely, in Korean only the other string will ever be used.
Don’t be misled either by the fact that, say, two sounds like it could only apply to the quantity 2: a language may require that 2, 12, 102 (and so on) are all treated like one another but differently to other quantities. Rely on your translator to know what distinctions their language actually insists upon.
It’s often possible to avoid quantity strings by using quantity-neutral formulations such as “Books: 1”. This will make your life and your translators’ lives easier, if it’s a style that’s in keeping with your application.
Note: A plurals collection is a simple resource that is referenced using the value provided in the name attribute (not the name of the XML file). As such, you can combine plurals resources with other simple resources in the one XML file, under one <resources> element.
file location:
res/values/filename.xml
The filename is arbitrary. The <plurals> element’s name will be used as the resource ID.
Required. This must be the root node.No attributes.
<plurals>
A collection of strings, of which, one string is provided depending on the amount of something. Contains one or more <item> elements.
attributes:
name
String. A name for the pair of strings. This name will be used as the resource ID.
<item>
A plural or singular string. The value can be a reference to another string resource. Must be a child of a <plurals> element. Beware that you must escape apostrophes and quotation marks. See Formatting and Styling, below, for information about to properly style and format your strings.
attributes:
quantity
Keyword. A value indicating when this string should be used. Valid values, with non-exhaustive examples in parentheses:
Value
Description
zero
When the language requires special treatment of the number 0 (as in Arabic).
one
When the language requires special treatment of numbers like one (as with the number 1 in English and most other languages; in Russian, any number ending in 1 but not ending in 11 is in this class).
two
When the language requires special treatment of numbers like two (as with 2 in Welsh, or 102 in Slovenian).
few
When the language requires special treatment of “small” numbers (as with 2, 3, and 4 in Czech; or numbers ending 2, 3, or 4 but not 12, 13, or 14 in Polish).
many
When the language requires special treatment of “large” numbers (as with numbers ending 11-99 in Maltese).
other
When the language does not require special treatment of the given quantity (as with all numbers in Chinese, or 42 in English).
example:
XML file saved at res/values/strings.xml:
<?xml version="1.0" encoding="utf-8"?><resources><pluralsname="numberOfSongsAvailable"><!--
As a developer, you should always supply "one" and "other"
strings. Your translators will know which strings are actually
needed for their language. Always include %d in "one" because
translators will need to use %d for languages where "one"
doesn't mean 1 (as explained above).
--><itemquantity="one">%d song found.</item><itemquantity="other">%d songs found.</item></plurals></resources>
int count = getNumberOfsongsAvailable();Resources res =getResources();String songsFound = res.getQuantityString(R.plurals.numberOfSongsAvailable, count, count);
When using the getQuantityString() method, you need to pass the count twice if your string includes string formatting with a number. For example, for the string %d songs found, the first count parameter selects the appropriate plural string and the second count parameter is inserted into the %d placeholder. If your plural strings do not include string formatting, you don’t need to pass the third parameter to getQuantityString.
Formatting and Styling
Here are a few important things you should know about how to properly format and style your string resources.
Escaping apostrophes and quotes
If you have an apostrophe or a quote in your string, you must either escape it or enclose the whole string in the other type of enclosing quotes. For example, here are some stings that do and don’t work:
<stringname="good_example">"This'll work"</string><stringname="good_example_2">This\'ll also work</string><stringname="bad_example">This doesn't work</string><stringname="bad_example_2">XML encodings don't work</string>
Formatting strings
If you need to format your strings using String.format(String, Object...), then you can do so by putting your format arguments in the string resource. For example, with the following resource:
<stringname="welcome_messages">Hello, %1$s! You have %2$d new messages.</string>
In this example, the format string has two arguments: %1$s is a string and %2$d is a decimal number. You can format the string with arguments from your application like this:
Resources res =getResources();String text =String.format(res.getString(R.string.welcome_messages), username, mailCount);
You have a bona fide Git repository and a checkout or working copy of the files for that project. You need to make some changes and commit snapshots of those changes into your repository each time the project reaches a state you want to record.
Remember that each file in your working directory can be in one of two states: tracked or untracked. Tracked files are files that were in the last snapshot; they can be unmodified, modified, or staged. Untracked files are everything else – any files in your working directory that were not in your last snapshot and are not in your staging area. When you first clone a repository, all of your files will be tracked and unmodified because you just checked them out and haven’t edited anything.
