你的手机上没有地方可以读取System.out.println();

相反，如果你想查看某些结果，要么查看你的logcat/控制台窗口，要么让Toast或Snackbar(如果你在较新的设备上)出现在设备的屏幕上，并显示消息:) 这就是我所做的，当我必须检查例如它在开关情况代码!祝你编码愉快!：）

Correction: On the emulator and most devices System.out.println gets redirected to LogCat and printed using Log.i(). This may not be true on very old or custom Android versions. Original: There is no console to send the messages to so the System.out.println messages get lost. In the same way this happens when you run a "traditional" Java application with javaw. Instead, you can use the Android Log class: Log.d("MyApp","I am here"); You can then view the log either in the Logcat view in Eclipse, or by running the following command: adb logcat It's good to get in to the habit of looking at logcat output as that is also where the Stack Traces of any uncaught Exceptions are displayed. The first Entry to every logging call is the log tag which identifies the source of the log message. This is helpful as you can filter the output of the log to show just your messages. To make sure that you're consistent with your log tag it's probably best to define it once as a static final String somewhere. Log.d(MyActivity.LOG_TAG,"Application started"); There are five one-letter methods in Log corresponding to the following levels: e() - Error w() - Warning i() - Information d() - Debug v() - Verbose wtf() - What a Terrible Failure The documentation says the following about the levels: Verbose should never be compiled into an application except during development. Debug logs are compiled in but stripped at runtime. Error, warning and info logs are always kept.

使用Log类。输出与LogCat可见

我将把这个留给更多的访问者，因为对我来说，这是关于主线程无法System.out.println的一些事情。

public class LogUtil {

private static String log = "";
private static boolean started = false;
public static void print(String s) {
    //Start the thread unless it's already running
    if(!started) {
        start();
    }
    //Append a String to the log
    log += s;
}

public static void println(String s) {
    //Start the thread unless it's already running
    if(!started) {
        start();
    }
    //Append a String to the log with a newline.
    //NOTE: Change to print(s + "\n") if you don't want it to trim the last newline.
    log += (s.endsWith("\n") )? s : (s + "\n");
}

private static void start() {
    //Creates a new Thread responsible for showing the logs.
    Thread thread = new Thread(new Runnable() {
        @Override
        public void run() {
            while(true) {
                //Execute 100 times per second to save CPU cycles.
                try {
                    Thread.sleep(10);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                //If the log variable has any contents...
                if(!log.isEmpty()) {
                    //...print it and clear the log variable for new data.
                    System.out.print(log);
                    log = "";
                }
            }
        }
    });
    thread.start();
    started = true;
}
}

用法:LogUtil。println("这是一个字符串");

最近我在Android Studio 3.3中发现了同样的问题。我关闭了其他Android工作室项目，Logcat开始工作。上面这个公认的答案根本不符合逻辑。

对我有效的解决方案:

在Logcat中。(如果尚未显示Logcat，则显示Logcat。点击查看菜单——>工具窗口——>Logcat)。它显示为System。不是像你期望的那样System.out.println。如果你还没有重新构建应用程序。

在图片中，黄色突出显示的是系统。输出“Hello again”。