Why doesn't "System.out.println" work in Android?

Question

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.

Use the Log class. Output visible with LogCat

Yes it does. If you're using the emulator, it will show in the Logcat view under the System.out tag. Write something and try it in your emulator.

Of course, to see the result in logcat, you should set the Log level at least to "Info" (Log level in logcat); otherwise, as it happened to me, you won't see your output.

if you really need System.out.println to work(eg. it's called from third party library). you can simply use reflection to change out field in System.class:

try{
    Field outField = System.class.getDeclaredField("out");
    Field modifiersField = Field.class.getDeclaredField("accessFlags");
    modifiersField.setAccessible(true);
    modifiersField.set(outField, outField.getModifiers() & ~Modifier.FINAL);
    outField.setAccessible(true);
    outField.set(null, new PrintStream(new RedirectLogOutputStream()); 
}catch(NoSuchFieldException e){
    e.printStackTrace(); 
}catch(IllegalAccessException e){
    e.printStackTrace(); 
}

RedirectLogOutputStream class:

public class RedirectLogOutputStream extends OutputStream{
    private String mCache;

    @Override
    public void write(int b) throws IOException{
        if(mCache == null) mCache = "";

        if(((char) b) == '\n'){
            Log.i("redirect from system.out", mCache);
            mCache = "";
        }else{
            mCache += (char) b;
        }
    }
}

it is not displayed in your application... it is under your emulator's logcat