As you edit files, Git sees them as modified, because you’ve changed them since your last commit. You stage these modified files and then commit all your staged changes, and the cycle repeats.
The main tool you use to determine which files are in which state is the git status command. If you run this command directly after a clone, you should see something like this:
$ git status
On branch masternothing to commit, working directory clean
This means you have a clean working directory – in other words, there are no tracked and modified files. Git also doesn’t see any untracked files, or they would be listed here. Finally, the command tells you which branch you’re on and informs you that it has not diverged from the same branch on the server. For now, that branch is always “master”, which is the default; you won’t worry about it here. Chapter 3 will go over branches and references in detail.
Let’s say you add a new file to your project, a simple README file. If the file didn’t exist before, and you run git status, you see your untracked file like so:
$echo'My Project' > README
$ git status
On branch masterUntracked files: (use "git add <file>..." to include in what will be committed) READMEnothing added to commit but untracked files present (use "git add" to track)
You can see that your new README file is untracked, because it’s under the “Untracked files” heading in your status output. Untracked basically means that Git sees a file you didn’t have in the previous snapshot (commit); Git won’t start including it in your commit snapshots until you explicitly tell it to do so. It does this so you don’t accidentally begin including generated binary files or other files that you did not mean to include. You do want to start including README, so let’s start tracking the file.
In order to begin tracking a new file, you use the command git add. To begin tracking the README file, you can run this:
$ git add README
If you run your status command again, you can see that your README file is now tracked and staged to be committed:
$ git status
On branch masterChanges to be committed: (use "git reset HEAD <file>..." to unstage) new file: README
You can tell that it’s staged because it’s under the “Changes to be committed” heading. If you commit at this point, the version of the file at the time you ran git add is what will be in the historical snapshot. You may recall that when you ran git init earlier, you then ran git add (files) – that was to begin tracking files in your directory. The git add command takes a path name for either a file or a directory; if it’s a directory, the command adds all the files in that directory recursively.
Let’s change a file that was already tracked. If you change a previously tracked file called “CONTRIBUTING.md” and then run your git status command again, you get something that looks like this:
$ git status
On branch masterChanges to be committed: (use "git reset HEAD <file>..." to unstage) new file: READMEChanges not staged for commit: (use "git add <file>..." to update what will be committed) (use "git checkout -- <file>..." to discard changes in working directory) modified: CONTRIBUTING.md
The “CONTRIBUTING.md” file appears under a section named “Changed but not staged for commit” – which means that a file that is tracked has been modified in the working directory but not yet staged. To stage it, you run the git add command. git add is a multipurpose command – you use it to begin tracking new files, to stage files, and to do other things like marking merge-conflicted files as resolved. It may be helpful to think of it more as “add this content to the next commit” rather than “add this file to the project”. Let’s run git add now to stage the “CONTRIBUTING.md” file, and then run git status again:
$ git add CONTRIBUTING.md
$ git status
On branch masterChanges to be committed: (use "git reset HEAD <file>..." to unstage) new file: README modified: CONTRIBUTING.md
Both files are staged and will go into your next commit. At this point, suppose you remember one little change that you want to make in CONTRIBUTING.md before you commit it. You open it again and make that change, and you’re ready to commit. However, let’s run git status one more time:
$ vim CONTRIBUTING.md
$ git status
On branch masterChanges to be committed: (use "git reset HEAD <file>..." to unstage) new file: README modified: CONTRIBUTING.mdChanges not staged for commit: (use "git add <file>..." to update what will be committed) (use "git checkout -- <file>..." to discard changes in working directory) modified: CONTRIBUTING.md
What the heck? Now CONTRIBUTING.md is listed as both staged and unstaged. How is that possible? It turns out that Git stages a file exactly as it is when you run the git add command. If you commit now, the version of CONTRIBUTING.md as it was when you last ran the git add command is how it will go into the commit, not the version of the file as it looks in your working directory when you run git commit. If you modify a file after you run git add, you have to run git add again to stage the latest version of the file:
$ git add CONTRIBUTING.md
$ git status
On branch masterChanges to be committed: (use "git reset HEAD <file>..." to unstage) new file: README modified: CONTRIBUTING.md
While the git status output is pretty comprehensive, it’s also quite wordy. Git also has a short status flag so you can see your changes in a more compact way. If you run git status -s or git status --short you get a far more simplified output from the command.
