import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.KeyEvent;
import android.webkit.WebSettings;
import android.webkit.WebView;
import android.webkit.WebViewClient;

public class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    WebView myWebView = (WebView) findViewById(R.id.myWebView);
    myWebView.loadUrl("la url");
    myWebView.setWebViewClient(new MyWebViewClient());
    WebSettings webSettings = myWebView.getSettings();
    webSettings.setJavaScriptEnabled(true);
}


private class MyWebViewClient extends WebViewClient {

}

public boolean onKeyDown(int keyCode, KeyEvent event) {
    WebView mWebView;
    mWebView = (WebView) findViewById(R.id.myWebView);
    if (event.getAction() == KeyEvent.ACTION_DOWN) {
        switch (keyCode) {
            case KeyEvent.KEYCODE_BACK:
                if (mWebView.canGoBack()) {
                    mWebView.goBack();
                } else {
                    finish();
                }
                return true;
        }
    }
    return super.onKeyDown(keyCode, event);
}

}