How to get CPU usage statistics on Android?

ATTENTION: This answer is old and does NOT work on newer versions of Android due to enhanced security mechanisms.

For complete CPU usage (not for each process) you can use:

    /**
 * 
 * @return integer Array with 4 elements: user, system, idle and other cpu
 *         usage in percentage.
 */
private int[] getCpuUsageStatistic() {

    String tempString = executeTop();

    tempString = tempString.replaceAll(",", "");
    tempString = tempString.replaceAll("User", "");
    tempString = tempString.replaceAll("System", "");
    tempString = tempString.replaceAll("IOW", "");
    tempString = tempString.replaceAll("IRQ", "");
    tempString = tempString.replaceAll("%", "");
    for (int i = 0; i < 10; i++) {
        tempString = tempString.replaceAll("  ", " ");
    }
    tempString = tempString.trim();
    String[] myString = tempString.split(" ");
    int[] cpuUsageAsInt = new int[myString.length];
    for (int i = 0; i < myString.length; i++) {
        myString[i] = myString[i].trim();
        cpuUsageAsInt[i] = Integer.parseInt(myString[i]);
    }
    return cpuUsageAsInt;
}

private String executeTop() {
    java.lang.Process p = null;
    BufferedReader in = null;
    String returnString = null;
    try {
        p = Runtime.getRuntime().exec("top -n 1");
        in = new BufferedReader(new InputStreamReader(p.getInputStream()));
        while (returnString == null || returnString.contentEquals("")) {
            returnString = in.readLine();
        }
    } catch (IOException e) {
        Log.e("executeTop", "error in getting first line of top");
        e.printStackTrace();
    } finally {
        try {
            in.close();
            p.destroy();
        } catch (IOException e) {
            Log.e("executeTop",
                    "error in closing and destroying top process");
            e.printStackTrace();
        }
    }
    return returnString;
}

Have fun with it :)

You can read /proc/stat and parse the file contents. The first line is like:
cpu 79242 0 74306 842486413 756859 6140 67701 0
The meanings of the columns are as follows, from left to right:

 - 1st column : user = normal processes executing in user mode
 - 2nd column : nice = niced processes executing in user mode
 - 3rd column : system = processes executing in kernel mode
 - 4th column : idle = twiddling thumbs
 - 5th column : iowait = waiting for I/O to complete
 - 6th column : irq = servicing interrupts
 - 7th column : softirq = servicing softirqs

Average idle percentage :
X % = ( idle * 100 ) / ( user + nice + system + idle + iowait + irq + softirq )
You can compute the difference in idle between time deltas, and figure CPU usage.

You can reference the "DevTools" project.

Using ActivityManager you can get lots information, such as ActivityManager.RunningAppProcessInfo, ActivityManager.RunningTaskInfo, ...

But I am not sure the result will same as 'top' command.

see ActivityManager