$ git status -s
M READMEMM RakefileA lib/git.rbM lib/simplegit.rb?? LICENSE.txt
New files that aren’t tracked have a ?? next to them, new files that have been added to the staging area have an A, modified files have an M and so on. There are two columns to the output – the left hand column indicates that the file is staged and the right hand column indicates that it’s modified. So for example in that output, the README file is modified in the working directory but not yet staged, while the lib/simplegit.rb file is modified and staged. The Rakefile was modified, staged and then modified again, so there are changes to it that are both staged and unstaged.
Often, you’ll have a class of files that you don’t want Git to automatically add or even show you as being untracked. These are generally automatically generated files such as log files or files produced by your build system. In such cases, you can create a file listing patterns to match them named .gitignore. Here is an example .gitignore file:
$ cat .gitignore
*.[oa]*~
The first line tells Git to ignore any files ending in “.o” or “.a” – object and archive files that may be the product of building your code. The second line tells Git to ignore all files that end with a tilde (~), which is used by many text editors such as Emacs to mark temporary files. You may also include a log, tmp, or pid directory; automatically generated documentation; and so on. Setting up a .gitignore file before you get going is generally a good idea so you don’t accidentally commit files that you really don’t want in your Git repository.
The rules for the patterns you can put in the .gitignore file are as follows:
Blank lines or lines starting with # are ignored.
Standard glob patterns work.
You can end patterns with a forward slash (/) to specify a directory.
You can negate a pattern by starting it with an exclamation point (!).
Glob patterns are like simplified regular expressions that shells use. An asterisk (*) matches zero or more characters; [abc] matches any character inside the brackets (in this case a, b, or c); a question mark (?) matches a single character; and brackets enclosing characters separated by a hyphen([0-9]) matches any character between them (in this case 0 through 9). You can also use two asterisks to match nested directories; a/**/z would match a/z, a/b/z, a/b/c/z, and so on.
Here is another example .gitignore file:
# no .a files
*.a
# but do track lib.a, even though you're ignoring .a files above
!lib.a
# only ignore the root TODO file, not subdir/TODO
/TODO
# ignore all files in the build/ directory
build/
# ignore doc/notes.txt, but not doc/server/arch.txt
doc/*.txt
# ignore all .txt files in the doc/ directory
doc/**/*.txt
GitHub maintains a fairly comprehensive list of good .gitignore file examples for dozens of projects and languages at https://github.com/github/gitignore if you want a starting point for your project.
If the git status command is too vague for you – you want to know exactly what you changed, not just which files were changed – you can use the git diff command. We’ll cover git diff in more detail later, but you’ll probably use it most often to answer these two questions: What have you changed but not yet staged? And what have you staged that you are about to commit? Although git status answers those questions very generally by listing the file names, git diff shows you the exact lines added and removed – the patch, as it were.
Let’s say you edit and stage the README file again and then edit the CONTRIBUTING.md file without staging it. If you run your git status command, you once again see something like this:
$ git status
On branch masterChanges to be committed: (use "git reset HEAD <file>..." to unstage) new file: READMEChanges not staged for commit: (use "git add <file>..." to update what will be committed) (use "git checkout -- <file>..." to discard changes in working directory) modified: CONTRIBUTING.md
To see what you’ve changed but not yet staged, type git diff with no other arguments:
$ git diff
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.mdindex 8ebb991..643e24f 100644--- a/CONTRIBUTING.md+++ b/CONTRIBUTING.md@@ -65,7 +65,8 @@ branch directly, things can get messy. Please include a nice description of your changes when you submit your PR; if we have to read the whole diff to figure out why you're contributing in the first place, you're less likely to get feedback and have your change-merged in.+merged in. Also, split your changes into comprehensive chunks if you patch is+longer than a dozen lines. If you are starting to work on a particular area, feel free to submit a PR that highlights your work in progress (and note in the PR title that it's
That command compares what is in your working directory with what is in your staging area. The result tells you the changes you’ve made that you haven’t yet staged.
If you want to see what you’ve staged that will go into your next commit, you can use git diff --staged. This command compares your staged changes to your last commit:
It’s important to note that git diff by itself doesn’t show all changes made since your last commit – only changes that are still unstaged. This can be confusing, because if you’ve staged all of your changes, git diff will give you no output.
For another example, if you stage the CONTRIBUTING.md file and then edit it, you can use git diff to see the changes in the file that are staged and the changes that are unstaged. If our environment looks like this:
$ git add CONTRIBUTING.md
$echo'test line' >> CONTRIBUTING.md
$ git status
On branch masterChanges to be committed: (use "git reset HEAD <file>..." to unstage) modified: CONTRIBUTING.mdChanges not staged for commit: (use "git add <file>..." to update what will be committed) (use "git checkout -- <file>..." to discard changes in working directory) modified: CONTRIBUTING.md
Now you can use git diff to see what is still unstaged
$ git diff
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.mdindex 643e24f..87f08c8 100644--- a/CONTRIBUTING.md+++ b/CONTRIBUTING.md@@ -119,3 +119,4 @@ at the ## Starter Projects See our [projects list](https://github.com/libgit2/libgit2/blob/development/PROJECTS.md).+# test line
and git diff --cached to see what you’ve staged so far (–staged and –cached are synonyms):
$ git diff --cached
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.mdindex 8ebb991..643e24f 100644--- a/CONTRIBUTING.md+++ b/CONTRIBUTING.md@@ -65,7 +65,8 @@ branch directly, things can get messy. Please include a nice description of your changes when you submit your PR; if we have to read the whole diff to figure out why you're contributing in the first place, you're less likely to get feedback and have your change-merged in.+merged in. Also, split your changes into comprehensive chunks if you patch is+longer than a dozen lines. If you are starting to work on a particular area, feel free to submit a PR that highlights your work in progress (and note in the PR title that it's
Git Diff in an External Tool
We will continue to use the git diff command in various ways throughout the rest of the book. There is another way to look at these diffs if you prefer a graphical or external diff viewing program instead. If you run git difftool instead of git diff, you can view any of these diffs in software like Araxis, emerge, vimdiff and more. Run git difftool --tool-help to see what is available on your system.
Now that your staging area is set up the way you want it, you can commit your changes. Remember that anything that is still unstaged – any files you have created or modified that you haven’t run git add on since you edited them – won’t go into this commit. They will stay as modified files on your disk. In this case, let’s say that the last time you ran git status, you saw that everything was staged, so you’re ready to commit your changes. The simplest way to commit is to type git commit:
$ git commit
Doing so launches your editor of choice. (This is set by your shell’s $EDITOR environment variable – usually vim or emacs, although you can configure it with whatever you want using the git config --global core.editor command as you saw in Chapter 1).
The editor displays the following text (this example is a Vim screen):
# Please enter the commit message for your changes. Lines starting
# with '#' will be ignored, and an empty message aborts the commit.
# On branch master
# Changes to be committed:
# new file: README
# modified: CONTRIBUTING.md
#
~
~
~
".git/COMMIT_EDITMSG" 9L, 283C
You can see that the default commit message contains the latest output of the git status command commented out and one empty line on top. You can remove these comments and type your commit message, or you can leave them there to help you remember what you’re committing. (For an even more explicit reminder of what you’ve modified, you can pass the -v option to git commit. Doing so also puts the diff of your change in the editor so you can see exactly what changes you’re committing.) When you exit the editor, Git creates your commit with that commit message (with the comments and diff stripped out).
Alternatively, you can type your commit message inline with the commit command by specifying it after a -m flag, like this:
$ git commit -m "Story 182: Fix benchmarks for speed"[master 463dc4f] Story 182: Fix benchmarks for speed 2 files changed, 2 insertions(+) create mode 100644 README
Now you’ve created your first commit! You can see that the commit has given you some output about itself: which branch you committed to (master), what SHA-1 checksum the commit has (463dc4f), how many files were changed, and statistics about lines added and removed in the commit.
Remember that the commit records the snapshot you set up in your staging area. Anything you didn’t stage is still sitting there modified; you can do another commit to add it to your history. Every time you perform a commit, you’re recording a snapshot of your project that you can revert to or compare to later.
Although it can be amazingly useful for crafting commits exactly how you want them, the staging area is sometimes a bit more complex than you need in your workflow. If you want to skip the staging area, Git provides a simple shortcut. Adding the -a option to the git commit command makes Git automatically stage every file that is already tracked before doing the commit, letting you skip the git add part:
$ git status
On branch masterChanges not staged for commit: (use "git add <file>..." to update what will be committed) (use "git checkout -- <file>..." to discard changes in working directory) modified: CONTRIBUTING.mdno changes added to commit (use "git add" and/or "git commit -a")$ git commit -a -m 'added new benchmarks'[master 83e38c7] added new benchmarks 1 file changed, 5 insertions(+), 0 deletions(-)
Notice how you don’t have to run git add on the “CONTRIBUTING.md” file in this case before you commit.
To remove a file from Git, you have to remove it from your tracked files (more accurately, remove it from your staging area) and then commit. The git rm command does that, and also removes the file from your working directory so you don’t see it as an untracked file the next time around.
If you simply remove the file from your working directory, it shows up under the “Changed but not updated” (that is, unstaged) area of your git status output:
$ rm PROJECTS.md
$ git status
On branch masterYour branch is up-to-date with 'origin/master'.Changes not staged for commit: (use "git add/rm <file>..." to update what will be committed) (use "git checkout -- <file>..." to discard changes in working directory) deleted: PROJECTS.mdno changes added to commit (use "git add" and/or "git commit -a")
Then, if you run git rm, it stages the file’s removal:
$ git rm PROJECTS.md
rm 'PROJECTS.md'$ git status
On branch masterChanges to be committed: (use "git reset HEAD <file>..." to unstage) deleted: PROJECTS.md
The next time you commit, the file will be gone and no longer tracked. If you modified the file and added it to the index already, you must force the removal with the -f option. This is a safety feature to prevent accidental removal of data that hasn’t yet been recorded in a snapshot and that can’t be recovered from Git.
Another useful thing you may want to do is to keep the file in your working tree but remove it from your staging area. In other words, you may want to keep the file on your hard drive but not have Git track it anymore. This is particularly useful if you forgot to add something to your .gitignore file and accidentally staged it, like a large log file or a bunch of .a compiled files. To do this, use the --cached option:
$ git rm --cached README
You can pass files, directories, and file-glob patterns to the git rm command. That means you can do things such as
$ git rm log/\*.log
Note the backslash (\) in front of the *. This is necessary because Git does its own filename expansion in addition to your shell’s filename expansion. This command removes all files that have the .log extension in the log/ directory. Or, you can do something like this:
Unlike many other VCS systems, Git doesn’t explicitly track file movement. If you rename a file in Git, no metadata is stored in Git that tells it you renamed the file. However, Git is pretty smart about figuring that out after the fact – we’ll deal with detecting file movement a bit later.
Thus it’s a bit confusing that Git has a mv command. If you want to rename a file in Git, you can run something like
$ git mv file_from file_to
and it works fine. In fact, if you run something like this and look at the status, you’ll see that Git considers it a renamed file:
$ git mv README.md README
$ git status
On branch masterChanges to be committed: (use "git reset HEAD <file>..." to unstage) renamed: README.md -> README
However, this is equivalent to running something like this:
Git figures out that it’s a rename implicitly, so it doesn’t matter if you rename a file that way or with the mv command. The only real difference is that mv is one command instead of three – it’s a convenience function. More important, you can use any tool you like to rename a file, and address the add/rm later, before you commit.
Q: If Java uses the pass-by reference, why won’t a swap function work?
A: Java does manipulate objects by reference, and all object variables are references. However, Java doesn’t pass method arguments by reference; it passes them by value.
Take the badSwap() method for example:
public void badSwap(int var1, int var2)
{
int temp = var1;
var1 = var2;
var2 = temp;
}
When badSwap() returns, the variables passed as arguments will still hold their original values. The method will also fail if we change the arguments type from int to Object, since Java passes object references by value as well. Now, here is where it gets tricky:
If we execute this main() method, we see the following output:
X: 0 Y: 0
X: 0 Y: 0
X: 100 Y: 100
X: 0 Y: 0
The method successfully alters the value of pnt1, even though it is passed by value; however, a swap of pnt1 and pnt2 fails! This is the major source of confusion. In the main() method, pnt1 and pnt2 are nothing more than object references. When you pass pnt1 and pnt2 to the tricky() method, Java passes the references by value just like any other parameter. This means the references passed to the method are actually copies of the original references. Figure 1 below shows two references pointing to the same object after Java passes an object to a method.
Figure 1. After being passed to a method, an object will have at least two references
Java copies and passes the reference by value, not the object. Thus, method manipulation will alter the objects, since the references point to the original objects. But since the references are copies, swaps will fail. As Figure 2 illustrates, the method references swap, but not the original references. Unfortunately, after a method call, you are left with only the unswapped original references. For a swap to succeed outside of the method call, we need to swap the original references, not the copies.
Figure 2. Only the method references are swapped, not the original ones
O’Reilly’s Java in a Nutshell by David Flanagan (see Resources) puts it best: “Java manipulates objects ‘by reference,’ but it passes object references to methods ‘by value.'” As a result, you cannot write a standard swap method to swap objects.
Tony Sintes is a principal consultant at BroadVision. Tony, a Sun-certified Java 1.1 programmer and Java 2 developer, has worked with Java since 1997.
Figure 1. After being passed to a method, an object will have at least two references
Figure 2. Only the method references are swapped, not the original ones
if(view instanceofImageView){ImageView imageView =(ImageView) view;// do what you want with imageView}elseif(view instanceofTextView){TextView textView =(TextView) view;// do what you want with textView}elseif...
An Android Toast is a small message displayed on the screen, similar to a tool tip or other similar popup notification. A Toast is displayed on top of the main content of an activity, and only remains visible for a short time period. This screenshot shows how a Toast looks like on the screen:
The Toast is shown at the bottom of the screen in the above screenshot (but you can change that).
Creating a Toast
Here is an Android Toast example:
Toast toast = Toast.makeText(getApplicationContext(),
"This is a message displayed in a Toast",
Toast.LENGTH_SHORT);
toast.show();
The Toast.makeText() method is a factory method which creates a Toast object. The method takes 3 parameters. First the methods needs a Context object which is obtained by calling getApplicationContext(). Note: The getApplicationContext() method is a method that exists inside activities, so the above code has to be located in an Activity subclass to work.
The second parameter is the text to be displayed in the Toast. The third parameter is the time duration the Toast is to be displayed. The Toast class contains two predefined constants you can use: Toast.LENGTH_SHORT and Toast.LENGTH_LONG. You will have to experiment with these two values to see which fits your situation better.
Toast Positioning
You can change the positioning on the screen of a Toast message using the setGravity() method. Here is a ToastsetGravity() example:
toast.setGravity(Gravity.CENTER, 0, 0);
The first parameter of the setGravity() method specifies the overall position of the Toast. You can use the following constants in the Gravity class to specify the overall position:
TOP
BOTTOM
LEFT
RIGHT
CENTER
CENTER_HORIZONTAL
CENTER_VERTICAL
Each of these constants defines the position in either the X or Y direction, except for the CENTER constant which implies centered both horizontally and vertically. You can combine these constants using the | (or) operator, like this:
The two other parameters of the setGravity() method are an X and Y offset to the position defined by the Gravity constant. If, for instance, you need the Toast to be displayed at the top, centered horizontally, but 20 pixels down from the top position, you would use this setGravity() call:
It is possible to define a custom View for your Toast. To do so, first you must create a layout XML file for the custom View. Here is an example Toast layout XML file:
Put this layout XML file into your Android project’s /app/src/main/res/layout directory and name the file my_toast.xml .
To use this layout XML file with a Toast you write this code:
LayoutInflater inflater = getLayoutInflater();
View toastLayout = inflater.inflate(R.layout.my_toast,
(ViewGroup) findViewById(R.id.toast_root_view));
TextView header = (TextView) toastLayout.findViewById(R.id.toast_header);
header.setText("Message for you:");
TextView body = (TextView) toastLayout.findViewById(R.id.toast_body);
body.setText("You have got mail!");
Toast toast = new Toast(getApplicationContext());
toast.setGravity(Gravity.CENTER, 0, 0);
toast.setDuration(Toast.LENGTH_LONG);
toast.setView(toastLayout);
toast.show();
First you obtain the LayoutInflater. You use that to inflate (create) the View defined by your the layout XML file named my_toast.xml (referred to by R.layout.my_toast).
Notice the findViewById(R.id.toast_root_view) call as the second parameter to the inflate() method call. This finds the root ViewGroup in the my_toast.xml layout XML file (the root ViewGroup has the id toast_view_group). This call is necessary for the inflated View to know what the root ViewGroup inside the inflated View is.
Once the View is created, you obtain the two TextView components from the View and set their texts.
Finally, you create a Toast object, set its gravity (position / alignment), its duration, its View and then show it.
The 
谢嘉琪 